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A vertical border with a pattern of concentric circles and dots on a light beige background. A large, stylized letter "T" in the center of a rectangular frame. WATERMARK A black and white illustration of a telescope mounted on a brick structure, with a starry night sky in the background. SIX INCH ACHROMATIC TELESCOPE. MADE BY MR. HENRY FITZ OF THE CITY OF NEW YORK, AND HAVING BEEN USED FOR THE PURPOSE OF OBSERVING THE SKY OVER THE CITY OF NEW YORK AND NEAR OF THE SAME CITY, IT IS 5 FEET IN LENGTH AND THE TOTAL COST OF THE INSTRUMENT ABOUT $1000. IT SHOWS THE MOON AND PLANETS WITH THEIR APPARENT 3RD & 6TH STARS IN THE TRAPEZUM OF ORION'S, SEPARATES LARETIS, SEANOROMEO, AND OTHER CLOSE STARS FROM EACH OTHER, AND SHOWS THE OBSERVATORY OF LEWIS M. RUTHERFORD, ESQ. IN THE CITY OF NEW YORK. third. 1863. A PRIMARY ASTRONOMY, FOR SCHOOLS AND FAMILIES: ADAPTED TO THE CAPACITY OF YOUTH, AND ILLUSTRATED BY NEARLY TWO HUNDRED ENGRAVINGS. BY HIRAM MATTISON, PROFESSOR OF NATURAL PHILOSOPHY AND ASTRONOMY IN THE PALOUL SEMINARY; AUTHOR OF THE ELEMENTARY ASTRONOMY, ASTRONOMICAL MAPS, ETC. ETC. NEW YORK: HUNTINGTON AND SAVAGE, MASON AND LAW, 21 PARK ROW. (Contains the Actor House.) 1851. Depicted at the Club Office No. 20th May. 1851. Entered according to Act of Congress, in the year 1857, By HIRAM MATTISON, In the Clerk's Office of the District Court for the Southern District of New York. 03-20551 Submitted by RICHARD VALENTINE, 41 Durlst, New York. **CONTENTS.** **PART FIRST.** PRELIMINARY OBSERVATIONS AND DEFINITIONS. I. History of Astronomy. 5 II. The Modern, or Copernican System. 9 III. Geometrical Definitions. 14 IV. The Sphere and the Plane. 18 V. Of the Circle and the Ellipse. 30 VI. The Terrestrial and Celestial Spheres. 24 **PART SECOND.** OF THE SOLAR SYSTEM. VII. Bodies that compose the System. 31 VIII. Names of the Planets, Signs, etc. 33 IX. Distances of the Planets from the Sun. 38 X. Distances of the Planets from each other. 46 XI. Magnitude, Density, and Gravitation of the Planets. 45 XII. Revolution of the Planets around the Sun. 46 XIII. Revolutions of the Planets around each other. 50 XIV. Direct and Retrograde Moons, Planets Stationary, etc. 53 XV. Diurnal Revolutions of the Planets, Time, etc. 57 XVI. The Moon's Phases and its Revolution about the Earth. 62 XVII. Form and Position of the Planetary Orbits, Nodes, etc. 69 XVIII. Of Transits, when one Body passes another, the Seasons, etc. 73 XIX. Seasons of the different Planets; Telescopeous Views, etc. 82 XXI. Telescopeous Views of the Planets continued. 87 XXII. The Moon's Phases continued. 92 XXIII. Revolutions of the Moon around the Earth. 98 XXIV. The Moon's Changes continued. 101 XXV. The Moon's Phases continued, etc., of the Moon 106 XXVI. Eclipse of the Sun. 113 XXVII. eclipses of the Moon continued 120 XXVIII. The Moon's Phases continued, eclipses, etc.. 125 XXIX. Satellites of Saturn, Herschel, and Neptune 131 XXX. Of the Tides 131 XXXI. Of Swing and Neap Tides 138
4 SUGGESTIONS TO TEACHERS, ETC.
XXXII. Of Comets 188
XXXIII. Of the Sun 145
XXXIV. General Remarks upon the Solar System 152
PART THIRD
OF THE SEDIMENTARY HEAVENS.
XXXX. The Fixed Stars--their Number, Placements, etc. 152
XXXV. Double, Variable, and Temporary Stars 139
XXXVI. Omnipresent Stars 161
XXXVII. Of the Atmosphere, Winds, Clouds, Storms, etc. 165
SUGGESTIONS TO TEACHERS AND STUDENTS. 1. The course prior should generally be committed to memory, as an answer to the questions. 2. In beginning the course the class should be carefully studied; and the student should be prepared, if called upon, to make drawings similar to those found in the lesson upon the blackboard. 3. In studying the illustrations, the pupil should generally imagine himself in the position of the observer represented in each book being "North"; the bottom South; the left hand East; and the right hand West. 4. The teacher should call upon several members of the class, in succession, to sketch the diagrams as they occur in the lesson, and to show how they illustrate the point under consideration--this explanation being in all cases ex tempore. 5. To facilitate such inquiries, important lessons or terms should receive special attention; and to facilitate such inquiries, important words are not only pronounced and defined, but are traced to their original sources. 6. Finally, whenever practicable, let the objects or phenomena described in the lessons be looked up and observed as they really appear in nature. This will invigorate the subject with new interest, and will enable students to know more of this most wonderful and most sublime of all human studies. New York, January, 1851. # PRIMARY ASTRONOMY. ## PART I. PRELIMINARY OBSERVATIONS AND DEFINITIONS. ### LESSON I. HISTORY OF ASTRONOMY. **1. QUESTION—What is Astronomy?** **ANSWER—It is the science of the Heavenly Bodies—the Sun, Moon, Planets, Comets, and Stars.** **2. What does it teach respecting these bodies?** Their names, distances, magnitudes, and motions, and the causes of Day and Night, the Seasons, Eclipses, Tides, and various other phenomena.* **3. Was Astronomy known in the ancient?** It was; and was taught in Egypt, Chaldea, India, China, and Greece long before the birth of Christ. **4. What were the occupations of the first Astronomers?** They were shepherds and herdsmen. **5. By what were they led to this study?** By observing the movements of the sun, moon, and stars, while watching their flecks from year to year in the open fields.* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 18 6 PRIMARY ASTRONOMY. ANCIENT ASTRONOMERS OBSERVING THE HEAVENS. A black and white illustration of a group of people observing the sky with telescopes. The background shows a landscape with hills and trees. G. Where do modern Astronomers make their observations? In what are called Observatories. (An observatory is a place erected for the purpose of observing the heavens. The most celebrated in this country is the one erected by Professor James Ferguson, at Alexandria, Virginia; another is that erected by Mr. John Harrison, at Philadelphia; one at Cambridge, Nova, &c. An effort is now being made (1850) to erect one on Long Island Sound, near New Haven, Connecticut; another is about to be erected at Rutherford, La., may be seen in the rear of his residence, corner of Second Avenue and Eleventh street, New York City.) Dr. DICK'S OBSERVATORY, YORK DUNTER, SCOTLAND. 7. What other advantages have modern Astronomers? They are assisted by many useful instruments, the most important of which is the Telescope. * From the Greek tele, at a distance, and skope, to see. PRIMARY ASTRONOMY. 7 [This is a picture of the Great Refracting Telescope of the Cincinnati Observatory. It is a copy of a photograph taken by the author of the "Neddermorgen," July, 1846. The original (which is an enlargement of a drawing) was made by Mr. William H. Pickering, and our artist has given us an exact miniature of it in this engraving.] MODERN ASTRONOMERS USING A TELESCOPE. This Telescope has an objective (placed in a tube) 30 inches long, and an eyepiece (which is twelve inches in diameter), with which it magnifies ten times; and with this instrument, Mr. Pickering has discovered many new stars, and has made his first discovery of a planet revolving round the sun. Q. What did the ancients think of the form of the earth? Instead of a globe inhabited on all sides, they believed it to be a plane, inhabited on one side only. Q. What did they think of her motions, and those of the heavenly bodies? They believed the earth to be at rest in the center of the universe, and that the sun, moon, and stars actually revolved around her, from east to west, as they appear to do every twenty-four hours. [The ancient astronomers were very cleverly deceived by appearances. Seeing but a small portion of the earth at any one time, they were easily misled in their conclusions. They supposed that the apparent daily revolution of the heavenly bodies was outward from them. In the same manner, they imagined that the sun moved towards them every day, especially when carried swiftly forward without any apparent cause (as when one sees a ship in a stormy or rainy sea); and when for a time they forget their own motion.] Q. What other motions did the ancients in respect to the figure of the world? That it was longest from east to west. [It appeared that travelling east or west had no effect upon the apparent position of the stars, while going north and south did. Hence the conclusion that the earth was flat, and that it was supported by some solid body. This was called the modern system, (antique and lunatic,) the whole of which means length, and the former breadth.] Q. What was the ancient system of Astronomy called, and why? A black-and-white illustration showing a telescope being used to observe celestial objects. S PRIMARY ASTRONOMY. The *Ptolemaic* Theory, from Ptolemy,† an Egyptian philosopher, who flourished in the second century of the Christian era. (1) The word Ptolemy was a common name for the successive Egyptian princes, as at Ptolemais, in Egypt, in the days of the Roman Empire. PTOLEMAIC THEORY OF THE STRUCTURE OF THE UNIVERSE. A diagram showing the structure of the universe according to Ptolemaic theory. It depicts the earth as a sphere with a flat disk (the "celestial sphere") above it, which is divided into four equal parts by two perpendicular lines. Each part is further divided into twelve smaller segments, each containing a planet. The sun is located at the center of the celestial sphere, and the moon is shown orbiting around it. The stars are depicted as points scattered throughout the sky. 2. The ancients supposed the earth to be surrounded by eight crystal-like arches or spheres, called "spherae," which were supposed to be solid and unmovable. In this way they accounted for their falling down upon the earth, while the light of the sun and moon was supposed to be reflected from them. The sun was supposed to be near the earth. The moon was supposed to be in the first sphere, the sun in the fourth, and so on up to the eighth sphere. This theory was called "the system of Ptolemy" because all three particulars. 3. This Ptolemaic theory was embraced by some men and proved satisfactory, one of which was Leon Battista Alberti, who wrote a treatise on it. In his effort to explain this mys- terious system he made many mistakes. He supposed that the earth was a sphere. The earth is seen as a plane, resting upon the head of a huge pyramid, which, in turn, rests upon a vast mountain. With this view, men and stars are always revolving around the earth. This absurd theory was abandoned about 1500 years ago, but revived again in the sixteenth century, or 300 years ago.] * Pronounced Toot-ee-kwa'-go. † Tole'-mee. ‡ Fa'-no. PRIMARY ASTRONOMY. 9 LESSON II. THE MODERN OR COPERNICAN SYSTEM. A diagram showing the Copernican system with the sun at the center, Earth and other planets orbiting around it. The diagram includes concentric circles representing different orbits and lines indicating the direction of rotation. 12. Describe the modern or true theory. It represents the sun as the fixed center, around which the earth and other planets revolve. 13. What does it teach respecting the form of the earth? That it is a Sphere or Globe. 14. To what does it attribute the apparent westward revolution of the sun, moon, and stars? To the actual revolution of the earth eastward upon its axis every twenty-four hours. 10 PRIMARY ASTRONOMY. (If a fly were placed upon artificial globe in a room where several lamps were suspended, and the globe were made to revolve, he would be likely to conclude, if he could see nothing but the lamp-lights, that he was revolving round the lamp-lights, and not upon the globe, and had no suspicion of his own motion. In the same manner the observer of the heavens may be deceived.) **15. What is the modern system of Astronomy called, and why?** The **Copernican Theory**, after Copernicus, a Prussian astronomer, who taught it. **16. When and where was Copernicus born?** In Thorn, in Prussia, in 1473. **17. When did he begin to propagate his theory?** About the year 1510. (This was about 8 years after the discovery of America by Columbus, or 340 years ago.) **18. Was this theory entirely unknown before?) It is supposed to have been taught by Pythagoras,† a Greek philosopher, about 500 years before Christ. **19. Who next taught it after Copernicus?** Galileo, the inventor of the Telescope. **20. How were his doctrines received?** They were pronounced erroneous, and he was obliged to renounce them. (The following is his renunciation, made June 28, 1632: * "A Galileo, in the seventeenth year of his age, having been persuaded by some persons that the earth does not move with regard to the sun, and that the sun moves with regard to the earth, and that the earth does not move with regard to the sun, and that the sun moves with regard to the earth, and that the earth does not move with regard to the sun, and that the sun moves with regard to the earth, and that the earth does not move with regard to the sun, and that the sun moves with regard to the earth, and that the earth does not move with regard to the sun, and that the sun moves with regard to the earth, and that the earth does not move with regard to the sun, and that the sun moves with regard to the earth, and that the earth does not move with regard to the sun, and that the sun moves with regard to the earth, and that the earth does not move with regard to the sun, and that the sun moves with regard to the earth, and that the earth does not move with regard to the sun, and that the sun moves with regard to the earth, and that the earth does not move with regard to the sun, and that the sun moves with regard to the earth, and that the earth does not move with regard to the sun, and that the sun moves with regard to PRIMARY ASTRONOMY. 11 moon, and stars, which are seen to be round, are round; and as all worlds of the same kind we see the shape are round, the inference is that the earth also is round.] 29. What proof from the appearance of the earth's surface? The convexity $^{\circ}$ of the earth's surface shows it to be a globe or ball. 29. How do we know that the earth's surface is convex? It is evident from the fact that the masts of a ship, approaching us from sea, are always seen before the hull; and the higher the observer is on the shore, the more of the ship will he see. CONVEXITY OF THE EARTH'S SURFACE. A diagram showing a ship approaching from sea level, with its masts visible before its hull. The observer on land sees more of the ship as they get closer. [Here the observer upon the shore at A sees only the top-masts of the ship, while those at B can see both masts and part of the hull. This shows that the earth is flat. Now if the water between A and B were exactly flat instead of convex, this would not be so. The observer at A would see only one mast, and at B only two masts, as well as B. The advantage of B over A, in connection with his elevation, shows that the sur- face of the waters is convex between A and the ship.] 29. What additional proof have we that the earth's surface is con- vex? In constructing aqueducts, it is found necessary to have the middle elevated a little above the plane of the ends, otherwise the water will run over in the middle before the ends are full. 8 Convex-conv., revolving toward a globular form; roundness. 1 It would be well, where there is time, during residence, for some member of the Class to draw this and similar figures used for illustration upon the Blackboard, and explain the same in the hearing of the class. 1 Ac-cw-ncvr., a structure for conveying water from one place to another, as across rivers, etc. 12 PHIARY ASTRONOMY. CONVENTS DISCOVERED IN THE BUILDING OF AQUEDUCT. A diagram showing a circular structure with various lines and labels indicating different parts of the building. [In the cut, the top of the aqueduct is perfectly flat; but the water, on being let in, conforms to the general figure of the globe, unless spread in the middle, and runs round the circumference. The interior of this new work is found to be about eight inches in a mill.] 25. What further proof can you offer that the earth is a globe or half a sphere? As we go north upon its surface the stars in the south seem to go down, till they sink below the horizon. [The same is true of those northern stars when we go north, and this is precisely what would occur if the earth were a sphere.] According to the observations of the ancients, at a near the star observed about 40 degrees north of the equator, at which time it was seen to rise in the north, till at 80 degrees it seems to sink toward the south, till at 90 degrees it goes down or out of sight. This is exactly what would happen if the earth be between the star and the observer. This elevo- tion and descent of the sun is caused by the observer's change of position, rather than by any change in the nature of the sun itself. For every degree he rises north or south, he sees one degree more or less of the sun; it follows that the apparent elevation of the star and the latitude of the observer are always equal. (This is a very important fact, which is not often known.) 26. What is the fifth proof that the earth is a globe? The shadow of the moon, when seen upon the surface of the moon, in an eclipse, is convex; which shows the earth, from which the shadow is projected, to be convex also. [It is not pretended that this reasoning is altogether logical and conclusive, for it does not follow that all bodies must be spherical; but it may serve as a sug- gestion. Still, as the earth revolves, and all its positions come into view at some time or other, it is evident that no body could have been made so spherical as to allow of any other form being allowed one such a shadow under any circumstances.] * Hoar'zon.* PRIMARY ASTRONOMY. 13 5. In this cut the shadow of the earth on the moon is exhibited. At A the ob- server sees the moon, and casts a shadow ac- cordingly upon the moon. The shadow of the ob- ject is triangular or prismatic, but that of the earth is circular; but the shadow of the earth is not circular, indicating that it is a globe. A. An observer may see on the upper side of the moon (as at B) a part of its shadow, looking at it through C. A diagram showing a triangular shadow cast by an object on the moon, with a circular shadow cast by the Earth. CONVEYOR OF THE EARTH'S SHADOW. 27. State the last proof that the earth is a globe. It is certain from the fact that many ships have sailed quite around it, going west- ward till they came to the place where they started. [Profoundly thought, a Portuguese, was the first person who sailed round the world; he did so in 1519. It is now quite common for ships to sail round the world, and to visit all parts of the Cape of Good Hope, thus circum- navigating the globe.] 28. What proof have we that the earth revolves or turns over? The apparent revolution of the sun, moon, and stars, from east to west, every twenty-four hours, shows the real revolution of the earth from west to east in the same time. [1. That the earth revolves upon its axis may be inferred from the fact that a sphere is well adapted to such a revolution, and that it is impossible for man to project a ball into space without giving it some motion in one direction or another. 2. That the heavenly bodies appear to revolve westward, is no proof that they are revolving about their own axes; for it is possible that they might be revolving about other axes than at rest, as already shown at Question 6.] - Copernicus tells us that he was first led to this conclusion by observing that when he looked at the stars in different directions, they seemed to move in opposite directions; and that when he turned his body from one direction to another, he saw them move in accordance with his motion. This was sufficient evidence to convince him that the earth was revolving about its own axis, and that it was necessary for him to recede as he rapidly spun from it; and as trees and other objects seem A diagram showing a ship sailing around the world. SHIPS SAILING AROUND THE WORLD. 7 14 **PRIMARY ASTRONOMY.** to glide by us, when, on riding swiftly past them, we lose the consciousness of our own motion. 3. For the sun and fixed stars to revolve daily around the earth would require an immense force, which is not found in nature. The sun, therefore, must have moved to fly onward at the rate of 35 million of miles per hour, or 60,400 miles per second; and the fixed stars must have run at the rate of 14,000 millions of miles per second, or 70,000 miles per second; so that they would appear to us to move with the velocity of light, viz., 186,000 miles per second, or 30,000 miles per second, in each direction. 4. Now let us consider what would happen if the sun and stars were to move in the same direction as we do, viz., towards the north pole of the earth. In this case they would appear to us to move in opposite directions, viz., towards the south pole of the earth. This would be a very remarkable phenomenon, and one which might easily be observed by any one who had a good telescope. 5. Now let us suppose that the sun and stars were to move in opposite directions to our own motion. In this case they would appear to us to move in the same direction as we do. This would be a very remarkable phenomenon, and one which might easily be observed by any one who had a good telescope. 6. Now let us suppose that the sun and stars were to move in opposite directions to our own motion. In this case they would appear to us to move in the same direction as we do. This would be a very remarkable phenomenon, and one which might easily be observed by any one who had a good telescope. 7. Now let us suppose that the sun and stars were to move in opposite directions to our own motion. In this case they would appear to us to move in the same direction as we do. This would be a very remarkable phenomenon, and one which might easily be observed by any one who had a good telescope. 8. Now let us suppose that the sun and stars were to move in opposite directions to our own motion. In this case they would appear to us to move in the same direction as we do. This would be a very remarkable phenomenon, and one which might easily be observed by any one who had a good telescope. 9. Now let us suppose that the sun and stars were to move in opposite directions to our own motion. In this case they would appear to us to move in the same direction as we do. This would be a very remarkable phenomenon, and one which might easily be observed by any one who had a good telescope. 10. Now let us suppose that the sun and stars were to move in opposite directions to our own motion. In this case they would appear to us to move in the same direction as we do. This would be a very remarkable phenomenon, and one which might easily be observed by any one who had a good telescope. LESSON III. GEOMETRICAL DEFINITIONS.* 29. What is a Solid or Body? It is any thing having length, breadth, and thick- ness. 30. Describe a surface. It is the outside or exterior of a body, and has length and breadth only. 31. How are surfaces distinguished? Into Plane, Convex, and Concave. (A surface may also be convex, concave, hard or soft; but the question has reference to its shape only.) 32. What is a Plane Surface? * As some knowledge of geometrical terms and figures is necessary in the study of astronomy two lessons upon the subject are here re- sorted for the benefit of those students who have not studied Geometry. An introduction to some of its first principles in this connection will probably awaken interest in it among all students; but it is hoped that further argument with this beautiful and sublime study. 14 PRIMARY ASTRONOMY. 15 It is a surface that is perfectly flat, like the top of a table or the side of a wall. (L) We may imagine what is called a plane, to extend off beyond the plane surface at any distance, and to continue indefinitely extending. When a plane or a line is extended in this way, it is said to be perpendicular. (2) A plane surface is one which has two plane surfaces; or between two lines, like the plane of a circle. A sheet of tin, laid across a small wire or rod, forms a plane surface. The plane surface may be horizontal, whether horizontally, perpendicularly, or otherwise; and the place which the tin occupies on the surface of the earth, or the circle upon which it is laid. (3) An object which the tin would touch or cut, if extended outward to the horizon, or infinity, is at the plane of the sheet, or the circle upon which it is laid. (4) When a plane surface is placed so that they would never meet or cut each other, however far they might be extended (the two sides of a board, or two sheets of tin placed equidistant from each other every point, represent parallel planes). (5) When are Planes said to be Perpendicular? When one stands exactly upright upon the other; or when they cut each other at right angles. (6) In the figure one plane is placed horizontally, and another plane is placed perpendicularly to it. Therefore perpendicular to each other; however they should be inclined in the observer's eye. (7) When are Planes said to be Inclined? When they intersect or cut each other obliquely. (8) What is meant by the Angle of Inclination? It is the angle contained between A diagram showing two planes at different angles. One plane is horizontal and the other is inclined at an angle. * A plane surface is to be distinguished from a plane.* PERPENDICULAR PLANES. INCLINED PLANES. 16 PRIMARY ASTRONOMY. between the two surfaces of the planes nearest each other. (The space A and B in the adjoining cut represents the Angle of Incidence.) **37. What is meant by the Area of a Plane Figure?** It is the amount of surface combined therewith. **38. What is a Concave Surface?** It is one that is hollowed out like the outside of a bowl. **39. Describe a Concave Surface.** It is one that is hollowed out like the inside of a bowl. **40. What is a Sphere?** It is a Globe or Ball, every part of the surface of which is equidistant from a point within called its center. (Hereon the word globular, or round, like a sphere.) **41. What is a Hemisphere?** It is the half of a sphere or globe. (In discussing this term, both Eastern and Western, and North- ern and Southern hemispheres, but in America the term is only applied to the northern portion of the horizon.) **42. Describe a Spheroid.** It is a body resembling a sphere, but yet not perfectly round or spherical. **43. How many kinds of Spheroids are there?** Two: the Oblate and the Oblong. **44. What is an Oblate Spheroid?** It is a globe that is slightly flattened, as if pressed on opposite sides. (This is a very difficult figure to represent upon paper, but it may be seen in perspective in the Telescope will be at no loss for an illustration.) A diagram showing two views of a spheroid, labeled "A" and "B", with arrows indicating angles of inclination. ANGLE OF INCINERATION. A diagram showing two views of a concave surface, labeled "CONVEX" and "CONCAVE", with arrows indicating angles of inclination. COVERED AND CONCAVE SURFACES. A diagram showing two views of a sphere, labeled "SPHERE". SPHERE. A diagram showing two views of an oblate spheroid, labeled "OBLATE SPHEROID". OBLATE SPHEROID. * A'BE-A' * Spes'ion. PRIMARY ASTRONOMY. 17 **45. Describe an Oblong Spheroid.** It is an elongated sphere. (This figure, like an Oblate Spheroid, admits of various deformations, but is always a sphere.) It may be much or less slightly elongated, and the ends may be alike or differently terminated, or it may be a Circular Spheroid. **46. What is meant by the Axis of a Sphere?** It is the line, real or imaginary, around which it revolves. (A wire run through the center of a round apple would represent the axis of a spherical apple.) **47. What are the Poles of a Sphere?** The extremities of its axis, or the points where the axis cuts the two opposite surfaces. **48. Describe the Equator of a Sphere.** It is an imaginary circle upon its surface, midway between its poles, the plane of which cuts the axis perpendicularly, and divides the sphere into two equal parts or hemispheres. **49. By what other name is the Equator sometimes designated?** Sometimes called a Great Circle, because no larger circle can be drawn upon a sphere. **50. What is a Less Circle?** It is one that divides a sphere into two unequal parts. (Its two cut the circles represented in perspective, and each part contains half of the circumference of the sphere, whose full diameter is included; while the other part contains half of the circumference between the Equator and the Pole, and is consequently less than half of the circumference.) The extent of this Equation will be further illustrated in Lemma VI., where it will be seen that it is greater than a quadrant. * Den*-GIG-XL-TED. A diagram showing an equator (a great circle) on a sphere, with two lesser circles (less circles) intersecting it at right angles. One lesser circle is shown to be smaller than a quadrant. 2* 18 **PRIMARY ASTRONOMY.** **51. What are the Meridians of a Sphere?** They are lines drawn from pole to pole upon its surface. *Meridian.* Upon the Equator, they are equal, and are most distant at the Equator, as shown in the cut. Hence the length of a degree longitudinal depends entirely upon the latitude of the place where it is measured. It varies from 0° to infinity. **LESSON IV.** OF LINES AND ANGLES.* **52. What is a Point?** It is that which has no magnitude or extension, but simply position. *The common notion of a point is derived from the extremity of some slender body, such as the extremity of a common penknife. This being perpetualisable in space, it is called a point. But this is a confused point ; for by the definition, a point has no magnitude.* (NATURAL PHILOSOPHY.) **53. What is a Straight or Right Line?** It is the shortest distance between two points. *A right line.* **54. What is a Curve Line?** It is one that departs continually from a direct course. **55. What are Parallel Lines?** They are such as remain at the same distance from each other throughout their whole extent. **56. What are Oblique Lines?** Such as are not parallel, but incline to- ward or approach each other. *The judicious Teacher will find it advantageous to make a free use of the Blackboard during this and the two following lessons.*
What are Meridians of a Sphere? They are lines drawn from pole to pole upon its surface.
What is a Point? It is that which has no magnitude or extension, but simply position.
What is a Straight or Right Line? It is the shortest distance between two points.
What is a Curve Line? It is one that departs continually from a direct course.
What are Parallel Lines? They are such as remain at the same distance from each other throughout their whole extent.
What are Oblique Lines? Such as are not parallel, but incline to- ward or approach each other.
PRIMARY ASTRONOMY. 19 57. When two lines intersect each other, what is the space included between them called? AN ANGLE. An Angle. 58. How are Angles divided? Into three kinds; namely, the Right Angles, the Acute, and the Obtuse. 59. What is a Right Angle? It is one formed when one straight line intersects another perpendicularly. The angles on either side are equal, and are Right Angles. 60. What are Acute and Obtuse Angles ? When one right line intersects another obliquely, the smaller angle is called an acute angle, and the larger an obtuse angle. (An acute angle is always less, and an obtuse angle greater than a right angle.) There are four such angles in one quarter, and the latter nearly one half of a circle. In every case, the sum of all four angles is four right angles, that is, the sum of the one and the remainder of the three is four right angles. This is very evident, and the other very certain. We both these angles may vary to the amount of a degree. 61. What is a Triangle ? A figure bounded by straight lines, and having only three sides. 62. How many kinds of Triangles are there ? Four: the Equilateral, Isosceles, * Scalene*, and Right-Angled. 63. What is an Equilateral Triangle ? It is one having three sides equal. * Equilateral, from the Latin equus, equal, and lateralis, from linea, side. A triangle with all sides equal. * ISO-CE-LUS. † SCI-LE-TUS. 20 **PRIMARY ASTRONOMY.** **64. What is an Isosceles Triangle?** It is one having but two sides equal. (The term *Isoscelus* is from a Greek word, signifying equal, &c.) It is also called an Equilateral Triangle. Let the student exactly observe the derivation of words, and he will find that they are all derived from the same root. **65. Describe a Scalene Triangle.** It is one having no two sides equal. (The term *Scalene* is from the Greek skalenos, and signifies unequal, unequal.) **66. What is a Right-Angled Triangle?** It is one having a right angle. **67. What are its different sides called respectively?** The Base, the Perpendicular, and the Hypotenuse.* **LESSON V.** OF THE CIRCLE AND THE ELLIPSE. **68. What is a Circle?** It is a plane figure, bounded by a curve line, every part of which is equally distant from a point within called the center. **69. What are Concentric Circles?** Such as are drawn around a common center. (If they are in the same plane they must be drawn at different distances from each other, otherwise they would touch each other. If not drawn they are parallel, otherwise they are not.) * Hepten-`e`, ever, from a Greek word, which signifies to sublend or stretch : a line subtended (see 84) from the base to the perpendicular. A circle with a dot in the center. A CIRCLE. A concentric circle with another circle inside it. CONCENTRIC CIRCLES. PRIMARY ASTRONOMY. 21 **70. Must all Concentric Circles be in the same plane?** No: if drawn around the same common center they will be concentric, though they lie in different planes. (Here the three circles are in different planes, but at the same distance. They are concentric, not parallel.) **71. What is the Circumference of a Circle?** The curve line which bounds it. **72. What is the Diameter?** It is a right line passing through the center, and terminating each way in a point on the circumference. **73. Describe the Radius of a Circle.** It is a right line drawn from its center to any point in the circumference. (The plural of radius is radii, and as small proceed from a common center, light, which proceeds from a luminous point in all directions, is said to radiate; and the lines which represent these rays are called radii.) 22 PRIMARY ASTRONOMY. The 60th part of a minute, or the 756,000th part of a circle. (1. Upon all circles, whether great or small, as supposed to be divided into 360 equal parts called degrees, and marked lines, 360°. Each degree is again subdivided into 60 equal parts called minutes, and marked lines, 60′. Each minute is again divided into 60 equal parts, called seconds. The minutes are marked thus, °', and the seconds thus, ′", so that each second is one sixtieth of a minute. (2. To save the trouble of dividing a circle into degrees and minutes by means of a protractor, it will be sufficient to divide the circumference of a circle into 360 equal parts called degrees, and to mark these divisions with a line or arc. The division of a semicircle or half-circle, divided into 180 equal parts called degrees, is shown in the following figure. To measure an angle as at B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z (fig. 1), place the point of the protractor upon the line P Q so that the center of the protractor may coincide with the point where the line P Q meets the circumference of the circle at A. At this point draw a line parallel to P Q through the center of the protractor. This line will intersect the circumference of the circle at some point which we shall call A'. Then measure along this line from A' to any point on the circumference of the circle which is desired to be measured. This point will be found at some distance from A' which will represent the number of degrees contained in the angle at B. Thus we see that if we wish to measure an angle at B we must first find out how many degrees it contains. We do this by measuring along the circumference of the circle from A' to any other point on the circumference which represents its magnitude in degrees. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in minutes. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in seconds. This process is continued until we have measured all parts of the angle at B. (3. In order to avoid confusion between angles and arcs we shall use different symbols for them. An angle is represented by a capital letter such as A or B or C or D or E or F or G or H or I or J or K or L or M or N or O or P or Q or R or S or T or U or V or W or X or Y or Z (fig. 1). An arc is represented by a small letter such as a b c d e f g h i j k l m n o p q r s t u v w x y z (fig. 1). Thus we see that an angle is represented by a capital letter and an arc by a small letter. (4. It will be seen that when we wish to measure an angle at B we must first find out how many degrees it contains. We do this by measuring along the circumference of the circle from A' to any other point on the circumference which represents its magnitude in degrees. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in minutes. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in seconds. This process is continued until we have measured all parts of the angle at B. (5. In order to avoid confusion between angles and arcs we shall use different symbols for them. An angle is represented by a capital letter such as A or B or C or D or E or F or G or H or I or J or K or L or M or N or O or P or Q or R or S or T or U or V or W or X or Y or Z (fig. 1). An arc is represented by a small letter such as a b c d e f g h i j k l m n o p q r s t u v w x y z (fig. 1). Thus we see that an angle is represented by a capital letter and an arc by a small letter. (6. It will be seen that when we wish to measure an angle at B we must first find out how many degrees it contains. We do this by measuring along the circumference of the circle from A' to any other point on the circumference which represents its magnitude in degrees. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in minutes. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in seconds. This process is continued until we have measured all parts of the angle at B. (7. In order to avoid confusion between angles and arcs we shall use different symbols for them. An angle is represented by a capital letter such as A or B or C or D or E or F or G or H or I or J or K or L or M or N or O or P or Q or R或S或T或U或V或W或X或Y或Z (fig. 1). An arc is represented by a small letter such as a b c d e f g h i j k l m n o p q r s t u v w x y z (fig. 1). Thus we see that an angle is represented by a capital letter and an arc by a small letter. (8. It will be seen that when we wish to measure an angle at B we must first find out how many degrees it contains. We do this by measuring along the circumference of the circle from A' to any other point on the circumference which represents its magnitude in degrees. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in minutes. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in seconds. This process is continued until we have measured all parts of the angle at B. (9. In order to avoid confusion between angles and arcs we shall use different symbols for them. An angle is represented by a capital letter such as A或B或C或D或E或F或G或H或I或J或K或L或M或N或O或P或Q或R或S或T或U或V或W或X或Y或Z (fig. 1). An arc is represented by a small letter such as a b c d e f g h i j k l m n o p q r s t u v w x y z (fig. 1). Thus we see that an angle is represented by a capital letter and an arc by a small letter. (10. It will be seen that when we wish to measure an angle at B we must first find out how many degrees it contains. We do this by measuring along the circumference of the circle from A' to any other point on the circumference which represents its magnitude in degrees. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in minutes. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in seconds. This process is continued until we have measured all parts of the angle at B. (11. In order to avoid confusion between angles and arcs we shall use different symbols for them. An angle is represented by a capital letter such as A 或B 或C 或D 或E 或F 或G 或H 或I 或J 或K 或L 或M 或N 或O 或P 或Q 或R 或S 或T 或U 或V 或W 或X 或Y 或Z (fig. 1). An arc is represented by a small letter such as a b c d e f g h i j k l m n o p q r s t u v w x y z (fig. 1). Thus we see that an angle is represented by a capital letter and an arc by a small letter. (12. It will be seen that when we wish to measure an angle at B we must first find out how many degrees it contains. We do this by measuring along the circumference of the circle from A' to any other point on the circumference which represents its magnitude in degrees. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in minutes. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in seconds. This process is continued until we have measured all parts of the angle at B. (13. In order to avoid confusion between angles and arcs we shall use different symbols for them. An angle is represented by a capital letter such as A 或B 或C 或D 或E 或F 或G 或H 或I 或J 或K 或L 或M 或N 或O 或P 或Q 或R 或S 或T 或U 或V 或W 或X 或Y 或Z (fig. 1). An arc is represented by a small letter such as a b c d e f g h i j k l m n o p q r s t u v w x y z (fig. 1). Thus we see that an angle is represented by a capital letter and an arc by a small letter. (14. It will be seen that when we wish to measure an angle at B we must first find out how many degrees it contains. We do this by measuring along the circumference of the circle from A' to any other point on the circumference which represents its magnitude in degrees. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in minutes. We then proceed to measure along this same line from A' to another point on the circumference which represents its magnitude in seconds. This process is continued until we have measured all parts of the angle at B. (15. In order to avoid confusion between angles and arcs we shall use different symbols for them. An angle is represented by a capital letter such as A 否B 否C 否D 否E 否F 否G 否H 否I 否J 否K 否L 否M 否N 否O 否P 否Q 否R 否S 否T 否U 否V 否W 否X 否Y 否Z (fig. 1). An arc is represented by a small letter such as a b c d e f g h i j k l m n o p q r s t u v w x y z (fig PRIMARY ASTRONOMY. 23 **84. What is a Chord?** It is a straight line within a circle, joining the extremities of an arc. The chord is the shortest line subtended (from one end) under, and near, to, strictly, because it means stretched under the same angle with the radius of the circle. Hence, all arcs and as many chords, the longer arc is a large one, and the shorter arc is a small one. But such division of the circle, whatever great or small, is an arc, and the line joining the extremity of each arc, respectively, is a chord. **85. What is an Ellipse?** It is an oblong figure, like an oblique view of a circle. **86. In what respect does it differ from a Circle?** Its diameters are unequal ; and it has two points called its *Foci*, around which, as centers, the figure is described. **87. How are the unequal diameters of an Ellipse distinguished ?** The longer is called its *Major Axis* and the shorter its *Minor Axis*. [The longer is sometimes called the *Transverse Axis* and the shorter the *Longitudinal Axis*. The former are more simple and perspicuous and therefore more generally used.] **88. What is meant by the Eccentricity of an Ellipse?** It is the distance between the center and either focus. [All circles drawn around two different points in centers are similar geometric circles.] *For* $c$ in the plural of *Focis.* $A$ *x* $c$ is the plural of *Ax-ia.* The eccentricity of an ellipse is that distance from its center. Hence a circle that varies in its distance from the center is called an ellipse. Those who depart from the usual round of thought and custom are called eccentric persons. A diagram showing a triangle labeled "ARC AND CHORD" with three points labeled C, D, E. The arc CDDE is shown as an ellipse. A diagram showing a point labeled "FOCUS OF AN ELLIPSE." Below this point, there are two lines labeled "MAJOR AXIS" and "MINOR AXIS," intersecting at right angles. A diagram showing two points labeled "FOCIS" on opposite sides of a circle, with arrows pointing towards them. The text reads "ECCENTRICITY OF AN ELLIPSE." 24 PRIMARY ASTRONOMY. LESSON VI. THE TERRESTRIAL AND CELESTIAL SPHERES. **80. Can you repeat the definition of a Sphere given in a preceeding lesson? (Question 40.)** **90. What then do you understand by a Terrestrial Sphere?** The Earth or Globe which we inhabit. (Though the earth is not strictly speaking a sphere, as that figure is defined at 41, yet it is so called because it is the nearest approach to a perfect one, in account of its near approach to that figure, and as a matter of convenience. A common artificial sphere is called a globular globe.) **91. What is the Axis of the Earth?** The imaginary line about which it revolves (46). **92. What are the Poles of the Earth?** The extremities of her axis where they cut or pass through the earth's surface (47). (The wise upon whom an artificial globe turns represents the earth's axis and the circumference of the globe as the Pole and the Equator.) **93. What is the Equator of the Earth?** An imaginary circle drawn around it, from east to west, at an equal distance from each Pole, and dividing it into two equal parts, called Hemispheres (48). (See definition and illustration, page 12.) **94. What is Latitude?** Distance North or South of the Equator. **95. How is it reckoned?** From the Equator each way, in Degrees, Minutes, and Seconds. (A line drawn from the Equator to the Pole cannot be more than a quarter of a circle, or 90° ; it is obvious that no place can have more than 90° of latitude ; or, in other words, all places upon the earth's surface must be between the Equator and 90° of latitude, either north or south.) The "extremes," therefore, are "terres" pertaining to the earth; hence St. Paul says, "To Coasts." "Terres" signifies "the terrestrial pole," not bodies terrestrial; but the glory of the celestial is one, and the glory of the terrestrial is another. PRIMARY ASTRONOMY. 25 **96. What are Parallels of Latitude?** Circles either North or South of the Equator, and running parallel to it. We see that there are two kinds of parallels between the Equator and the Poles, though upon most maps they are generally, if not quite, only one (for every degree). **97. What are the Tropics?** Two parallels of latitude, each 23° 28' from the Equator. **98. What are they called respectively?** The Southern is called the Tro- pic of Cancer, and the Northern the Tropic of Capricorn. The Tropics lie on the E. and W. in the mi- nuted figure. The Tropics never dip so sharply beyond any point on the earth further from the Equator than the tropic. Between these they remain level regularly, leaving no space for the equinoxes and solstices. The first of March, reaching the Tropic of Capricorn on the 21st of June, coming back on the 21st of September, and reaching the Tropic of Cancer on the 21st of December, is the time when we see this region and return to it again. The next year will be exactly like this. The Tropics will be seen at their greatest distance from the Equator during the summer of your year. The cause of this ap- pearance of the Tropics will be explained in a subsequent chapter. **99. What are the Polar Circles?** They are two parallels of latitude, 66° 34' from the Poles. (See cut.) **100. What are they called respectively?** The Northern is called the Arctic, and the Southern the Antarctic Circle. (These circles are shown at F F in the preceding figure.) **101. How do the Tropics and Polar Circles divide the surface of the Earth?** Into five parts, called Zones. A new property signifies a grade; but this term is here used in an accommodated sense, on only three of these five divisions at all resemble a grade. The parts out cut off by these lines are called zones. **102. How are these Zones classified?** A diagram showing two circles representing parallels of latitude around a central circle representing the Earth's axis. One circle is labeled "Parallalia" and is divided into four equal parts with labels "E" and "W". The other circle is labeled "Tropicae polaris" and is also divided into four equal parts with labels "E" and "W". The diagram shows that both circles are parallel to each other and run horizontally across the Earth's surface. 3 26 **PRIMARY ASTRONOMY.** Into Torrid, Temperate, and Frigid.† **102. How many of each?** One Torrid, two Temperate, and two Frigid. **104. Where are they located, respectively?** The torrid, between the Tropics ; the temperate, between the Tropics and the Polar Circles ; and the frigid, between the Polar Circles and the Poles. **105. What are Meridians?** Imaginary lines drawn from Pole to Pole over the earth's surface. (Meridian means the straight line of right angle and the point of intersection with the earth's surface.) These lines divide the earth into Hemispheres and Continents. We may imagine Meridians to pass through every meridian circle on the earth's surface. They meet in the Poles, and are furthest apart at the Equator.) **106. What is Longitude?** It is distance East or West from any given Meridian. **107. How is it reckoned?** Both East and West, in Degrees, Minutes, and Seconds. (A degree of longitude at the Equator consists about 60 miles but is less and less as we move northwards the poles, at which points it is nothing. A degree of latitude is about 60 miles on all parts of the globe.) **108. What is the Meridian called from which we commence to measure longitude?** The First Meridian. (On European charts and globes longitude is usually reckoned from the Royal Observatory in Greenwich near London; but in this country it is often reckoned from a point in Ireland.) The first meridian was drawn by Sir George Airy in 1835, and is now reckoned longitudinally from the same Meridian, and all charts and globes were corrected accordingly. * * * † Ton-azn, parched, dried with heat. † Fais-azn, cold, wanting heat or warmth. A diagram showing a globe with meridians marked. The equator is shown as a horizontal line dividing the globe into two hemispheres. The North Pole is marked with a star symbol. The South Pole is also marked with a star symbol. The equator is divided into four equal parts by meridians running north-south. These meridians are labeled "Meridians" around their circumference. PRIMARY ASTRONOMY. 27 **109. What is the greatest Longitude that any place can have?** One hundred and eighty degrees (180°). **110. What is meant by the Sexable Horizon?** It is that circle which terminates our view, or where the horizon appears to end. **111. What is the Rational Horizon?** It is an imaginary plane, below the visible horizon and parallel to it, which, passing through the earth's center, divides it into upper and lower hemispheres. * 3 These hemispheres are distinguished as uper and lower with reference to the observer only. * The upper hemisphere includes all points within the earth, or about 400 miles from the radius; and this is called the upper hemisphere. * The lower hemisphere includes all points without the cutted arch at the same point; and this is called the lower hemisphere. * It seems impossible to conceive of any plan that will show us the rational horizon, because we cannot see it, and we should on the horizon of this planet.) **112. What are the Zenith and Nadir Points!** The Zenith is the point directly overhead, and the Nadir the point directly under our feet. * 3 These dimensions must be remembered, are merely relative. As the earth is a sphere, every point on its surface may be considered as being equally distant from the Nadir toward its center. So with the directions Up and Down: one is from the center toward the Nadir; and two are directions which are up in one direction and down in another. This shows how difficult it is to determine these points until the mind has become familiar to the conception of Up and Down, as far as they relate to our own position on the earth's surface. We must remember that we are bound to the earth's surface by our own weight, and that we cannot move away from it except by means of some other force than gravity. Therefore we must consider these directions as relative to our own position on the earth's surface, and not as absolute directions, but merely relative directions. Even this direction is relative to our own position on the opposite direction; and yet, for as absolute directions in conceived, what is East to one, an East to another? * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V E * C E A T I V A. LIVE B. D C. UP AND DOWN, EAST AND WEST 28 PRIMARY ASTRONOMY. **113. What is nearest by Space in Astronomy?** It is the interval or void between the earth and the heavenly bodies, and extending onward beyond them all in every direction. [Space has no limita, in or other words, is boundless or infinite. Suppose six persons were to start from as many different points upon the earth's surface, on for instance, the circumference of a circle, they would all arrive at the same point in the next figure. Let them ascend or diverge from the earth in straight lines, pursuing their course until they reach the heavens. They will find that each one, from each other with the speed of light, requiring fifteen million of years, some of these persons will never reach the heavens, while others will reach them before their advancement. Should they chance to meet another world in the line of their course, they would not be able to return to this earth again, but would remain there and beyond it space would still involve them towards explore in immeasurable depths. And then they might go on forever, without changing their position in respect to the heavens, and thus continue to travel through space without end; so space without center or circumference, an eternal ocean without bot- tom.] **114. What is the Celestial Sphere?** It is the apparently concave surface of the heavens, in which the stars seem to be set, and which surrounds the globe on every side. (The relation of the Terrestrial to the Cele- stial Sphere may be understood by the manner described in fig. 10. The Terrestrial sphere is divided into all directions, as they seem to fill the whole expanse of space.) **115. What is the Equinoctial?** It is the Celestial Equator, or the plane of the earth's equator extended in every direction to the starry heavens. **116. What is Declination?** It is apparent distance either north or south of the Equinoctial. [Declination is the same to the heavens that latitude is to the earth.] **117. What is Right Ascension?** A diagram showing a celestial sphere with a line drawn from a point on Earth to a point on the horizon. This line represents Right Ascension. THE EQUINOCTIAL PRIMARY ASTRONOMY. 29 It is distance east of a given point, and is reckoned on the Equinoctial quite around the heavens. [In one respect Right Ascension in the heavens is like longitude on the earth: in another respect it is not so. In the first place, it is reckoned from the vernal equinox, which is at 0°, while longitude is reckoned both east and west of the first meridian, and can only amount to 180°. Right Ascension is reckoned only eastward, and consequently may amount to 360°. Secondly, the vernal equinox is fixed by observation, but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on the earth is determined by observation also; but the meridian of any place on 30 PRIMARY ASTRONOMY. **135. What are the first grand divisions of the Heavenly Bodies?** The Solar System and the Sidereal Heavens. **136. What constitutes the Solar System?** It includes the sun, and all the worlds that revolve around him. (This system is worked out by him from the Laws of Nature, which regulate the System of the Sun.) **137. What are the Sidereal Heavens include?** All those bodies that lie around and beyond the Solar System, in the region of the Fixed Stars. (L. The word Sidereal is from the Latin, meaning to signify relating to the stars. The Sidereal Heavens are those beyond the fixed stars. The outer regions of the Solar Sys- tem to the Sidereal Heavens is called the Galaxy. All the stars in this region appear only as a star at a distance from each other, and not as a body with a definite form. The Solar System is drawn in such a way that the Sidereal Heavens are seen around and at a distance from it in every direction.) **138. What part of this book have you now gone over?** PART FIRST, which consists of Preliminary Observations and Definitions. **139. What do you find upon in your next Lesson?** PART SECOND, which treats of the Solar System. A diagram showing the Solar System and the Sidereal Heavens. The Solar System is depicted as a circle with a central dot representing the Sun, surrounded by smaller circles representing planets orbiting around it. The Sidereal Heavens are shown as a larger circle surrounding the Solar System, with additional smaller circles representing distant stars or galaxies. A diagram showing the Solar System and the Sidereal Heavens. The Solar System is depicted as a circle with a central dot representing the Sun, surrounded by smaller circles representing planets orbiting around it. The Sidereal Heavens are shown as a larger circle surrounding the Solar System, with additional smaller circles representing distant stars or galaxies. PRIMARY ASTRONOMY. 31 PART II. OF THE SOLAR SYSTEM. LESSON VII. BODIES THAT COMPOSE THE SYSTEM. 130. Of what bodies does the Solar System consist? Of three classes; namely, the Sun, Planets, and Comets. 131. How is the Sun distinguished? As the fixed center of the system, around which the other bodies revolve; and as the largest and only self-luminous body in the system. 132. Describe the Planets. They are those large globes or worlds that revolve around the sun, and receive their light and heat from him. 133. How are the Planets divided? Into Primary and Secondary. 134. What are the Primary Planets? Such as revolve around the sun only, as their center of motion. 135. What are the Secondaries? They are small planets that revolve around the primaries, and accompany them in their revolution around the sun. [The Secondary Planets may be seen near their respective Primaries in the cut, page 9. They are often called Moons or Satellites. A satellite is a follower of a primary (or some other) body.] 32 PRIMARY ASTRONOMY. **136. What is the Orbit of a Planet?** It is the path it pursues in its revolution around the sun. (The Orbits of the planets are represented by lines which circle in the top column re- ferred to.) **137. What do you understand by the Interior and Exterior Planets?** The Interior are those whose orbits lie within, and the Exterior those whose orbits lie without, the orbit of the earth. Some astronomers speak of these two classes respectively, as Inferior and Superior. The reason seems to be, that all those nearer the sun than the earth are nearer than she is to the sun, and therefore inferior to her; while, on the other hand, those that are above or beyond her, are her superiors. In this respect, however, we must observe, that though the inferior planets are nearer to the sun with respect to the earth's orbit, it is obvious that interior and exterior are the terms applied to them according to their distance from each other. The Apollons superior planets, Mercury and Venus, which are much larger, inferiors. **138. What are the Asteroids?** They are the twelve small planets revolving between the orbits of Mars and Jupiter. (Several signified asteriodes, as is implied to these small planets because of their considerable size, have been discovered since this work was written; but, through ordinary instruments we are always roughly distinguished from the rest.) **139. What are Comets?** They are a class of bodies distinguished for their long train of light, their various shapes, and the great ac- centuity (50%) of their orbits. A Comet is a body which appears in the upper part, page 30, to which the student is referred. In the miniature representation of the Solar System, on the same page, page 30, it is shown as a comet. **140. Are the Planets and Comets self-luminous, or do they shine merely by reflection?** They are all opaque* bodies, and shine only as they are illuminated by the sun. That the planets and comets are opaque, is obvious from the fact that the solar rays cannot penetrate them; but whether they shine merely by reflection or by self- * o-raxt*, dark, obscure. PRIMARY ASTRONOMY. 33 Hence the various phases or appearances of the planets. Again: whenever, by any means, the light of the sun is interrupted or cut off, the planet, thus deprived of its borrowed rays, comes to shine. Hence what is called eclipses of the Moon. LESSON VIII. NAMES OF THE PLANETS, SIGNS, ETC. 141. How many Primary Planets are there? Twenty: (Nine of them have been discovered within a few years, and it is not improbable that there are several others, of the family of the Asteroids, that will hereafter be dis- covered.) 142. What are the Names of the Primary Planets ? Beginning at the sun, and passing outward, they are—
Planets Name
MERCURY Aphrodite
VENUS Juno
EARTH Atlas
MARS Phaethon
VENUS Atlas
PLUTO Pallas
LEPIDOSPERMAE Saturnus
MARS Hercules and
HIEROPOLEIS Helios
(To this table for the student to commit entire memory. To the teacher in whom he thinks they have once seen, it will help to fix in his mind the relative positions of the planets, and greatly increase the acquisition of further knowledge respecting them.) 143. Why is this ? After heathen gods and goddesses. 144. Why is this ? Because Astronomy was first studied by Pagan * * * * * * * * * **Pagan's** own, the Greek Asteroid, discovered May 11, 1800. See definition of Asteroid, Question 158. † The Asteroid discovered Sept. 18, 1850. ‡ This planet was first called Le Perrier, but is now more generally known by the name of Neptune. 34 **PRIMARY ASTRONOMY.** nations, who named the planets then known after their imaginary divinities. (1.) What was Mercury's name? what is called **Merkuropg.** **140. Who was Mercury, in Mythology ?** He was the messenger of the gods, and the patron of thieves and dishonest persons. **140. What does his Astronomical Sign signify?** It denotes his caduceus or rod, with serpents twined around it (2). (1. Mercury was represented as very quick, and skilful in interpreting and ex- pressing the thoughts of others. He was also a messenger, and a spy. When he visited London, addressed the people, and wrought a miracle, they said, "The gods have sent him." In the same way, when he went to Rome, he brought back the statue of son Jupiter, and Paul Reverer, because he was such a quick speaker.) See Arts XIX. & 5-6. (2. The meaning of Mercury was not of word or nature, born by Mercury in an sign of quality and office. In modele of the body, the head represents power; the serpent, wisdom; and the two wings, agility and speed.) The original form of this sign may be understood by the marked cut, in which the present astronomical symbol (Q) is inscribed. **147. Who was Venus ?** The Goddess of Beauty and Love. **148. What is her Sign ?** It is a Mirror or Looking-glass, which she is represented as carrying in her hand (3). (Ancient mirrors were made of bronze or silver; highly polished on one side, and covered with glass of whatever was brought before them. Hence it is said in the Book of Kvision, where God showed Solomon a vision of his own face in a mirror; that is, the favor of men, and the face of it is seen, of the looking- glass. The ancient Egyptians had mirrors made of glass; but these mirrors had a handle stucked, as represented in the cut, which very much resembles the size of the planet.) *All these symbols should be drawn in rotation upon the Blackboard during class time; so that each student may see them clearly. It will be well, therefore, for the student to observe each sign carefully, that he may be prepared to draw and explain it if called upon.) MIRROR OF VENUS. PRIMARY ASTRONOMY. 35 **149. What Sign represents the Earth?** She has two ; one representing a sphere and its equator (♀), and the other (♂) the four quarters of the globe. **150. Describe Mars and his Sign.** Mars was the God of War, and his sign (♂) represents an ancient shield or buckler, crossed by a spear. (Quintus Fabius was not known to the ancients, so they had no special name for this planet. He fought with short swords and spurs, and defended themselves with the shield, carried on the back, which is represented in the emblems of war. The original form of the sign of Mars is preserved.) **151. Who was Pluto?** The "Queen of all the Flowers." **152. Describe Vesta and her Sign.** She was the Goddess of Fire, and her sign is an altar (♀), with a fire blazing upon it. **153. Who was Ixus?** The beautiful waiting-maid of Juno. Her sign ( ) is a rainbow. (It was the special office of Ixus to stir up strife and discord among men, and to reconcile them when they were at odds.) **154. Who was Marte?** The first wife of Jupiter, and the Goddess of Prudence and Sagacity. **155. Describe Hera and her Sign.** Hera presided over children and youth, and was cup-bearer to Jupiter. Her sign (♀) is a cup. (Who was celebrated for her beauty ? but having one day a stately and splendid triumph, she fell into a fit of melancholy, and thus she became a mother of sorrow, and doomed to harass and drive the people of the house of heaven.) **156. Who was ASTREA, and what is her Sign?** She was the Goddess of Justice, with the sign of a balance ( ). A stylized illustration of a shield crossed by a spear. 36 A P R I L 1850 # ASTRONOMY. 1. Mythology taught that Jupiter left heaven, during the golden ages, to reside on Earth, and to govern mankind; but he returned to heaven, and continued a constellations of stars. Virgo and Libra in the zodiac still represent him. 2. The female figure, holding a pair of scales, in the seat of arms of several of the United States, is a representative of Justice. 3. Described Jero and her Sign. She was the reputed Queen of Heaven; and her sign (? is an ancient eunuch), crowned with a star—attainable by beauty only. 4. Described Dione and her Sign. She was the Goddess of Grain and Harvest, and her sign (?) is a sickle. 5. Described Pallas and her Sign. She was the Goddess of Wisdom and of War. She was represented as carrying a spear, which she brandished* terribly in time of battle; hence her sign (? is the head of a spear. 6. Who was Hygeia ? The Goddess of Health, and the daughter of Eucleas, the father of the healing art. (Our word Hygeia, which signifies the laws of health, &c., is derived from the Hebrew Hygeia.) 7. Who was Patroclus ? She was one of the three Syrnes—a sea nymphs of rare beauty. They were all admirable singers; hence a lyric (?) is her appropriate sign. I. The three Syrnes, Patroclus, Hygeia, and Leucosla, were represented as dwellers on the sea; they were supposed to be born from the sea by their enchanting songs. Hence whenever one enters or adores to ride in, often called a "Syrnian" boat. 2. As this place was discovered at the Naples Observatory, in Italy, it was quite appropriate to name it after one of the Syrnes, that mythology based on the coast of a neighboring island. * Brandish-ed to wave, shake, or flourish. † Syrene, a youthful godling, inhabiting some particular locality. ‡ Erna, a harp. A stylized illustration of a female figure holding a pair of scales. A stylized illustration of a female figure with a sickle. A stylized illustration of a female figure holding a spear. A stylized illustration of a female figure named Hygeia. A stylized illustration of a female figure named Patroclus. PRIMARY ASTRONOMY. 37 **163. After whom is Clio, the last discovered Asteroid, named, and what is her Sign?** After Clio, one of the Muses. Her sign is a star, with a sprig of laurel over it ( ). **164. Who was Jupiter?** He was the reputed father of the gods, and the King of Heaven. **165. What does his Sign (2) represent?** It was originally the Greek letter ζ, zeta—the same as our Z—the initial of the Greek word zeus, the name for Jupiter. **166. Describe Saturn and his Sign.** Saturn, called by the Romans Saturnus, Cronos, presided over time and chronology. He is represented as an old man, with wings, bald excepting a forelock, with a scythe in one hand and an hour-glass in the other. His sign (5) represents a scythe. **167. How many names has Herschel?** Three. **168. What are they?** Georgium Sidius, Urania, and Herschel. **169. Why is it called Georgium Sidius?** It was so called by its discoverer, in honor of George III. of England, who patronized or assisted him. **170. What does Urania mean?** It is from Urania, the name of a heathen goddess, A depiction of Saturn, an old man with wings and a bald head excepting a forelock, holding a scythe in one hand and an hourglass in the other. OLD SATURN, OR CHRONOS. **171. How many names has Herschel?** Three. **172. What are they?** Georgium Sidius, Urania, and Herschel. **173. Why is it called Georgium Sidius?** It was so called by its discoverer, in honor of George III. of England, who patronized or assisted him. **174. What does Urania mean?** It is from Urania, the name of a heathen goddess, 4 38 PRIMARY ASTRONOMY. who was represented as presiding over **Astronomy**, or the study of the heavens. [From this we have our word astronomy, which signifies a description of the heavens.] **176. Why is this planet ever called Herschel?** In honor of Sir John Herschel, its discoverer. (Describing the discovery of this planet by Sir J. Herschel, the planet, this one is more popularly known by the name of Herschel than by any other.) **177. What, then, is the meaning of his Sign ?** It consists of the letter H, with a planet suspended from the cross-bar (§), to indicate that Dr. Herschel was its discoverer. **178. Describe Neptune and his Sign.** Neptune was the god of the Seas, but the astronomical sign is composed of a L and a V, united, with a planet suspended from the haftline of the V (§) to indi- cate that Le Verrier was its discoverer. **179. What Sign is generally used for the Moon?** She is known by various representations, according as she is new, halfgrown, or full; thus, ☉, ☉, ☉. **180. What is the astronomical emblem of the Sun?** It is a Shield or buckler— ☉, ☉, ☉. (As the ancient often kept their bucklers high, so to dazzle the eyes of their enemies this instrument was selected as an appropriate emblem of the Sun.) LESSON IX. DISTANCES OF THE PLANETS FROM THE SUN. **175. Will you give the distances of the several Planets, in miles, commencing at the Sun ?** * Where circumstances permit, and the Teacher is favorable to such exercises, these statistics may be learned to good advantage by con- cert recitation.
Planets Distances from Sun
Moon 238,000 miles
Venus 22,000 miles
Earth 12,000 miles
Mars 144,000 miles
Jupiter 880,000 miles
Saturn 1,200,000 miles
Uranus 2,800,000 miles
Neptune 4,500,000 miles
PRIMARY ASTRONOMY. 39
Mercury 87 millions Jupiter 408 millions.
Venus 61 Saturn 900
The Earth. 95 Hercules 1,800
Mars. 145 Le Verrier or Nop
Asteroids, from 235 to 275 tune 1,850
The distances of the stars from the sun are given in round numbers. The design is to impart a subtly correct general idea, without overloading the memory. The following will exhibit to the eye the comparative distances of the planets. A diagram showing the relative sizes and distances of various celestial bodies, including Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and asteroids. The scale is not precise but serves to illustrate the relative sizes and distances of these bodies. 2. To meet his conception of these vast distances, the student may imagine a mile laid down from the sun to the orbit of Neptune. Now if the train proceeded from the sun at the rate of one mile per second, it would reach Mercury in 152 years; the Earth in 361 years; Jupiter in 8,684 years; Saturn in 3,423 years; Uranus in 19,700 years; and Neptune in 30,700 years. It would take nearly 100 million times across the continent, from Boston to Oregon! It is now about 5,400 years since the earth was formed. In that time it has completed its revolution around the sun once and has passed through all the stages of its development. It has been a day and night since the creation of man. It would take nearly 100 million years for a planet which would require about 1,000 years to complete its revolution around the sun to arrive at Neptune nearly as far as it is now from the sun. 176. What effect would it have upon the apparent magnitude of the Sun if we were to go to Mercury? He would appear much larger. 177. Suppose we were to go out to Hercules or Neptune? He would appear much smaller. 178. Why is this? Because the apparent magnitude of objects depends much upon the distance from which they are viewed. [Note: This text appears to be discussing astronomical concepts related to apparent magnitudes and distances.] CLEAR AND REMOTE VIEWS OF THE SAME ORBIT. Let A represent the position of an observer upon the earth, so that the Sun appears 2° or about half a degree in diameter. Now it is obvious that if the observer advances 40 PRIMARY ASTRONOMY. to R (half way), the object will fill an angle to its eye noise as large as it filled when viewed from V. Again, if the words from A to C, the object will appear but half as large. Hence we rule that the apparent magnitude is increased as the distance is diminished, and diminished as the distance is increased. 3. These principles are further illustrated in the following col, in which the observed magnitudes of the planets, corresponding with the comparative distances of Saturn, Herschel, and Neptune. THE SUN VIEWED FROM DIFFERENT POINTS. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A B C How the first observer on the left stands upon future, and the sun fills a comparatively large angle, as shown by A. From Herschel the angle is smaller, and at Neptune it is still less. 3. By applying the principles thus illustrated to the sun, as viewed from the several points indicated in the preceding col., we shall find that he will observe him from each of these points. The following cut represents the comparative apparent magnitudes of the sun as seen from the different planets. THE SUN AS SEEN FROM THE DIFFERENT PLANETS. From N. H. S. Jupiter. Mercury. Venus. Earth. Mars. Jupiter. Saturn. Herschel. Neptune. 4. The relative apparent magnitudes of the sun, as seen from different points in the Solar System, is as follows:
From Mercury From Venus From Earth From Mars From Jupiter From Saturn From Herschel From Neptune
Venus 81° 75° 60° 50° 40° 30° 20° 10°
Mars 75° 65° 55° 45° 35° 25° 15° 5°
Jupiter 65° 55° 45° 35° 25° 15° 5°
Saturn 55° 45° 35° 25° 15° 5°
Herschel 45° 35° 25° 15° 5°
Neptune
Let us continue our imaginary journey outward beyond Neptune, toward the Sun. The Sun will then appear to us as a very bright star; and if we look at it through a telescope, we shall see that it is composed of a number of stars; and this view, will appear only as a sparkling star; and the final result will expand to View as we approach them, till they assume all the magnitude and splendor of the sun himself.] LESSON X. LIGHT AND HEAT OF THE PLANETS. 179. From what source are the Light and Heat of the Planets derived? A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three different observers at points A, B, and C. A diagram showing the Sun's position relative to three不同的observers点A、B和C。41 From the Sun. **180. What effect does the variation in their distances have upon their light and heat?** It must make a great difference in their respective temperatures. [1.] The following cut is designed to illustrate the PHILOSOPHY OF THE DIFFUSION OF LIGHT. A diagram showing three squares labeled 1, 2, and 3, with arrows indicating the direction of light passing through them. The first square (1) has a smaller area than the second square (2), which in turn has a smaller area than the third square (3). The arrows indicate that the light spreads out more in the larger squares. The text below the diagram states: "Here the light is seen passing in straight lines from one on the left toward the several planes on the right. It is also shown that A, B, and C receive equal quantities of light, but that D receives less than A, B, and C. This is because D is farther away from the source of light than A, B, and C. The light falling upon D is spread over four times such surface as B, and nine times as much as A. Therefore D receives only $\frac{1}{4}$th and $\frac{1}{9}$th of the light received by A and B respectively." The reason why this statement is true can be explained by considering the fact that the distance between any two points is inversely proportional to the amount of light they receive. For example, if point A is twice as far from the source of light as point B, then point A will receive only half as much light as point B. Similarly, if point C is three times as far from the source of light as point B, then point C will receive only one-third as much light as point B. Therefore, we can conclude that the amount of light received by any point is directly proportional to its distance from the source of light. This means that if we increase the distance between any two points, then the amount of light they receive will decrease proportionally. **5. The student may not exactly understand this last statement. The square of any plane is equal to its area. Therefore, if we take any two points on a plane, say A and B, and draw a line connecting them, then we can see that the square of AB is equal to its area. Now let us consider what happens when we move these points along a line. As we move these points along a line, we can see that the square of AB becomes smaller and smaller. This means that the amount of light received by these points decreases proportionally.** **181. What is the comparative Light and Heat of the planet Mer- cury?** It must be about 6½ times as great as that of our globe. **182. How high a Temperature would that be?** About 325 degrees, or 113° hotter than boiling water. [Taking the average temperature of our globe at 60 degrees, that of Mercury would be 60 minus 325 or 365. But water boils near the level of the sea at 212, and this boiling point is about 365 degrees.] **183. What must be the Light and Temperature of Neptune? Only about $\frac{1}{2}$ part as great as that of our globe.** *4* 42 PRIMAELY ASTRONOMY. (1. The comparative light and heat of the planets, the earth being 1, is as follows:
Mercury 61 Jupiter 30
Venus 5 Saturn 18
The Earth 1 Hercules 18
Mars 1 Regulus 18
The Asteroids
These last are degrees of colours which we can form no just concep- tion of. It is evident that the light and heat received by the respective planets at various low curves may compare to modify either extreme of the scale, but that the extremes of the scale cannot be compared with each other, because they are not in the same ratio. That proves the light and strength of the body of the planet from the insensible rays of the sun, and that it is not possible to give any idea of its comparative magnitude, except by a reasoning medium to govern the light for a great distance around them, and consequently upon their otherwise cold and dark heavens.) LESSON XI. MAGNITUDE, DENSITY, AND GRAVITATION OF THE PLANETS. 184. What is meant by the Magnitude of a planet? Its size, bulk, or dimensions. 185. What is the Diameter of a planet? (See p.2.) 186. State the diameters of the several planets so far as known.
(The magnitudes of spherical bodies are to each other as the cubes of their diameters.) Then $83,000^3 = 83,000 \times 83,000 \times 83,000 = 59,666$ million cubed feet. Then $59,666^3 = 59,666 \times 59,666 \times 59,666 = 234$ million cubed feet. Then $234^3 = 234 \times 234 \times 234 = 13$ million cubed feet. Then $13^3 = 13 \times 13 \times 13 = .5$ million cubed feet. Then $5^3 = .5 \times .5 \times .5 = .1$ million cubed feet. Then $1^3 = .1 \times .1 \times .1 = .0$. Then $1^3 = .0 \times .0 \times .0 = .0$. Then $1^3 = .0 \times .0 \times .0 = .0$. Then $1^3 = .0 \times .0 \times .0 = .0$. Then $1^3 = .0 \times .0 \times .0 = .0$. Then $1^3 = .0 \times .0 \times .0 = .0$. Then $1^3 = .0 \times .0 \times .0 = .0$. Then $1^3 = .0 \times .0 \times .0 = .0$. Then $1^3 = .0 \times .0 \times .0 = .0$. Then $1^3 = .0 \times .0 \times .0 = .0$. Then $1^3 = .0 \times .0 \times .0 = .0$. Then $1^3 = .0 \times .0 \times .0 = .0$. Then $1^3 = .0 \times .0 \times .0 = .0$. Then $1^3 = .0 \times .0 \times .0 = .0$. Then $1^3 = .0 \times .0 \times . PRIMARY ASTRONOMY. 43 About $\frac{3}{4}$ that of the Earth. **180. How do Jupiter, Saturn, Herschel, and Neptune compare with our globe?** Jupiter is more than 1,400 times as large; Saturn 1,000 times; and Herschel and Neptune each 90 times. [This subject may be illustrated by the following cut, exhibiting the Comparative Magnitude of the Sun and Planets.] A diagram showing the relative sizes of the Sun (S) and four planets: Mercury (M), Venus (V), Earth (E), Mars (M), Jupiter (J), Saturn (S), and Neptune (N). The Sun is depicted as a large circle, while the planets are shown as smaller circles. The distances between the planets are indicated by lines connecting them to the Sun. The student may think it almost incredible that what appears only as a star in the heavens should be larger than the mighty globe upon which he dwells; but when he considers that the diameter of the Sun is about 93 million miles, and that of the Earth only about 7,920 miles, he will see how great this difference really is. The other three appeared nearly equal, as illustrated under Question 179, he will see that they could not be seen at all if they were not very large bodies! DENSITY. **195. What is meant by Density?** Compactness or closeness of parts. (1). For example, earth is less dense than iron, and iron is more dense than common salt. 2. Elasticity and solidity are not the same; for while ice is more solid than lead, it is for ice loose, and is consequently lighter. **191. What can you say of the Density of the planets?** They differ in the compactness of the substances of which they are composed. **192. What is the Density of Mercury?** Almost the same as lead, or three times the average density of our globe. **193. State the Density of Venus and Mars.** It is about the same as that of the Earth. 44 PRIMARY ASTRONOMY. **194. How is it with the other large planets?** Jupiter and Helechel have but $\frac{1}{3}$ and Saturn $\frac{1}{6}$ the density of our globe; the first two answering to water, and the latter to cork. **GRAVITATION.** **195. What is Gravitation?** The tendency of all bodies toward each other. (Gravitation is a species of attraction, and is often called attraction, or the attraction of gravitation.) **196. Give an example of the gravity of gravitation.** It is seen when bodies raised from the earth, and left without support, fall to its surface. (All substances fall toward the earth's center by reason of their weight.) The spherical quadrant would almost instantaneously fall into the earth by its own di- rection. Hence when three men, standing on a table, drop one of them, he drops both the others; but drop each a man, they all fall toward the same point; hence, the earth at- tracts them all to herself.) **197. What constitutes the Weight of any substance?** It is the force of attraction or gravitation. **198. Upon what does the amount of this force depend?** Upon the quantity of matter* in the bodies attracting, and their distances from each other. **199. Is it true that a cubic foot of cork weighs as much as a cubic foot of lead?** Because, being much less dense, it contains much less matter to be attracted. * Matter, in philosophy, signifies any visible or tangible substance— as earth, wood, stone, water, etc. **PRIMARY ASTRONOMY.** 45 **209. Suppose the Earth was only half as dense as she is, how would it affect the weight of bodies on her surface? It would be reduced one-half.** **201. Suppose a stone weighing four pounds at the Earth's sur-face, were taken down half way to her center, what would it weigh there?** About two pounds. In this case the diameter of the earth is divided into four equal parts, and the weight of the stone at the whole stratum amounts to four pounds. When the stone reaches B, the weight will be reduced to one-half, because it has lost its force by reason of each other. This act at C and D diminishes the weight still further, until at E, where it is one-fourth of its former weight. At F, one-half the globe is at the same distance downward; conse- quently the weight will be one-eighth of its former weight at B as at A, and the stone would weigh two pounds.* **202. What would it weigh at the Earth's center?** Nothing. **203. Why?** Because the attractive force would be the same in all directions. **204. Would an object weigh the same on the surface of the sun as on that of the Earth?** It would not. It is intended merely to assert that the attractive force is not the same. If a body were actually weighed upon the surface of each plane, by scales, it would weigh the same on both planes. But suppose we take a body which is so light as to be equal to that of the body to be weighed, whether it were more or less. With it we spread it over the surface of each plane, and find that it does not alter either the vertical or horizontal attraction; but that only the vertical attraction is altered, and that only the vertical attraction with which we could weigh objects accurately on the planets.* **205. Why not?** Because they vary in bulk and density, and conse- quently in their attractive forces. A body weighing 100 lbs., on Mars, would weigh 75 lbs. on Mercury, 60 lbs. on Venus, 50 lbs. on Mars, 32 lbs. on Io, or Jupiter; 18 lbs. on Saturn; and about 10 lbs. on our Earth. A person weighing 130 lbs. on the Earth, would consequently weigh 75 lbs. on * This question proceeds upon the supposition that a well be sunk 2,000 miles deep without filling with water. 46 PRIMAECY. ASTRONOMY. Mars, 351 on Jupiter, etc. The attractive force of the Aetherite is so slight, that if a man of ordinary muscular strength were transported to one of them, he could probably lift a负荷 of land from his seat without difficulty. The attraction of the Sun upon the planets cannot be considered both its act and death. Though one planet was large again as another, still if it were half as large as another, it would not be able to attract it with any considerable force besides its own weight. At Jupiter, for instance, we see the densest in the curbs, his sit- uation being such that he can only attract those bodies which are nearest to him. These bodies were precisely the same, their attractive force, or the weight of bodies on their respective sides, being equal. 206. How does Distance affect the attractive force of bodies? The rule upon this subject is, that the attraction is in inverse proportion to the square of the distance. If this should not be easily understood by the pupil, he may turn back to Question 190 and the Note. Light and gravitation are governed by precisely the same law, so far as they affect our existence; so that the illustration of one, as given in page 71, will answer for both. 207. Suppose three bodies, whose distances from the Sun were as 1, 4, and 8, what would be their relative attraction? $$\frac{1}{1^2}$$ and $$\frac{1}{4^2}$$ LESSON XII. REVOLUTIONS OF THE PLANETS AROUND THE SUN. 208. Are the planets at rest or in motion? They are all in motion, or revolution. 209. Describe their motions. Each planet has two revolutions: one around the Sun, called its Periodical* revolution; and another on its own axis, called its Diurnal** revolution. 210. What is the time required for a planet to revolve around the Sun called Its Periodic Time. * Periodic-al/on, occurring at regular intervals or at stated times; hence weekly, monthly, or quarterly publications are called Periodicals. † Diurn-al', from the Latin diversus, daily. PRIMARY ASTRONOMY. 47 **211. In what direction do the planets revolve around the Sun?** Eastward, or toward that part of the heavens in which the Sun appears to rise. **212. Can you state the Periodic Times of the several planets ?** Mercury ........................................ 0 yrs. 88 da. Jupiter ........................................ 11 yrs. 317 da. Venus ........................................ 0 = 25° Saturn ........................................ 29 = 175° The Earth ........................................ 1 or 261° Helmed ........................................ 94 The Moon ........................................ 22° Neptune ........................................ 164° (As the period of a planet constitutes its year, it follows that Horace's word "annus" means "year".) **213. What is the hourly motion of the planets in their orbits?** From 11,000 to 110,000 miles. **214. Which have the most rapid motion?** Those nearest the Sun. **215. What is the hourly motion of the Earth?** About 63,000 miles. It may seem incredible to the student that the panchora globe is flying through space at such a velocity, but it is only a fraction of a mile per second (about one mile per hour). Like many other astounding facts in astronomy, its truth can easily be demonstrated. The distance between the Sun and Mercury is about 36 million miles (question 173), it is obvious that this whole distance must be traversed by Mercury in about two hours. The distance between the Sun and Venus is about 63 million miles (question 173), it is evident that this whole distance must be traversed by Venus in about three hours. Between the Sun and Mars is about 93 million miles (question 173), it is evident that this whole distance must be traversed by Mars in about four hours. Between the Sun and Jupiter is about 484 million miles (question 173), it is evident that this whole distance must be traversed by Jupiter in about six hours. Between the Sun and Saturn is about 934 million miles (question 173), it is evident that this whole distance must be traversed by Saturn in about eight hours. Between the Sun and Uranus is about 2,864 million miles (question 173), it is evident that this whole distance must be traversed by Uranus in about ten hours. Between the Sun and Neptune is about 4,504 million miles (question 173), it is evident that this whole distance must be traversed by Neptune in about twelve hours. So far as the Earth is not propelled by machinery like a steamboat, or borne upon a carriage like a horse, it moves with great rapidity, especially as every thing upon its surface, and the atmosphere by which it is sur- rounded, move with it. **216. What keeps the planets revolving so steadily in their orbits?** It is the result of two distinct influences, called the **centripetal** and **centrifugal** forces. * centripetal - e-cipal, from centrum, center, and pasio, to move toward-tending to the centre. * centrifugal - e-cipal, from centrum, and pasio, to fly from the center. In both these cases we agree with Dr. Webster, that "the centripetal force would be natural and easy," while "the centrifugal force would be difficult." The third syllable, as in *Greece* or *Greece* would be natural and easy."
Mercury 8,000 miles. Astron.
Venus 7,000 Juno 1,400 **
The Earth 1,910 ** Panopeus**
Mars** 7,000 ** Victoria**
Fler** Ceres** 1,600 **
Vesta** Pallas** 2,100 **
Inferno** Jupiter** 2,100 **
Metis** Saturn** 7,000 **
Hobe** Hercules** 83,000 **
Hygeia**:Neptune:**
83,000 **:
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48 PRIMARY ASTRONOMY. **217. What is the Centripetal Force?** It is the mutual attraction of the Sun and planets, which prevents them from wandering off from their orbits. **218. Doctrine of the Centripetal Force.** It is the tendency to fly off from the Sun, produced by the planets revolving around him. [Diagram showing CENTRIFUGAL and CENTRIPETAL forces] L (is the Sun we have an arc of the Earth's orbit). The situation of the sun, when he is at his greatest distance from us, is called apogee; while the tendency to follow the line C D is called centripetal force. This force, combining, gives the planet what may be called its centripetal motion. It is represented by C E; and it must cease its motion in the centre of the Sun, if continued to revolve from age to age. If this force were not continually exerted, the planet would at once fall near the Sun; and if this centrifugal force were destroyed, all the planets would be driven away from him, and leave the Solar System forever. Thus we see that there is no real and confusion of the poet: The Sun with such a force as he has, Plants and men run through towards him; But let any agent that can do so, Be taken away, they will be lost. **219. Why do the planets wander out the Sun resolve most rapidly?** Because the nearer the Sun the greater the attraction (206), and the more centrifugal force is necessary to balance it. This mechanism of the Solar System strikingly displays the wisdom of the great Creator. He has made a system of planets, which revolves round him without error; and in order that these forces might be exactly balanced, God has imparted to each planet a certain amount of centripetal force, which keeps it in its orbit. Thus they neither fall to the Sun nor go one way or another beyond the limits of their orbits. They are kept in their orbits by these two great forces, and steadily revolving from age to age. **220. What three great principal or laws are found to prevail throughout the Solar System?** The first is, that all the planets and comets revolve in elliptical orbits. **221. State the Second law of planetary motion.** PRIMARY ASTRONOMY. 49 The Radius Vector of a planet describes equal areas in equal times. **223. What is meant by the Ra- dius Vector?** It is an imaginary line join- ing the center of the Sun and the center of the planet, in any part of its orbit. [In the circle lines extending from the Sun to the planets we represent the radius vector in different positions.] **223. What is meant by the Ra- dius Vector describing equal areas in equal times?** That it sweeps over the same surface in an hour, when a planet is near the Sun and moves swiftly, as when it is furthest from the Sun and moves most slowly. [On the preceding cut the twelve stelae, numbered 1, 2, 3, &c., over each of which is written a number, represent the planets.] **224. What is the Third great law?** That the squares of the periodic times of any two planets are proportioned to the cubes of their mean dis- tances from the Sun. [The student will understand that the square of any number is the product of that number multiplied by itself; thus, 2 × 2 = 4, 3 × 3 = 9, &c.; and that when one number is multiplied by itself, and the product multiplied by the same number again, we have obtained its cube; thus, 2 × 2 × 2 = 8, 3 × 3 × 3 = 27, &c.] The mean distances of the planets, when one is sacrificed the other can be deduced from it.] **225. Who discovered these three great laws?** Kepler, a German astronomer, after whom they are called Kepler's Laws. [Kepler was a disciple of Tycho Brahe, a noted astronomer of Denmark, and was afterwards appointed to succeed him. His residence and observatory were at Weilheimburg, in Germany.] * Vac-'ron', from solo, to carry — a radius carried around.* A diagram showing a circle with twelve points labeled with numbers (1 through 12) connected to a central point (labeled "Sun"). Each point has a line extending outward to another point on a smaller circle labeled with numbers (1 through 6). The lines connecting these points are labeled with numbers (1 through 6), indicating that each line represents a planet's radius vector. The numbers on the outer circle range from 1 to 12, while those on the inner circle range from 1 to 6. The diagram illustrates how Kepler's third law relates to the distances of planets from the Sun and their periods of revolution.]5 50 PRIMARY ASTRONOMY. LESSON XIII. ASPECTS, SIDEREAL AND SYNOCLIC REVOLUTIONS, ETC. 226. What is meant by the Aspects of the planets? Their positions with reference to each other. 227. When are planets said to be in Conjunction? When they are in the same longitude in the heavens. (1.) They are on the same meridian, or, in other words, at the same hour of the day. In this case they are in the same hemisphere of the Earth, and are said to be in conjunction. 228. What are the Sun, Venus, and Mars shown as viewed from the Earth, in the following figure? (See fig. 1.) ![Diagram showing Sun, Venus, and Mars viewed from Earth] In the sign of conjunction 229. What is an Inferior Conjunction? It is when the planet is between the Earth and the Sun. (See Venus at "inferior" in the cut.) (This conjunction is called inferior because the dark side of the planet is toward the Sun, just as does shine with the "inferior" light.) 230. What is a Superior Conjunction? It is when a planet is beyond the Sun, and its illuminated side is toward us. (See Venus at "superior.") 231. When are planets in Quadrature? When they are 90° apart. (Mars would be in quadrature with the Earth and Venus in the above figure, if placed at D.) A diagram showing Mars at D, which represents quadrature with both the Earth and Venus. PRIMARY ASTRONOMY. 51 When in opposite directions in the heavens, one toward and the other from the Sun. [In the lower part of the last cut Mars and Venus are in opposition. This aspect is denoted by the sign Σ, as there shown.] **232. What is meant by the Sidereal Revolution of a planet?** It is the complete revolution from any given point in its orbit around to the same point again. **233. Why is it called a Sidereal Revolution?** From *sidereus*, because such complete revolution is determined by observations upon the fixed stars. **234. What is a Synodic Revolution?** It is from one conjunction to the same conjunction again. (1. In the adjoining cut the revolution of the Earth about the Sun is represented by a circle C, round to the same point again, and then proceeding to another point, and so on.) 2. Suppose the Earth and Mercury to move in the same direction, and that C (where Mercury would be at inferior conjunction) is taken as the starting point, and proceed in the direction of the arrows. In this case, after two revolutions, they will reach the same point again; but as the Earth takes 365 days for one revolution, while Mercury takes 88 days for one revolution, she will only have reached the point D when Mercury has passed through E. Hence, though they will not be in conjunction, and a synodic revolution will take place, yet their conjunction will not occur until Mercury has made three revolutions since his conjunction with the Earth. He starts on, however, at his conjunction with the Earth, i.e., 88 days from time he left C the second time, he overtakes her at D, which is 365 - 88 = 277 days from time he left C. 3. From this illustration, it will be seen that the synodic revolution of a planet must always exceed its sidereal revolution by some number of days. 4. The term synod signifies a meeting or convention ; and the synodal revolution of a planet is that which occurs when it meets with another planet at a given point in its orbit. By noting the synodic as a meeting revolution, its meaning will not be forgotten. **235. Which requires the most time, the Sidereal or the Synodic Revolution?** The synodic. * See Question 127 and Notes. A diagram showing a circle labeled "C" representing the Earth's orbit around the Sun. Inside this circle is another smaller circle labeled "D," representing Mercury's orbit around the Earth. Arrows indicate movement from "C" to "D" and back to "C." The text explains that Mercury's synodic revolution exceeds its sidereal revolution by some number of days. 52 PRIMARY ASTRONOMY. **236. What familiar illustration can you give?** At twelve o'clock the hour and minute-hands of a clock or watch are together; but at one o'clock, when the minute-hand has made a complete revolution, and points to XII again, the hour-hand has gone forward to I, and the minute-hand will not overtake it till about five minutes afterward. **237. Which revolution then, represents the Sidereal and which the Synodic?** The revolution of the hour-hand from XII to XII again, is like the sidereal revolution of a planet; and when it overtakes the hour-hand, it becomes a synodic revolution. **238. Name both the Sidereal and Synodic periods of some of the planets.**
A diagram showing the Sun, Venus, and Mars viewed from Earth. A diagram showing Mars at D, representing quadrature with both the Earth and Venus.
Mercury Sidereal Synodic
88 days 115 days
Venus 225 594
Mars 1 year 1 year
Jupiter 11 517 599
Saturn 29 173 378
Herschel 64 3091
Nepster 10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
**239. Why is the Synodic period of the most distant planet nearest to the periodic time of the Earth?** Because, being remote from the Sun, they move very slowly, and the Earth soon overtakes them after performing her periodic revolution. A diagram showing the synodic periods of the interior planets. PRIMARY ASTRONOMY. 53 [Suppose the Earth and Mercury to be in conjunction, as shown at A B. In 361 days the Earth performs her annual or periodic revolution, and returns to the point C1; but in 360 days the Earth has only performed through one fifth part of his orbit, or about 42° of the point C1; and in 43 days the Earth overshoots him on the line D. It is on this account that the synodic period of Mercury is only 871 days, or 2 years and 10 months less than the period of time of the Earth.] LESSON XIV. DIRECT AND RETROGRADE MOTIONS, PLANETS STATIONARY, ETC. **240. When is a planet's motion said to be Direct?** When it is eastward among the stars. (All the planets actually revolve counterclockwise, as stated at 91), and they generally appear to us as moving westward.) **241. When is a planet said to be Retrograde?** When it seems to go back or westward in the ecliptic. **242. What is the cause of this apparent variation in the course of the planets!** It is due to the rapid revolution of the interior plan- et around the Sun, and to the changes in the position from which we view them, produced by the Earth's revolution. The Sun will be at A when the Earth is at a and Venus at R, she will appear to be at S. The Sun will be at B when the Earth is at b and Venus at T, she will appear to be at U. The Sun will be at C when the Earth passes from A to F and Venus from S to V, she will appear to be at G and the amount of her apparent contrary motion will only be half what it was before. **243. What is meant by the Arc of Retrogradation!** A diagram showing a circle with two smaller circles inside it. The larger circle represents the Earth's orbit around the Sun (S). The smaller inner circle represents Mercury's orbit around the Sun (M). The outer circle has three points labeled A, B, and C. Point A is where Mercury is closest to the Sun (S) and farthest from the Earth (E). Point B is where Mercury is farthest from the Sun (S) and closest to the Earth (E). Point C is where Mercury is between points A and B. An arrow pointing clockwise indicates retrograde motion. 29 54 PRIMARY ASTRONOMY. It is the portion of the ecliptic through which a planet seems to retrograde. [In the preceding figure it would be the arc C1.] **214. When is a planet said to be Stationary?** When it appears to move neither east nor west among the stars. 1. At this time, when Venus is at D, she will be moving toward the Earth, and at D she will be going past the Earth; so that she will appear to remain stationary at C. C. See also 213. 2. Some late writers have called this a stationary motion; for instance, one who, "Where is a planet's mean in its stationary?" We were not before aware that such a question could be asked about a stationary planet." See Clark's Astronomy, p. 156, and Smith's Illustrations, p. 38. **215. What is meant by the greatest Eastern and Western Elongations of a planet?** It is the greatest apparent distance east or west of the Sun at which it is ever found. [In the last cut the point B would represent the greatest eastern and the greatest western elongation of Venus.] **216. What is the greatest angular distance to which Venus ever deviates from her mean position?** She varies from 45 to 48 degrees. **217. What does this variation in her elongations indicate?** That she revolves in an elliptical orbit. [L. It will be obvious, without illustration, that she is further from the Sun at one time than at another so as to exhibit all the difference in her elongation, she em- braces all angles between 0° and 180°.] The eccentricity of her orbit is observed by comparing the difference between her mean place and her mean elongation, which is only about 35°. Her orbit therefore, is very nearly a circle.] **218. When is Venus Evening Star?** When she is west of the Sun, and rises before him. [For more on this see 219, or come, from her inferior to her superior conjunction. See cut, page 30.] **219. When is she Evening Star?** When she is east of the Sun, and remains above the horizon after he has gone down. [From her superior to her inferior conjunction she is east of the Sun, and Morning Star.] PRIMARY ASTRONOMY. 55 VENUS AS MORNING AND EVENING STAR. A diagram showing the positions of Venus relative to the Sun and Earth. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z (Let the student hold the book up erect of him, and he will see one way Venus is alternately Morning and Evening Star. Let the plane A B represent the middle or equator of the Earth, and let the plane C D represent the middle or equator of the Earth in its apparent position. The Sun is shown at three different points; when it is at A, Venus is rising in the east; when it is at B, she is setting in the west; and when it is seen reviving around him from west to east, or in the direction of the arrow. Now let us suppose that Venus is at her greatest altitude at 3 o'clock, and that she rises at 6 o'clock, and rises below him at 9 o'clock. She remains therefore, he, Mercury star. On the other hand, when she is just out of the Sun's face, at 2 o'clock, she risings in the west after the Sun has set, and then sets in the east before the Sun rises. In this case, Venus would be at her greatest altitude at sunrise at 3 o'clock, and would remain so until sunset at 6 o'clock. Then she would rise again in the west after the Sun has set, and then set in the east before the Sun rises. It is evident that Venus must have a daily rotation, with the Sun on the meridian of the observer, as represented at L. I may readily watch Venus through her whole course by means of a telescope. 4. Venus may sometimes be seen at mid-day, either out of wind of the Sun; and by this means we can observe her daily rotation, and also her distance from the Sun. 250. How long is Venus alternately Morning and Evening Star? For 292 days, or from one conjunction to another. 251. What did the ancients think of the Morning and Evening Star? They supposed they were two different stars. 252. What did they call them? They called the Morning Star Phosphor, and the Evening Star Hesperus. 56 PRIMARY ASTRONOMY. NATURAL APPEARANCE OF VENUS IN DIFFERENT POSITIONS. [1. On the left Venus is seen in the east at Morning Star, at various distances from the Sun, and on the right as Evening Star. It is worth observing how much student to mortals when Venus is at the time he is hearing this lesson, and to watch her for a few weeks, and see her move merely by degrees, until she becomes a great star.] **253. What is the greatest diameter of Mercury?** It varies from 16 to 29 degrees. [This proves the orbit of Mercury also to be elliptical.] **254. Is Mercury often seen?** He is not. **255. Why not?** Because generally so near the Sun as to be hid from our view by his beams. **256. In what months must Mercury be seen, if at all?** In March and April, August and September. [By considering as Atmone, you can ascertain when he is at his greatest elongation, and when he is at his greatest distance from the Sun. When he is at his greatest elongation, he must be looked for as the east, before sunrise. It will be worth while to see him.] **257. How do you account for the apparent retrograde motion of the Earth's Moon?** It was caused wholly by the change of the Earth's position, in revolving around the Sun. PRIMARY ASTRONOMY. 57 EQUINOXIAL MOTION OF THE EXTERIOR PLANETS. A diagram showing the equinoxial motion of the exterior planets. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune are represented by circles with arrows indicating their direction of motion. The arrows are labeled "A" and "B", with "A" being the direction of motion when the planet is at its perihelion (closest point to the Sun) and "B" being the direction of motion when the planet is at its aphelion (farthest point from the Sun). The diagram also shows the relative positions of the planets in relation to each other. (1.) Suppose the Earth at A, and the planet Neptune at B, we would then appear to be at C among the stars; but as Neptune moves but a little from B toward F, while the Earth remains stationary at A, we shall see Neptune appear to move from B toward C. Whatever Neptune may have moved, however, from B toward F, will yet to reduce the amount of retrogradation. It is evident from this figure, that the more distant an exterior planet is, and the slower it moves, the less will be the arc of retrogradation, and the longer it will be before it appears to be at C again. 3. The students will now see the philosophy of the following table, which may be seen as one of the facts and the time of revolution of the principal planets.
Planet Time Days
Mercury 10 80
Venus 220 42
Mars 680 73
Jupiter 1700 6
Saturn 2900 139
Uranus 3600 139
Neptune 4400 100
LESSON XV. DIURNAL REVOLUTIONS OF THE PLANETS, TIME, ETC. 258. What is meant by the Diurnal Revolution of a planet? Its revolution upon its own axis, causing day and night. (The equality with which the Earth revolves upon her axis, is referred to in the first lesson on Astronomy. It was said that "the Earth turns round her own axis once in twenty-four hours." Let us now see how this is done. Look at your watch; you will find that you have two hands: one for your hours and minutes, and one for your days and nights. Now observe carefully what happens to these hands during a day. You will find that they go around twice in twenty-four hours. This means that every day has two nights and two days. This is because every day has two nights and two days. This is because every day has two nights and two days.) 259. On which side of a planet is Day? On the side toward the Sun. 260. Where is it Night? On the side opposite the Sun. 58 PRIMARY ASTEONOMY. PHILOSOPHY OF DAY AND NIGHT. A diagram showing the Earth's rotation and revolution around the Sun. **261.** What, then, does the time of a planet's revolution upon its axis constitute? Its day ; including a day and a night. **262.** What is meant by a Solar Day ? It is the time elapsing from the Sun's crossing a meridian, to his coming to the same meridian again. **263.** How long does this require? Twenty-four hours. **264.** What is a Sideral Day ? The time required for the apparent revolution of a star from the meridian around to the same meridian again. **265.** What is the length of a Sideral Day ? Twenty-three hours, 56 minutes, and 4 seconds. **266.** What, then, is the difference between a Solar and Sideral Day ? About four minutes, the solar day being the longest. **267.** Will some one of the class explain the cause of this by a diagram upon the blackboard ? SOLAR AND SIDERAL TIME. * G SOLAR DAY, 24 h. 58 min. 4 s. SIDERAL DAY, 23 h. 50 m. 4 s. SUN ON THE MERIDIAN [1. To the man at A the Sun (B) is exactly on the meridian, or it is twelve o'clock noon. The Earth passes on from B to D, and in the same time revolves on her axis.] PRIMARY ASTRONOMY. 59 When she reaches D. The man, who has stood on the same meridian, has made a revolution of 360°, and is now at C. He will have to wait another 360° before he can reach B. Therefore, a twelve-hour day begins the next day before, but the sun is now east of the meridian; he must wait another four hours for the Earth to run a little further eastward, and bring the Sun again to the meridian. 2. It is obvious that if the Earth was not revolving around the Sun, his whole work would be done in one day only, and he would never see completion of each solar day, being the Sun on the meridian. 268. What is the annual difference between Solar and Sidereal Time? It amounts to one day in every 3651. 269. Why is this? Because it takes 360 actual revolutions of the Earth, as measured by the fixed stars, to produce 3652 natural days. 270. How does Longitude on the Earth affect our local time? Every 15° east makes it an hour earlier, and every 15° west makes an hour later. 271. Why is this? Because, if the Sun pass through 360° every 24 hours, he must pass over 15° each hour; as 360° ÷ 24 = 15°. 272. When is sunrise at New York what time is it 90° east of New York? Twelve o'clock. 273. When is it 12 o'clock at New York what time is it 30° west of that point? Ten o'clock. 274. In what time does each planet revolve upon its axis; or, in other words, what is the length of a day upon each of the planets?
Mercury 24 hours
The Earth 24 =
Mars 24 =
The Astersoids unknown. Jupiter 12 = hours. Saturn 10 = hours. Herschel and Neptune, unknown. 275. How was it ascertained that the planets revolve on their respective axes? 60 PRIMARY ASTRONOMY. By observing the motion of spots upon their surfaces, by the aid of the telescope. **276. In what direction do the planets rotate on their respective axes ?** Eastward ; or in the same direction that they revolve in their orbits. (1. In the sun we have no axis of the Earth's orbit, and therefore no rotation about its axis, but it revolves around the Sun. The arrows show the direction in which it revolves.) 2. By relating the book up such as he, and looking straight through it to the Sun, one will see that the Sun "rises" or first appears in the east. It is be- cause of this that we say that the Sun rises in the east. At a certain round into the light, and sees the Sun rise. **277. Upon what four planets are the days nearly equal?** Mercury, Venus, the Earth, and Mars. **278. If Jupiter's day only ten hours longer, and his year equal to about twelve of our years (31 years, 31 days), how many days must he have in one of his years?** About 10,397. (In this computation we reckon 365 days to a year, and 24 hours for a day. If the stars were not so far away as they are, as is true in part of the Solar System, we would please reckon it for himself, and see if we are correct.) **279. How many days has Saturn in one of his years?** About 25,000. (18 years 35 days is 10,397 days of our time ; × 98 = 105,694 hours = 104 hours, or 104 days.) **280. What effect has the Rotatory motion of the planets upon their form or figure?** The centrifugal force produced thereby causes them to swell out at their respective equators, and to contract at their poles; thus giving them the form of oblate spheroids. * Be-fore, from rota, to revolve or more round a center. * Be-come, from rota, a wheel. PRIMARY ASTRONOMY. 61 [1. When fluids are left free to yield to the influx of atmosphere, at normally rising sea-level, they assume a spherical form, or a spheroidal form. Hence water, in falling from the clouds, assumes a spherical form, and is thus rendered more compact and more dense, than when it falls in drops. The same effect is produced by the air on its passage down, remains perfect till it globulates into raindrops. In the case of India-rubber, a pole is run through its center, and which it is to maintain, so that it may remain perfectly straight until it is cut off. But when in motion it will contract at its poles, and swell out at its equator, and thus assume a spheroidal form. This is due to the centrifugal force of the earth's rotation. It is also due to the centrifugal force of the earth's rotation. It is also due to the centrifugal force of the earth's rotation. It is also due to the centrifugal force of the earth's rotation. At rest. IN REVOLUTION. The air on its passage down, remains perfect till it globulates into raindrops. In the case of India-rubber, a pole is run through its center, and which it is to maintain, so that it may remain perfectly straight until it is cut off. But when in motion it will contract at its poles, and swell out at its equator, and thus assume a spheroidal form. This is due to the centrifugal force of the earth's rotation. It is also due to the centrifugal force of the earth's rotation. It is also due to the centrifugal force of the earth's rotation. It is also due to the centrifugal force of the earth's rotation. 5. For as the Earth is concerned, we are taught in the Holy Scripture—the best authority—by St. John (Rev. xvi. 17), "And I am coming soon." We are further assured that "the world was made by him," and that "all things were made by him," and that "all things were made by his word" (John viii. 38). And again (John xviii. 28), "I have spoken unto you before that death should come upon you." And again (John xviii. 28), "I have spoken unto you before that death should come upon you." 2014 What is the difference, no far as known, between the Equatorial and Polar Diameters of the several planets? The Earth 26 miles; Mars 25; Jupiter 6,000, and Saturn 7,500. (The oblongs of Jupiter and Saturn is plainly visible through a telescope, as well as their polar diameters appear to be equal.) ORIGINAL FORM. CHANGING. PRESENT APPEARANCE. The plain line in the middle figure shows the original form, and the dotted line its present form. The difference is the change produced by its rotation. When measured 6 62 PEIARYM ASTRONOMY. by the proper instruments, it is found, in the case of Jupiter, to amount to about $\frac{1}{3}$ of its average diameter; and that being 80,000 miles, $1$ is but little less than 6,000. **292. To what is the great obstinateness of Jupiter and Saturn at- tributable?** To their rapid revolutions upon their respective axes. CERENICAL force. Their motions are so much in opposition to every other move- ment that they cannot be brought into any regular, or even uniform motion by the water from the surface in which they move. In the same manner an increased motion on the axis would make them fly out of their orbits; and the same thing would happen around the Sun would cause them to leave their orbits, and go out of the Solar system. **293. If the Earth is oblate, what portions of its surface are nearest to its center?** Those about the Poles. **294. Does not the surface ascend then, on the whole, from the Poles to the Equator?** It does. **295. How, then, can rivers run from the Poles toward the Equa- tor? At great distance, without running up hill!** They cannot. [This meant would be impracticable in short distances, and where the bed of a river running south from a northern ascent instantly downwards; and yet there is no such descent in the Mississippi (see page 178), which runs north.] **296. Can you give an instance of a river running up hill?** The Mississippi is said to be higher at its mouth than it is some thousands of miles above. [The plausibility of this opinion may be illustrated by comparing the diameters of C and D with those of A and B the equatorial diameters. The entire difference between them is 95 miles, or 23 degrees, which is nearly equal to one degree of latitude. Now as the Earth's circumference is 25,000 miles, the dis- tance round each degree is nearly 110 miles. A fourth of that distance must be 275 miles; and in this way we find that a river rising at a point 110 miles in every 1,000 toward the equator. The Mis- sissippi rises at a point 110 miles in every 1,000 miles of north latitude inclusive; or 11 degrees; whilst at 60° miles to a degree, would amount to about 1,380] A diagram showing four points labeled A, B, C, and D. Point A is near the top left corner of the diagram. Point B is near the bottom right corner. Point C is near the top right corner. Point D is near the bottom left corner. A B C D PRIMARY ASTRONOMY. 63 miles. If, then, it runs a distance equivalent to 1,500 miles directly south (in a wind- ing course), it will have run 1,500 miles north, and so on. The circumference of the Earth at its equator is 24,800 miles; that at its poles is 24,700 miles. Therefore, the difference between these circumferences is 100 miles. In one month, then is 1,500 miles directly north. There is some philosophy, therefore, that the waters of the ocean are a great distance from either pole toward the equator, it must run up to. **287. What causes the waters to flow toward the Equator, if they have to ascend in so doing?** The centrifugal force imparted to them by the Earth's revolution. (See 280 and Illustrations.) **288. What, then, would be the result if the Earth should cease to rotate on its axis?** The waters of the equatorial regions would rush toward the Poles, till the Earth again became a perfect sphere. **289. What would be the effect if the rotation of the Earth upon her axis was greatly increased?** The waters of the globe would rush toward the equa- tor, and the weight of bodies there would be greatly diminished. The centrifugal force and the centrifugal force are mutual opposing powers, act- ing against each other. The present rotation of the Earth diminishes the weight of bodies at the equator by about ⅓ part, so that if the Earth had no such motion, bodies at the equator would weigh about ⅔ as much as those at the Poles. This is called "the centrifugal force." It is also called "the centrifugal power." 8. Should the Earth revolve on her axis every 84 minutes, the centrifugal force would be doubled; and this would cause all bodies to weigh double their actual weight; and if the centrifugal force was still further increased by a still more rapid rotation of the Earth, all bodies would weigh three times their actual weight; and so forth. But since we know that all bodies near the equator weigh less than those near the Poles, we may conclude that if we doubled our present rate of rotation of the Earth, all bodies near the equator would fly off from the surface, while the water flew from the continents; and that this would happen for ever after. LESSON XVI. THE ECLIPTIC, EQUINOX, SUNS, LONGITUDE, ETC. **290. What is the Equator?** It is the plane of the Earth's orbit, or of the path in which the Sun appears to move in the heavens. 11. If the student has not understood what is meant by "the plane of the Earth's orbit," let him turn back and review Questions 30 to 37, and notes. A diagram showing a circle representing Earth's orbit around a central point (representing Sun) with lines extending from this point to points on Earth's surface indicating different latitudes. 64 PRIMARY ASTRONOMY. 9. It is obvious that the centers of all circles or ellipses must be on the plane of such circles, and as the Earth revolves around the Sun, he, being in the center, must always remain in the same plane. The Earth's orbit; so that the true sphere and the apparent path of the Sun must coincide. **291. Why is the plane of the Earth's orbit called the Ecliptic?** Because eclipses of the Sun and Moon never take place except when the Moon is in or near this plane. **292. What is the position of the Ecliptic to persons north of the Equator?** It is south of us; runs east and west; cuts the center of the Sun and Earth; and may be imagined as indefinitely extended. (See Note 1 to Question 32.) [L] The precise position of this plane may always be known by the position of the Sun, which varies its distance from the Earth, but does not change its direction. In regard to this variation will be explained hereafter. The Ecliptic is a line drawn through points to represent the ecliptic, or plane of the Earth's orbit. It is an oblique curve, running from north to south, cutting at right angles both sides of the Sun and Earth on one side, and half on the other, as shown above. The circle, representing beyond this line, is to represent the plane or ecliptic indefinitely extended. **293. What is meant by Above and Below the Ecliptic?** The Northern is called the upper part of this Southern, the lower sides. (The student must bear in mind, however, that there is no absolute up or down in the universe. See 112 and 113.) The Ecliptic is called also the Equatorial. This term has reference only to the Earth, and not to any particular point on it. The positions upon that of the observer, as shown and illustrated at 25. Through this ecliptic is a permanent plane, and into which all stars are supposed to fall. But since we cannot see all stars at once, it has no absolute up or down; unless it be said he has seen them all at once. Hence it is said that we have a merely arbitrary, and grows out of our position near of the equator, which rule applies equally well to all stars (see 113). **294. What is the Zodiac?** It is an imaginary belt, 16° wide—namely, 8° on PRIMARY ASTRONOMY. 65 each side of the ecliptic—and extending east and west quite around the heavens. **CHORUS HORIZONTAL VIEW OF THE ECLIPTIC AND ZODIAC.** A diagram showing the Earth's orbit around the Sun, with the zodiacal signs represented by circular symbols. The Earth is shown in perspective, largest near its center, and growing smaller as her distance is increased. The arrows show her direction. (In this cut the interior dotted circle represents the Earth's orbit; the exterior the places her orbit reaches at different times of the year.) The Earth is shown in perspective, largest near her center and growing smaller as her distance is increased. The arrows show her direction. **285. What is meant by the signs of the Zodiac?** They are mere divisions of the circle, each of which constitutes one-twelfth part. (1) The student will consult the definition of a sign and see the illustration at Question 27. (2) The twelve signs of the zodiac are divided off in the east by the perpendicular line. **286. How are the different signs of the Zodiac designated?** By specific names given to each, and by corresponding symbols or signs. **287. What are the names of the twelve signs, and their astrological significations?**
Sign Name Significance
Aries (or the Ram) ♈ Liber (the Liberator)
Taurus (the Bull) ♉ Scorpio (the Scorpion) ♏
Gemini (the Twins) ♊ Sagittarius* (the Archer) ♐
Cancer (the Crab) ♋ Capricornus (the Goat) ♑
Libra (the Scales) ♎ Virgo (the Virgin) ♍
Scorpio (the Scorpion) ♏ Pisces (the Fishes) ♓
Capricornus (the Goat) ♑ Liber (the Liberator) ♌
Virgo (the Virgin) ♍ Sagittarius* (the Archer) ♐
Pisces (the Fishes) ♓ Liber (the Liberator) ♌
Liber (the Liberator) ♌ Sagittarius* (the Archer) ♐
Liber (the Liberator) ♌ Capricornus (the Goat) ♑
Liber (the Liberator) ♌ Virgo (the Virgin) ♍
Liber (the Liberator) ♌ Pisces (the Fishes) ♓
Liber (the Liberator) ♌ Sagittarius* (the Archer) ♐
Liber (the Liberator) ♌ Capricornus (the Goat) ♑
Liber (the Liberator) ♌ Virgo (the Virgin) ♍
(3. These names being from the Latin, their significations is added in brackets, and should be understood by the pupil.) In reading, however, it is only necessary to give them by name.
It is carefully observing these symbols, the student will detect a resemblance be- between them and those of the zodiacal signs.
* Sag.-ta'-a'-n-us, from sagitta, an arrow.
† Fv.-acca.
c*
(1) These names being from the Latin, their significations is added in brackets, and should be understood by the pupil.) In reading, however, it is only necessary to give them by name. It is carefully observing these symbols, the student will detect a resemblance between them and those of the zodiacal signs. * Sag.-ta'-a'-n-us, from sagitta, an arrow. † Fv.-acca. c* 66 PRIMARY ASTRONOMY. even several of them and the objects they represent. For instance, the sign for Aries represents his horse; he also with Taurus, etc.) **298. Why were these names given to the different signs?** Because the ancients imagined that the clusters of stars in each sign resembled the several objects after which they were named. (As this circle is the same circle as to this belt around the heavens. Not, as some have imagined, because it was a nose, but from the Greek word, an animal, because so many animals were represented within its limits.) **299. In what order are these signs arranged?** Beginning at Aries, they proceeded eastward around to Pisces. PERPENDICULAR VIEW OF THE ECLIPTIC. A circular diagram showing the positions of the zodiacal signs around the ecliptic. [1. On pages 64 and 65, we presented abject views of the ecliptic. The above is a perpendicular view. The circle shows the sun's path around the earth revolving round him; and in the discuses is shown the circle of the stary heavens. In this circle are placed all the signs of equal parts, or one of the twelve signs of the zodiac. The starry sphere is drawn in such a way as to correspond to months, to be inserted in that sign, and the symbol immediately opposite and within the sign.] **300. What influence have these signs upon health, vegetation, or any other terrestrial objects?** None at all. PRIMARY ASTRONOMY. 67 **ANCIENT ASTRONOMY.** [1. The ancients believed in a preternatural science, which they called astrology, and which was supposed to control influencing over the destinies of mortals. This science was so much esteemed by the ancients that they were led to meditate occasionally on the papers of an ab- manes, designed to show which part of the human body was most affected by the influence of the stars. In this representation of this foolish absurdity, what an absurdity! For, if we consider that the distance between the sun and the earth is about 93 million miles, and that between the sun and the moon about 250 million miles, it will appear that the distance between the sun and the earth is more than two hundred times greater than that between the sun and the moon. This is a very great distance indeed, and yet it is supposed that the influence of the sun upon the earth is more than two hundred times greater than that of the moon. This is a very great absurdity indeed. 2. This picture has been published in almost every book on astronomy. Now, however, we know that this is not true. We know that the sun does not have any more power over us than does the moon. This is because we do not understand how these things work. We do not understand why the sun has more power over us than does the moon. We do not understand why the sun has more power over us than does the moon. 3. It is also true that the sun has more power over us than does the moon. This is because we do not understand why the sun has more power over us than does the moon. We do not understand why the sun has more power over us than does the moon. **301. What is Celestial Longitude?** Distance east or of a given point in the heavens. **302. How is it reckoned?** From the first degree of Aries, around to the same point again, or to 360 degrees. [J. Ruppert Arcturus in his Meditations, reprinted page 60.] The Lupa pugil held her book up to the south pole of the earth, and then she looked at the plane of the ecliptic and the declination of its 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, 30th, CELESTIAL MECHANICS AND LOUISVILLE. Astronomy 1. The observer must stand on one side of the Earth in this figure; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at the other side of the Earth with his eyes open wide open; he must look at 2. The observer must stand on one side of half way around both sides and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of the earth and then turn back to the opposite side of 68 PRIMARY ASTRONOMY. 303. Upon what does the apparent Longitude of a planet depend? Upon its position in the ecliptic and the point from which it is viewed. 304. What is Geocentric Longitude? It is the apparent longitude of a planet when viewed from the Earth. 305. What is Heliocentric Longitude? It is the longitude of an object as seen from the Sun. GEOCENTRIC AND HELIOCENTRIC LONGITUDE. A diagram showing the positions of three celestial bodies (A, B, C) around a central body (D), with arrows indicating their apparent motions. The diagram also includes labels for different parts of the diagram. (In this cut, the planet B, when viewed from the Earth at A, seems to be in the sign Σ₂ ; but when viewed from the Sun, it appears to be in Τ₁. Again, when at C, her apparent motion is shown by an arrow pointing towards D, while at B, it points away from D.) The reader will not only perceive the differences between geocentric and heliocentric longitude, but will see why the latter moves more than the former whatever the true position of the planet. It is an easy thing, however, if one be known, to deduce the other from it. *Geocentric* means, from the Greek geo, the Earth, and kentron, center : from the Earth, as the center or point of observation. †Heliocentric* means, from the Greek helios, the Sun, and kentron, cen- ter. PRIMARY ASTRONOMY. 69 LESSON XVII. FORM AND POSITION OF THE PLANETARY ORBITS, NODES, ETC. 306. Are the orbits of the planets perfect circles? They are not, but are all more or less elliptical. 307. What is the point nearest the Sun called? The Perihelion.* 308. What is the most remote point called? The Aphelion†. 309. What is meant by the mean distance of a planet? It is the average between its greatest and least distances. [The distances given on page 30 are the mean distances.] 310. Do the planets revolve with a uniform velocity throughout their respective orbits? They do not. 311. In what part do they move most rapidly? When nearest the Sun. 312. And where most slowly? When most distant from the Sun. 313. Why? Because from the Aphelion to the Perihelion points the centripetal force combines with the centrifugal force to accelerate the planet's motion; while from Perihelion to Aphelion points, the centripetal force acts against the centrifugal force, and retarded it. *Per-ae-le-ion, from per, about, and Ae-lion, the Sun. †A-phe-le-ion, ago, from, and Ae-lion, the Sun. ‡To hasten or come to more fast. §To delay, hinder, or render more slow. 70 PRIMARY ASTRONOMY. [1. From A to B is the diameter, the central axis, represented by the line C D E F G H I J K L M N O P Q R S T U V W X Y Z. This axis is called the axis of revolution, because its motion is assymetrical ; but from B to A, the same series, shown by the letters, is reversed. The axis of revolution is always in the same plane, and the planet is revolved. Hence it comes to Appear that the Sun is at the center of the universe, and that the Earth is at its greatest.] In the statement of velocities on page 47, the mean of average velocity is given.) 314. Are the orbits of all the planets in the same plane? They are not. 315. As they all revolve around the Sun in one center, what is the consequence of their not revolving in the same plane? They cut or pass through the plane of the Earth's orbit. VENUS PASSING THE PLANE OF THE EARTH'S ORBIT. A diagram showing Venus passing through the plane of Earth's orbit. 316. What are the points called where a planet passes the ecliptic? The Nodes of its orbit. 317. How are the Nodes situated ? In opposite sides of the ecliptic, or 180° apart. (See preceding cut.) 318. What is meant by the line of the Nodes ? It is an imaginary line passing from one node to the other through the Sun's center. (See the line L N in the last cut.) 319. How are the Nodes distinguished ? PRIMARY ASTRONOMY. 71 Into ascending and descending. 320. Describe each. The ascending is that through which the planet passes coming above or north of the ecliptic; and the descending is through which it passes in returning south of the ecliptic. 321. What characters are used to denote each? The ascending is indicated by $\alpha$, and the descending by $\gamma$. (See last cut.) (These characters should be drawn upon a blackboard by the teacher, or some one of the class.) 322. Are the Nodes of all the planetary orbits in the same Longitude? They are not; but are distributed around the ecliptic. 323. How do we describe the position of the several planetary orbits? By taking the ecliptic as the standard, and recording their deviation from it. 324. How is this deviation ascertained? By marking the greatest distance from the ecliptic at which the planet is ever seen. 325. What is the deviation of the several orbits from the plane of the ecliptic?
Mercury 79 Ceres 11°
Venus 84 Fallos 84°
Mars 4 Hymenaeus 4
Flora 6 Parthenope 6
Vesta 7 Clio 8°
Iris 14 Aphrodite 14°
Metis 6 Saturn 2°
Helbe 15 Herschel 1°
Astrea 9 Neposim 1°
Juno 13
73 PRIMARY ASTRONOMY. INCLINATION OF THE ORBITS OF THE SEVERAL PLANETS TO THE PLANE OF THE ECLIPTIC. A diagram showing the inclination of the orbits of the planets to the ecliptic. The Earth's orbit is shown as a circle with a line extending from its center to the plane of the ecliptic. The other planets' orbits are shown as ellipses, with their centers also extending from the Earth's center to the plane of the ecliptic. The inclination of each planet's orbit to the ecliptic is indicated by the angle between the line connecting the Earth's center to the planet's center and the line connecting the Earth's center to the plane of the ecliptic. 1. In this cut the large line in the center represents the plane of the ecliptic, in which the Earth is placed. The dotted lines represent the orbits of the planets. 2. The dotted lines crossing the ecliptic at the Sun's center represent the planes of the orbits of Mercury, Venus, Mars, Jupiter, and Saturn. It will be seen that they are very nearly parallel to each other, and that they are all near to the ecliptic; that it is impossible to represent them in any other way without making them intersect one another, and that none are so near to the ecliptic, that it would be difficult to represent their positions at all, except upon a very large scale. PERSPECTIVE VIEW OF THE PLANETARY ORBITS. A diagram showing a perspective view of the planetary orbits. The Earth's orbit is shown as a circle with a line extending from its center to the plane of the ecliptic. The other planets' orbits are shown as ellipses, with their centers also extending from the Earth's center to the plane of the ecliptic. The inclination of each planet's orbit to the ecliptic is indicated by the angle between the line connecting the Earth's center to the planet's center and the line connecting the Earth's center to the plane of the ecliptic. 3. A drawing similar to that above may be found in Long's *Astronomy*, vol. II, 303; in Poggendorf's *Catalogue*, vol. I, p. 568; and in other works on astronomy. We have here formed an idea of the inclination of the planetary orbits, but we must guard against supposing that they are exactly perpendicular to the plane of the ecliptic, as we were obliged to represent in this cut. Instead of this they are di- rected towards it at different angles, according to their distances from it; and at no point do they approach nearer than about 1° or 2° from it. This is true for all our known planets, and for those which we suppose to exist beyond them; but there is one exception, namely, Uranus, which is situated at a distance from us much greater than any other known planet; and it is probable that its orbit will prove to be inclined at a still greater angle than that of any other known planet. (See Question 171.) But, with these exceptions, it is a good illustration.) 73 73 PRIMARY ASTRONOMY. 73 LESSON XVIII. OF TRANSITS. **326. What is a Transit?** The passage of a heavenly body over the meridian of any place, or across the disk* of the Sun. (This term is sometimes used with reference to terrestrial objects, as when we speak of the transit of a ship across the horizon; but here we use it only in its astronomical sense, transi- tio, transitorius, &c., are derived from the primitive word transitus.) **327. When do planets appear to pass over the Sun's face?** When they pass directly between us and him. **328. Do all the planets make transits across the Sun's disk?** They do not† **329. Why not?** Because the exterior planets can never get between the Earth and the Sun. (Let the student turn back to 257, and to this cite, page 9.) **330. What planets, then, make such transits?** Only Mercury and Venus. **331. Do these make a transit at every revolution?** They do not. **332. Why not?** Because the planes of their respective orbits do not lie in the plane of the ecliptic. (The student will see at once that the planets all revolve in the same plane, like rolling on many buttercups around an apple upon the top of a table; Mercury and Venus being nearer to the Sun than Mars, Jupiter, Saturn, Uranus, and Neptune; and since one of their orbits is above and the other below the ecliptic, they will generally appear to pass either above or below the Sun. To illustrate—) * Discs, the face or visible projection of a heavenly body. † That is, to our view; though they may to the inhabitants of the ex- terior planets. 74 PRIMARY ASTRONOMY. A diagram showing the Earth's orbit around the Sun with the nodes marked at A and B, and the Earth's position at different times of the year. Let the right line A, joining the Earth and the Sun in the above diagram, represent the line of nodes, and let the left line B, joining the Earth and the Sun, represent the line of transit. Now, if any planet passes through either of these lines, it may appear to be upon the Sun's disk; but if it is either above or below the ediplis, as represented by C and D, it will appear to pass either above or below the Sun, as shown at D and E respectively. **333. Under what circumstances, then, do transits occur?** When the Earth and an interior planet meet on the same side of the ecliptic ; the planet being at its node, and the Earth on the line of the nodes. In **PHILOSOPHY OF TRANSITS.** A diagram showing a planet passing in front of the Sun (transit) while on its node. This cut represents the ecliptic and nodes, with the orbit of an interior planet, his nodes, etc. The line of his node is as shown in the 10th of $\frac{1}{2}$ and the 10th of $\frac{1}{2}$. Now, if this planet passes through either of these lines (as shown in the 10th of $\frac{1}{2}$), he will seem to pass over one part of the Sun's face like a dark spot, as represented in the figure. On the other hand, Mercury is at his $2^{\circ}$ when it is in both the 10th of $\frac{1}{2}$ and 10th of $\frac{1}{2}$; when he will seem to pass down over one part of the Sun. **334. What are the Node Months of a planet?** The months in which the Earth passes the line of its nodes, and in which all its transits must occur. [As the Earth's revolution around its axis takes place in about 365 days, or 12 months, there are two nodes of a planet every year. When a planet is at one of these nodes, it follows that the Earth must pass the node in opposite months in the year.] **335. Which are the Node Months of Mercury?**
PRIMARY ASTRONOMY. 75 May and November. [All the transits of Mercury ever noticed have occurred in one or the other of these months, and for the reason usually assigned. The first ever observed took place November 10, 1631, when it was seen by 20 observers at the same place; and in all 20—8 to May, and 22 in November.] **320. When did the last transit of Mercury take place?** November 9, 1848. [The transit of Mercury was noticed by Michael at the Cincinnati Observatory, and by many others in America and in Europe. The writer had made all necessary preparations for observing it on this occasion, but it was too late. Over New York, Italy, unfortunately his sky was overcast with clouds, which hid the Sun from his view, and dissipated his hopes.] **321. When will the next occur?** November 11, 1854. [There are two less favorable during the present century: two in May, and three in November.] **322. Where does the time of Venus's Nodes lie?** In Gemini and Sagittarius. [It is evident that the node of Venus, in its orbit, is to be seen by considering the last figure, that Venus's second node must be in T., and her descending node in Z. .] **323. Which are Venus's Node Months?** December and June. **324. Why is this?** Because the Earth always passes Gemini and Sagittarius during these months. [It is evident that the last cut for us in the line of Venus's nodes is only one sign ahead of that of Mercury, the Earth will reach that point to the right of which Venus will be found in Gemini, and to the left of which she will be found in May or November, here should occur in June and December, as is always the case.] **324. When did the last transit of Venus occur?** December 6, 1822. **325. When will the next take place?** December 8, 1874. (Only four months of Venus have as yet been observed; namely, December 4, 1822; December 6, 1822; December 8, 1874; and December 9, 1874.) This observation was so interesting in showing that of 17th that he actually failed. In defining the return period of Venus's nodes we find them to be about six years. See "Uranographie Dictionary." *The next four will occur December 8, 1874; December 5, 1891; June 7, 1904; and June 5, 1915.* A diagram showing the positions of Venus's nodes relative to Gemini and Sagittarius.76 PRIMARY ASTRONOMY. LESSON XIX. THE SUN'S APPARENT MOTIONS, THE SEASONS, ETC. 343. What are the Sun's apparent motions? He appears to revolve daily from east to west around the globe. 344. By what is this apparent motion produced? (For an answer let the student consult his memory ; but if he fails, he may turn back to Question 14.) 345. What other apparent motion has the Sun? He appears to travel eastward through all the signs of the zodiac in about 365 days. 346. To what is this apparent motion attributable? To the revolution of the Earth in her orbit around the Sun. SUN'S APPARENT MOTION AROUND THE ECLIPTIC. A diagram showing the Sun's apparent motion around the ecliptic. The Sun is shown moving from east to west across the sky, passing through various constellations (Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pisces) over a period of about 365 days. The diagram also shows the Earth's rotation on its axis, with the North Pole pointing upwards and the South Pole downwards. The diagram is labeled with the names of the constellations and their corresponding months. AUTUMNAL CONGREGATION JULY 21 WINTER QUARTER SEPTEMBER 21 SPRING QUARTER MARCH 21 SUMMER QUARTER JUNE 21 VERNAL EQUINOX MARCH 21 AUTUMNAL EQUINOX SEPTEMBER 21 PRIMARY ASTRONOMY. 77 [Suppose the Earth to be at on the 30th of March; the Sun will appear to be at B in the opposite side of the ecliptic. At the Earth's motion in its orbit from A to C, the Sun will appear to be at D, which is in the same place with respect to the Earth as when the circle of the equinox every 360 days, or as often as the Earth revolves around him.] **347. What is the cause of the different seasons of the year—as** Spring, Summer, etc? It is owing to the inclination of the Earth's axis to the ecliptic, and her revolution around the Sun. **348. How much is the Earth's Axis inclined to the Ecliptic?** Twenty-three degrees and twenty-eight minutes. INCLINATION OF THE EARTH'S AXIS TO THE PLANE OF THE ECLIPTEC PLAN OF THE ECLIPTEC **349. Does the Earth's Axis always incline the same way and to the same amount?** It does. (1.) This general fact is shown in the table, pages 65 and 75, where her axis is seen passing through a point in the sky called the North Pole. The table shows that this point moves about 23 degrees north and south of a line drawn through two points in the sky called the Equator. The author is aware that the poles of the Earth have a slow motion round around the pole of the ecliptic, requiring 25,000 years for a single revolution; but regarding this motion as very slow compared with that of our planet, he has omitted it from his table, and it is not necessary for an explanation of the seasons, he has accordingly omitted them both. **350. What effect has this inclination and permanency of the Earth's Axis in the production of the Seasons?** As the Earth revolves around the Sun, it brings first one pole toward the Sam and then the other; thus causing a constant variation of light and heat and producing the seasons. On account of this fact we may refer to the opposite side where the Earth is shown at four different points, with her axis inclined to the line DP SP. When she is at A, she will receive more light than when she is at B; when she is at C, she will give out more heat than when she is at D; and when she is at E, she will receive less light than when she is at F. In short, her hemisphere toward us would have most heat while from B to C; her dome opposite one would have no heat. This variation in the light and heat received by our hemisphere causes us to experience different seasons. This is what causes the Seasons. Many very intelligent people in other respects, have an idea that the Earth's axis 7878 **PRIMARY ASTRONOMY.** "**wobble**" in their orbit is, as we bring first the Northern and then the Southern Hemisphere toward the Sun, and then again away from it, by reason of the axial effects at this by the simple revolution of the Earth in her orbit." **351. Is the Earth's orbit a Circle or an Ellipse?** It is elliptical, like all the planetary orbits. **352. Where do her Perihelion and Aphelion points lie?** Her Perihelion is in **Gemini**, and her Aphelion in **Scorpius**. **353. When is the Earth nearest to the Sun, or at Perihelion?** About the first of January. **354. When is she at her greatest distance?** Six months afterward, or July 3. **355. How is it, then, that it is cold in January, when the Sun is nearer than warm in July, when he is most distant?** It is because the Northern Hemisphere inclines toward the Sun in July, and from him in January. (The comparative amount of heat received in the Northern Hemisphere during July and January may be illustrated by the accompanying figure. In July, when the Sun is farthest north, he is represented to the eye. In January they are seen to strike the Northern Hemisphere obliquely; and consequently the sun's rays are less direct upon it than in July. The heat received by the rays is almost perpetually upon us, and so much more intense. Hence the variations of temperature which continue the season.) **356. What is the difference of the Earth's distance in July and January?** About three millions of miles. **357. Does not this in reality affect the general temperature of the Earth?** It undoubtedly does; but the variation of 3,000,000 miles is so slight when compared with the whole distance of the Sun, that the change of temperature produced thereby is imperceptible. A diagram showing the Earth's orbit around the Sun, with labels for different seasons (Summer and Winter) and positions (January and July). PEIMARY ASTEONOMY. 79 (The natural effect of this variation would be, so far as it had any influence, to make the Sun appear to rise and set at different times in different parts of the world, in the Northern and Southern. For instance, our summer solstice in January would slightly alter our summer solstice in December, and vice versa. But this is not the case. The Sun rises at the equinox, so also its increased distance in July would diminish the heat of our Summer, and at the same time enhance the cold of the corresponding Winter in the Southern Hemisphere. 258. What are the Equinoctial Points of the Earth's orbit? They are two points, 180° apart, where the Sun shines perpendicularly upon the equator; or, in other words, it is in the Equinoctial. (1. The Earth is shone at these points in the east, page 78. See A and B on Fig. 36.) These points are called the Equinoctial Points. At these two points, as represented in the cut, the days and nights will be equal all over the world. Hence the name equinocial, from the Latin equus, equal, and noctis, night.) 359. How are the Equinoctial Points distinguished? Into Vernal and Autumnal. (See fig. 36.) Because the Earth passes one on the 20th of March, when the Sun crosses the equator northward, and Spring begins; and the other on the 23d of September, when the Sun passes south of the equinoctial, and Autumn begins. (See cut, page 76.) 361. What effect has the inclination of the Earth's Axis and her Annual Revolution upon the apparent motion of the Sun? It causes him to appear to change his path in the heavens, coming up north nearly overhead in the Sum- mer, and dropping low down in the south in the Winter. 362. What is this variation of the Sun north and south of the Equator called? His declination. (See 116.) 363. How far does he Sun decline north and south of the Equator? Only 23° 38' on each side, or as far as the Tropics. (See 97 and 98, and Notes.) A diagram showing two points labeled A and B on a circle representing Earth's orbit around the Sun. Point A is labeled "Vernal" and point B is labeled "Autumnal." The diagram illustrates how these points are used to distinguish between Vernal and Autumnal Equinoxes.80 **PRIMARY ASTRONOMY.** [1.] The apparent position of the Sun with re- spect to the equator, at different seasons of the year, is shown in the diagram on the left. On the 21st June he has his greatest northern de- clination, and on the 21st December his greatest declination southward, at the Tropic of Cancer. From this time he ap- proaches the equator, and on the 21st of September, when he crosses it, and begins to decline southward. On the 21st of December he has his greatest southern declination, and on the 21st of March he begins to return toward the equator. This is shown by the arrows in the diagram. In March, and reaches its Summer Solstice again on the 21st June. From this time he continues to decline first north and then south of the equator until he arrives at his greatest northern declination, on the 21st June. 5. The Sun's declination may be easily measured by the shadow of a minute object upon the Earth's surface. This shadow will fall either directly perpen- dicularly, and exactly on the equator. On the 21st June, when the Sun has his greatest declination northward to A, or 23° 27' N as far as the Sun can go northward, his shadow falls directly upon the equator, and on the 21st December there would be no shadow falling upon it. But between these two points there would be a gradual change in the shadow from 23° 27' N to 0° C. That is, the equa- tor would receive a shadow varying from that of a line of perpendicular objects, for six months alter- nately. MEASURING THE SUN'S DECLINATION IN NORTHERN LATITUDE A diagram showing the Sun's declination in Northern Latitude.WATER SILTETTE DEC. 6 2. This cut shows how the student may measure the Sun's declination wherever he may be located north of the equator. In order to do this, he must find out where his latitude is situated. For example, if he be situated at B, on the 21st June, when the Sun has his greatest declina- tion northward, his shadow will fall directly upon the equator, and on this day he will have a declina- tion of zero degrees. From this time to June 21st the shadow gradually shortens, till on that day it reaches its shortest length at C. From this point it gradu- ally increases until it reaches its longest length in August, when it is extended to D again. The point A is just half way from B to C in equal measurement, through the distance on the plane in which we are situated. SUN'S DECLINATION. SHADOWS AT THE EQUATOR. G 23° 27' N B A C PRIMARY ASTRONOMY. 81 When the Sun is on the equinox, March 21st and September 23rd, the shadow will reach only to A and the angle CBA is half the top of the staff shown the western and eastern extremities of the shadow. The line AB is perpendicular to the horizon at each point, as marked in the figure. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with the middle of the place where the experiment is made. Let us now suppose that we have two rods, one at the top and bottom of the pole and the point A will exactly correspond with **361. What are **the Solarial Points?** They are those points in Earth's orbit where Sun ceases to decline north or south, and begins to return toward equinoctial (see page 70). In this case it may be observed that Sun is directly over- head in latitude B. This happens on June 21st or December 21st. On June 21st or December 21st, Sun appears to go southward while from December 21st to June 21st he comes northward; thus producing an agreeable change in latitude. This change in latitude is called **the solarial points**. **362. Why are these points distinguished?** Because Earth passes one on 23d of December, or in June; and another on 21st of June, or in Summer. By a little attention, it may be observed that Sun is most directly over- head in latitude B. This happens on June 21st or December 21st. On June 21st or December 21st, Sun appears to go southward while from December 21st to June 21st he comes northward; thus producing an agreeable change in latitude. This change in latitude is called **the solarial points**. **363. What is meant by **the Obligingity** of **the Ecliptic?** It is a circle which makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equinoctial makes an equино 82 PRIMARY ASTRONOMY. 368. How are the Axes of the other planets inclined to the Planes of their respective Orbits?
Mercury unknown Jupiter 2° 5'
Venus 7° 00' Saturn 30°
Mars 28° 40' Herschel and Neptune unknown.
INCLINATION OF THE AXES OF THE SEVERAL PLANETS TO THE PLANES OF THEIR ORBITS.
Venus Ecliptic. Mars.
A diagram showing the inclination of Venus' axis relative to its orbit. The angle between the ecliptic plane and the plane of Venus' orbit is approximately 24 degrees. A diagram showing the inclination of Mars' axis relative to its orbit. The angle between the ecliptic plane and the plane of Mars' orbit is approximately 28 degrees. A diagram showing the inclination of Earth's axis relative to its orbit. The angle between the ecliptic plane and the plane of Earth's orbit is approximately 23 degrees.
(The student must bear in mind, that the inclination above represented is not to the ecliptic (with the exception of the Earth), but to the planes of their several orbits; for instance.)
Venus PLANE OF VENUS' ORBIT PLANE OF THE ECLIPTIC
The orbit of Venus departs from the ecliptic 24°, as stated at 205, while her axis is in- clined to the plane of her orbit 24°, as shown in the above figure. This distinction lies in being definitely seen by the student. LESSON XX. SEASONS OF THE DIFFERENT PLANETS, TELESCOPIC VIEWS, ETC. 369. What influence has the inclination of a planet's axis upon its Seasons? It determines the extent of its zones; or, in other words, the amount of the Sun's declination north and south of its equator. (If this is not clear to the mind of the student, let him examine the first of the above diagrams.) In this case, Venus has an inclination of about 24°, so she inclines her observer (see Torvik Zone), and the smaller the Polar Circle.) A diagram showing the inclination of Venus' axis relative to its orbit. The angle between the ecliptic plane and the plane of Venus' orbit is approximately 24 degrees. PRIMARY ASTRONOMY. 83 **370. What influence has the Periodic Time of a planet upon its Seasons?** It determines their length. (As the nears of the several planets are permanent, they can have but four regular seasons in their year, however long it may be.) **371. What can you say of the Seasons of Venus?** Her Troops are within 1/4 of her Poles, making her Torrid Zone very wide. The Sun passes from one Tropic to the other about in 220 days, during which time she has her four seasons of 524 days each. **372. Describe the Seasons of Mars.** They are much the same as those of our Earth, except that they are longer. (As the year of Mars consists of 687 days, his four seasons must consist of 172 days each, or nearly 43 days each.) **373. Has Jupiter any change of Seasons?** Scarcely any. His axis being inclined to his orbit only 3° 5′, the Sun never departs more than 3° 5′ from his equator. **374. What effect does that have?** It causes perpetual summer at his equator, perpetual winter at his poles, and gives the intermediate regions an almost unchangeable temperature. **375. By what are the Seasons of Saturn distinguished?** His zones are much like those of our globe, but each of his seasons is about 71 years long. (He has no seasons because he is so remote from the Sun and other planets; and as that is about 20 years, each season would consist of about 71 years.) **376. How long are his Poles alternately in the light and shade?** For about fifteen years. **377. Have we any knowledge of the Seasons of Herschel and Neptune?** None, except that, being very remote from the Sun, their general temperature must be very low. 84 **PRIMARY ASTRONOMY.** **378. Are all the Primary Planets visible to the naked eye?** They are not. **379. Which of them can be thus seen?** Mercury, Venus, Mars, Vesta, Jupiter, and Saturn. (1) Mercury is a very small planet, situated at a distance of about 60 million miles from the Sun; under very favorable circumstances, in a star of the sixth or seventh magnitude; but, in a general thing, he is invisible, except by his light (or the light of the telescope). **380. How does Mercury appear to the naked eye?** As a star, always in the neighborhood of the Sun. **381. How does he look through a Telescope?** Like a globe or world, with numerous spots upon its surface. **382. What are these Spots supposed to be?** The natural divisions of the planet—as Continents, Islands, Mountains, &c. (Weberstr., an eminent German astronomer, measured several mountains upon the surface of this planet, one of which he found to be nearly seven miles in height.) **383. What is the natural color and appearance of Mercury?** Through a telescope he is a faint bluish tint, and exhibits a great variety of forms or appearances. **384. What is the natural appearance of Venus?** Her color is of a silvery white; and when at a distance from the Sun, either east or west, she is exceeding bright and beautiful. **385. How does she appear through a Telescope?** As she passes around the Sun she exhibits all the varying phases of the Moon. TELESCOPIC PHASES OF VENUS. A diagram showing the phases of Venus as observed through a telescope. The phases are labeled "EAST OF THE SUN" and "WEST OF THE SUN." The phases are numbered 1 through 6. EAST OF THE SUN WEST OF THE SUN E V W PHASES OF VENUS PRIMARY ASTRONOMY. 85 [1.] The telescopic appearance of Venus, at different points in her orbit, is represented in the last figure. At its first view she has her greatest customs and western exaltation, and her greatest declination to the north. In the next place, when she is in opposition, she represents her at her conjunctions. The spots on the face of the planet represent Venus's spots, which are supposed to be caused by the sun's rays being reflected from the surface of the planet. 2. Before the discovery of the telescope it was asserted, that if the Copernican theory were true, Mercury would never appear in the heavens. But this assertion was proved false by these phases; for it was evident that the theory was true. But no astronomer had ever seen any of these phases, nor had any of them found them exhibiting the very appearances required by the Copernican theory, in opposite directions. 386. Explain the cause of the different Phases of Mercury and Venus. It is because we see more of their enlightened sides at one time than at another. 387. What else does the Telescope reveal upon Venus ? A variety of spots, probably Islands, Continents, and Seas. SPOTS SEEN UPON THE DISK OF VENUS. 388. Has Venus any Mountains ? She has ; some of which are supposed to be over twenty miles in height. (Three elevations upon her surface have been estimated at 100, 116, and 19 miles, respectively.) 389. What can you say of her Atmosphere ? It is supposed to be very dense, and to surround the planet only to the depth of about three miles. (The atmosphere of our own globe is supposed to extend about forty miles from its centre.) The atmosphere of Venus is supposed to extend to about two hundred miles. 390. Why is it thought that the Spots seen upon Mercury and Venus are the great natural divisions of their surfaces ? Because such divisions would appear like spots, if viewed from a distance, and would vary as the planets A diagram showing different views of Venus with spots visible. A diagram showing different views of Mercury with spots visible. 86 PRIMARY ASTRONOMY. revolved, precisely as the spots vary upon Mercury and Venus. 391. How would the Continents, Islands, and Seas of our Globe appear at the distance of Mercury and Venus? As mere spots upon its surface, resembling those seen upon those planets. Distant Telescopical Views of the Earth.
A circular diagram showing three views of the Earth from different perspectives. A circular diagram showing three views of the Earth from different perspectives. A circular diagram showing three views of the Earth from different perspectives.
L. 3. 4. 3. 4. 4.
(Though we have here four different views of one same globe, No. 1 is a view of the Northern Hemisphere; No. 2 of the Southern; No. 3 of the Eastern Continent; No. 4, of the Western.) The first view (No. 1) is that which would be seen by an observer standing over every new position from which it is viewed; as in the Earth most near its apperance to the inhabitants of other worlds.) 392. How does Mars appear to the naked eye? Like a bright star of a reddish color. From this appearance, says Mr. Pickering, the student will find another group called the Hyades, one of which, called Aldebaran, is of a reddish hue, and resembles Mars in its brightness and in its motion round the sun; but it is much nearer to us, and with his sidelined sight upon it he appears much larger and brighter than Mars itself. See the position of Mars when in opposition, as illustrated on the map, page 30). 393. How does he appear through a Telescope ? He has a reddish hue, and exhibits slight phases, and a variety of spots upon his disk. 394. What is supposed to be the cause of the peculiar Color of Mars ? It is attributed to his extended and very dense atmospherical vapour. (When the sunlight passes through vapor or clouds in the morning or evening, the different rays of which it is composed are scattered, and the red rays only pass to this planet, so that it appears reddish; but it is supposed that the atmosphere of Mars may give him his crimson hue.) 395. What do astronomers think of the Spots upon his surface ? A circular diagram showing three views of the Earth from different perspectives. PEDIARY ASTRONOMY. 87 "Upon this planet," says Dr. Herschel, "we discern, with perfect distinctness, the outlines of what may be Continents and Seas." 396. What peculiar changes are seen to take place about his Pole? When it is Winter at his north pole, that part of the planet is white, as if covered with ice and snow; but as Summer returns to his Northern Hemisphere, the brightness about his north pole disappears. TELESCOPIC APPEARANCES OF MARS. [The right-hand figure represents Mars as seen at the Cincinnati Observatory, August 5, 1842. On the left is a view which was supposed to represent the sun. The middle view is from a drawing by Dr. Dick.] LESSON XXI. TELESCOPIC VIEWS OF THE PLANETS CONTINUED. 397. What peculiarities do the Atteriors present under the Telescope? A thin haze is seen around Pallas; and they are all of a pale ash-color, except Ceres, whose color is like that of Mars. 398. Describe the Telescopic appearance of Jupiter. His form is seen to be oblate; his color a light yellow; and his disk is streaked with several curious belts. 88 PRIMARY ASTRONOMY. **399. How are these Bells situated?** On both sides of his equator, and parallel to it. **400. What is their number?** Only two or three are generally seen, though more are sometimes visible. (1. Much depends upon the power of the telescope by which they are viewed. An ordinary telescope will show only one or two of them on each side of his equator; but those on the opposite side may be seen with such a telescope.) (2. The preceding cut represents the Bell of the 15th of February, 1873.) **401. Do these Bells appear permanent or fluctuating?** They sometimes continue without change for months, and at other times break up and change their forms in a few hours. **402. Are they regular or otherwise?** These Bells are irregular, both in form and apparent density; as both bright and dark spots appear them, and their edges are always broken and uneven. (The preceding cut offers a good idea of the appearance of these bells, and the spots seen in them.) **403. What are these Bells supposed to be?** They are thought to be openings in the atmosphere through which the body of the planet is seen. (1. The theory of Jupiter upon his axis is supposed to throw the clouds which float in his atmosphere into parallel streaks, leaving regular intervals between them, thus forming the Bells.) **404. How are the Spots in these Bells accounted for?** They are supposed to be caverns, mountains, or some- PRIMARY ASTRONOMY. 89 thing unknown to us, but permanently attached to the body of the planet. [One of these spots, first observed in 1655, disappeared, and reappeared in the same spot for more than forty years; showing conclusively that it was something permanent.] **405. What else do we mean in examining JUPITER through a Telescope?** Four small stars are seen near him, and revolving around him. [1. These are the satellites of Jupiter, of which we shall give a more particular account in the next chapter (p. 370).——2. The writer once saw four of these satellites at once, and very distinctly, through a telescope, on the night when he saw the comet of 1819. He then saw them revolve round Jupiter with great rapidity, like the planets in our system.——3. He also saw two other satellites, one of which he could see from one to four small stars near him, that will occupy different positions at different times.] 90 PRIMARY ASTRONOMY. [1. The writer has often seen with the telescope the body of the planet and its rings, and has been struck by the resemblance which they bear to each other, as well as by their appearance when viewed from different points of view. It is in asking how the rings, and about the body of the planet, are formed that this inquiry arises. It represents the planet as a solid body, and yet it is known that it is not so. It is a gaseous body, and therefore it cannot be solid. November, 1866.] TELESCOPIC VIEW OF SATURN. FISHERMAN'S VIEW OF THE RINGS OF SATURN. The writer has often seen with the telescope the body of the planet and its rings, and has been struck by the resemblance which they bear to each other, as well as by their appearance when viewed from different points of view. It is in asking how the rings, and about the body of the planet, are formed that this inquiry arises. It represents the planet as a solid body, and yet it is known that it is not so. It is a gaseous body, and therefore it cannot be solid. November, 1866.] 410. What is the thickness of these Rings? It is estimated at about 100 miles. 411. Are they supposed to be solid, like the body of the planet ? They are ; from the fact that they sometimes cast a strong shadow themselves upon the body of the planet; and at other times show the planet's shadow very distinctly upon their own surfaces. 412. Are they all rest or in motion ? They revolve eastward around the planet every 10½ hours; or in the time of its rotation upon his axis. (The writer has often seen with the telescope Saturn's rings, except that there are no gaseous in the case of Saturn to unite the center to the circumferences. This detail, however, is perfectly supplied by the law of gravitation.) 413. What changes are seen to take place in the appearance of these rings during their revolution ? The apparent ellipse of the rings seems to contract for about 7½ years, till it almost entirely disappears, when it begins to expand again, and continues to enlarge for 7½ years. A diagram showing a circular ring with a smaller circle inside it. **PRIMARY ASTEONOMY.** 91 **414. What other change has been noticed!** For fifteen years the part of the rings toward us seems to be thrown up, while for the next fifteen it ap- pears to drop below the apparent center of the planet. TELESCOPIC PHASES OF THE RINGS OF SATURN. The cause of these varying appearances of Saturn will be easily understood by ex- amining the following diagram. **415. How are these Rings opposed as respects Light and Shade?** The Sun shines, alternately, fifteen years upon one side, and fifteen upon the other. SATURN AT DIFFERENT POINTS IN HIS ORBIT Here observe, first, that Co axis of Saturn, like those of all the other planets, revolves round its own axis in the same direction as the Sun's rotation. At each point of its equator, and at right angles with his axis, they also must remain parallel to them- selves. E. This being true now, it is obvious that while the planet is passing from A to B, it must be seen from the Earth as a dark spot; but when it is passing from B to C, it will appear to a spinn, upon the upper or work side; and it requires about 30 years for the planet to pass from A to B, and another 30 years for it to pass from B to C. The time required on the rings will be 12 years. At A and B the rings are opposite towards the Sun, and also towards the Earth; at C and D they are opposite away from the Sun, and also away from the Earth. He was in August 1867, and he returned home in September 1868. He came to his exams in September 7, 1868, from which time he had been absent for two months, and then to expand. From that time to June, 1869, they will continue, when he will reach them to expand. From that time to June, 1870, they will continue, when he will reach 92 **PRIMARY ASTRONOMY.** his other equivalent at E, and the rings will be visible. From June, 1603, to Auguste- n, 1678, they will again appear, and from September, 758, to March, 1874, they will reappear, and from July, 1938, to January, 2000, they will appear once more. Between these years before. The first ring will appear on the left side of the Earth. The second ring will seem to expand from A to C, until it contracts from C to D. So, since from E to G and from G to A. Again: From A to B the front of the rings will appear above the horizon. The third ring will appear on the right side of the Earth. It will seem to contract from H to I, until it expands from I to J. Since from J to K and from K to L. The writer has often seen the stages of Saturn in different stages of expansion and contraction. In the year 1603, when he was a boy of ten years old, he saw that (at that time January), they appeared as a bright line of light, as represented at A in the diagram below. **416. What purposes do these Rings serve, as appendages to Saturn?** They reflect the sunlight upon his surface, as our Moon does upon the surface of the Earth. **417. How must they appear to a person upon the body of the planet, either north or south of its Equator?** Like two gorgeous arches of light, bright as the full Moon, and spanning the whole heavens from east to west. In the marked cut, the observer is sup- posed to stand near some 300 miles of the equator of Saturn. He sees two arches of light, which are like two horns of a ram. While a New Moon is shown in the upper arch, a Full Moon is at the end of this same time.] **418. How does the width of the two Rings compare with the diameter of the Moon?** The two rings united are nearly 13 times as wide as the diameter of the Moon. The diameter of Saturn's orbit, which is divided by 5,000, the diameter of the Moon, gives 131 as the result. **419. How does their distance from Saturn compare with that of the Moon from our globe?** The nearest ring is only one-hundredth part as far from the planet as our Moon is from us. (To find this distance divide Saturn's diameter by 50,000, the distance of the nearest ring; and we have the above result.) At distances of only 30,000 miles, our Moon would appear some forty times as A diagram showing three rings around Saturn. SCENE AROUND SATURN. PRIMARY ASTRONOMY. 93 Keep in she does at her present distance. How magnificent and inconceivably great, then, must these vast rings appear to us! At the same time the Moon's magnitude, only one-eighth part of her diameter.] **420. What else does the Telescope reveal in connection with stars?** Eight small stars are seen in his vicinity, which are found to be Moons revolving around him. These are seen only with good instruments. On one occasion the writer saw five of them at once through the instrument represented in the frontispiece; but the remainder were too faint to be distinctly perceived. **421. How does Herschel appear through a Telescope?** Like a small ash-colored globe, without rings, belts, or discernible spots. [On his six Moons he will speak in another lesson.] **422. What can you say about the Telesopic appearance of Neptunes?** In color and general appearance he resembles Herschel. [So far as is known, Neptune has no rings nor belts, and is situated by only one Moon.] LESSON XXII. OF THE SECONDARY PLANETS. How are the planets of our system divided (138)? What are the Primary Planets (144)? Describe the Secondary Planets (145). Which of these classes have you been considering in the last fourteen lessons? **423. How many Secondary Planets are there now known to be? Twenty.** **424. How are they distributed among the Primaries?** The Earth has one, Jupiter four, Saturn eight, Her- cules five, and Neptune two. **425. By what other names are the Secondary Planets known?** 94 PRIMARY ASTRONOMY. They are often called Moons or Satellites. (See Note to 135.) **426. How are they situated with reference to their respective Primaries?** They are placed at different distances; as the Primaries are placed with respect to the Sun. **427. What can you say of their motions ?** They revolve around their respective Primaries, from east to west, and at the same time accompanying them around the Sun. (The Moons of Herschel are said to be an exception to this remark, and to revolve backwards on their orbits, which is contrary to the laws of motion.) **428. How are these Orbits generally situated ?** In or near the plane of the equators of their respective Primaries. (Herschel is supposed to be an exception to this rule also : the orbits of his satellites lying almost at right angles with the plane of his orbit. It may be, however, that his observations have been made too late in the year ; for in the case with Venus, and that his Moons are, after all, in the plane of his equation.) THE ROYAL - HER DISTANCE, MAGNITUDE, ETC. **429. By what name does the Moon known as the ancients ?** The Romans called her Luna, and the Greeks Selene. (L. Postumius Laus we have our modern terms moon and lunacy; the former of which signifies pertaining to the Moon, and the latter a disease supposedly to be cured by looking at the Moon.) Selene, in Mythology, was the daughter of Helios, the Sun. Our English word moon, signifying "the light," of the Moon's surface—is from selene, her ancient name.) **430. How has the Moon generally been regarded by mankind ?** As the most interesting object in the heavens. (Our beauty has been celebrated in the poetry of every age.) **431. Why has she attracted so much attention ?** On account of her remarkable changes both of position and appearance. PRIMARY ASTRONOMY. 95 **432. How is she situated with respect to the Earth?** She is the nearest of all the heavenly bodies. **433. What is her average distance?** About 240,000 miles. **434. What is her Magnitude?** Her diameter is 2,160 miles. **435. How, then, does her bulk compare with that of the Earth?** She is only $\frac{1}{4}$th part as large. (The Moon's diameter is about 7,000 miles, and its radius is about 3,500 miles. The radius of the Earth is about 3,960 miles. Therefore, the Moon's diameter is about $\frac{7}{3.96} \approx 1.78$ times the Earth's diameter.) **436. How does her diameter compare with that of the Sun?** It is only about $\frac{1}{4}$th part as great. (288,000 x 5.91 = 491, or a little more than one hundred of the Moon's diameter is contained in the Sun.) **437. What is the apparent diameter of the Moon?** Thirty-one degrees and seven minutes (31° 7"). (To find this number, we must subtract from 360° the sum of the angles of the Sun and Moon when they appear together in the sky.) **438. How do the Sun and Moon compare in their apparent magnitudes?) They appear about a size. (As the mean angular diameter of the Sun is 27° 2', and that of the Moon is 2°, the difference can only be 25'. Both the Sun and Moon are therefore nearly equal in size.) **439. What is their real comparative bulk?** The Sun is seventy million times as large as the Moon. **440. How is it, then, that they appear so near of a size?** It is because the Sun is 400 times as far off as the Moon. (The adjoining cut shows that until as the Moon is, she is at large as near as A as the Sun gives it.) 96 PRIMARY ASTRONOMY. LESSON XXIII. REVOLUTION OF THE MOON AROUND THE EARTH. **441. In what direction does the Moon revolve around the Earth?** From west to east. **442. Is that her AKEFENT daily course in the heavens?** It is not. **443. What causes her apparent daily revolution westward?** The revolution of the Earth eastward upon its axis. (See Question 28, and Notes.) **441. What proof have we that the Moon actually revolves eastward?** By watching her for a single evening, we can perceive that while she seems to go over us westward, she is actually moving eastward among the stars. **445. How we any other way know this?** At Noon, when she is near the Sun in the west, and continues to separate from him till Full Moon, when she is in the east. From that time she approaches the Sun, till she meets and passes him from the west. **446. What is the Periodic Time of the Moon?** Her Sidereal Revolution is performed in 27½ days. **447. What is meant by her SENEAL REVOLUTION?** It is a revolution from any given point in her orbit around to the same point again. (See 234, and Notes.) **448. What is a Synodic Revolution of the Moon?** It is from one new Moon, or conjunction with the Sun, to another. **449. Can you state the difference between a Sidereal and a Synodic Revolution of the Moon, and explain it by a diagram?** The Sidereal is one complete revolution; but the PRIMARY ASTRONOMY. 97 motion of the Earth in her orbit renders it necessary for the Moon to perform a little more than a complete revolution each month, in order to come in conjunction with the Sun, and make a Synodic revolution. SINUSAL AND SYNODIC REVOLUTIONS OF THE MOON. A diagram showing the synodic revolution of the Moon. The Earth is shown in its orbit around the Sun, with the Moon at various positions relative to the Earth and Sun. The diagram shows that the Moon completes one synodic revolution in 29 days, 12 hours, and 44 minutes. (On the right the Earth is shown in her orbit, revolving around the Sun, and the Moon is shown in various positions relative to the Earth and Sun. The diagram shows that the Moon completes one synodic revolution in 29 days, 12 hours, and 44 minutes.) The Synodical Revolution is made up of two parts: (1) The Synodical Revolution of the Moon, or it is New Moon. As the Earth advances from D to E, the Moon passes through its first quarter phase, and then through its new moon phase. But she will still west of the Sun, and must pass on from B to C, or 1 day and 20 hours longer, before she can again come in conjunction with the Sun. This Synodical Revolution takes place every 29 days and 20 hours. (2) The Student will perceive that the difference between a Sidereal and Synodic revolution is due to the fact that the Moon does not always keep in step with the Earth's motion; namely, the revolution of the Earth around the Sun. See Question 367, with reference to this point. **450. What is the daily angular motion of the Moon eastward?** Thirteen degrees, ten minutes, and thirty-five seconds (13° 10' 35") per day. This motion is made for one day of twenty-four hours. In the adjoining table we have given this motion for each hour of twenty-four hours. It may be traced from her conjunction or new moon position to any other position at the same point again. This being a uniform revolution, requires only 29 days.) **451. What is the Moon's VELOCITY in her orbit, with respect to the Earth?** About 2,300 miles per hour.
































































This table represents a lunar calendar for a period of approximately three months. The dates are given in Roman numerals.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35" 13° 10' 35"
DAYSDAYSDAYSDAYSDAYSDAYSDAYSDAYSDAYSDAYSDAYSDAYSDAYSDAYSDAYSDAYSDAYSDAYS
9 A diagram showing the synodic revolution of the Moon. The Earth is shown in its orbit around the Sun, with the Moon at various positions relative to the Earth and Sun. The diagram shows that the Moon completes one synodic revolution in 29 days, 12 hours, and 44 minutes. (On the right the Earth is shown in her orbit, revolving around the Sun, and the Moon is shown in various positions relative to the Earth and Sun. The diagram shows that the Moon completes one synodic revolution in 29 days, 12 hours, and 44 minutes.) The Synodical Revolution is made up of two parts: (1) The Synodical Revolution of the Moon, or it is New Moon. As the Earth advances from D to E, the Moon passes through its first quarter phase, and then through its new moon phase. But she will still west of the Sun, and must pass on from B to C, or 1 day and 20 hours longer, before she can again come in conjunction with the Sun. This Synodical Revolution takes place every 29 days and 20 hours. (2) The Student will perceive that the difference between a Sidereal and Synodic revolution is due to the fact that the Moon does not always keep in step with the Earth's motion; namely, the revolution of the Earth around the Sun. See Question 367, with reference to this point. **450. What is the daily angular motion of the Moon eastward?** Thirteen degrees, ten minutes, and thirty-five seconds (13° 10' 35") per day. This motion is made for one day of twenty-four hours. In the adjoining table we have given this motion for each hour of twenty-four hours. It may be traced from her conjunction or new moon position to any other position at the same point again. This being a uniform revolution, requires only 29 days.) **451. What is the Moon's VELOCITY in her orbit, with respect to the Earth?** About 2,300 miles per hour.
DAYS DAYS DAYS DAYS DAYS DAYS DAYS DAYS DAYS DAYS DAYS DAYS
97 A diagram showing the synodic revolution of the Moon. The Earth is shown in its orbit around the Sun, with the Moon at various positions relative to the Earth and Sun. The diagram shows that the Moon completes one synodic revolution in 29 days, 12 hours, and 44 minutes. (On the right the Earth is shown in her orbit, revolving around the Sun, and the Moon is shown in various positions relative to the Earth and Sun. The diagram shows that the Moon completes one synodic revolution in 29 days, 12 hours, and 44 minutes.) The Synodical Revolution is made up of two parts: (1) The Synodical Revolution of the Moon, or it is New Moon. As the Earth advances from D to E, the Moon passes through its first quarter phase, and then through its new moon phase. But she will still west of the Sun, and must pass on from B to C, or 1 day and 20 hours longer, before she can again come in conjunction with the Sun. 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What is the Form of her Orbit?** With respect to the Earth it is elliptical, like those of all the other planets. **435. When is the Moon said to be in Apogee?** When in that part of her orbit nearest to the Earth. (The word Perigee is from the Greek περί, about, and γές, the Earth, and signifies near the Earth.) **434. When is the Moon in Apogee?** When at her greatest distance from the Earth. [Apogee is from ap-, from, and geo, the Earth.] **436. How many are the ANDES of the Moon's orbit?** They are the same as the Perigee and Apogee. (The singular of ap-odes is ap-oe.) **435. What is the Line of the Apogees?** It is a right line from one axis to the other. (This is simply the Major Axis of the elliptical orbit of the Moon. See Question 87, and note.) **437. Is the Line of the Apogees of the Moon's orbit fixed in the Eccentric, or does it change by direction? It revolves from west to east around the ecliptic in about nine years. [In this question an attempt is made to explain why the Moon's orbit is not circular, but that of an ellipse with its axes directly to the right and left, but at B, C, and D it is seen obliquely. This explanation is very pernicious when compared with what has been stated before. The Moon's orbit changes its direction once every nine years; and at the end of a year, when the Sun passes through B, C, and D, a new orbit is found to have reversed its direction. In this way, after nine years, we find that the apogees point north A will have made a complete revolution, and re- turned to its original position.] A diagram showing an elliptical orbit with points labeled A, B, C, D, E, F, G, H, I, J. MOITION OF THE APOGEE. **PRIMARY ASTRONOMY.** 99 **458. What is the true form of the Moon's path?** It is an irregular curve, always concave toward the Sun, and crossing the Earth's orbit every 13 degrees nearly. (1.) In the Earth's orbit still in her orbit, the Moon's path is concave toward the Sun, as in the ellipse to which she describes with respect to the Earth. (2.) If the Earth moved so slowly on her orbit that the Moon would describe a circle around her, the Moon would be at the same distance from the Earth at all times during one revolution, as shown by the adjoining figure. But this is not the case. The Moon describes a path which is concave toward the Sun, and at different distances from the Earth. This fact alone shows that the Moon's orbit never can recur into itself, but must always describe a new path. THE MOON'S ORBIT ALWAYS CONCAVE TOWARD THE SUN A B C D E 3. That the lunar orbit is always concave toward the Sun, may be demonstrated by the above diagram. Let the upper curve line A represent an arc of the Earth's orbit, equal to that passed through by the Earth during half a revolution. Now the radius of this arc is 600 miles less than that of the Earth's actual orbit, which is 480,000 miles within the Earth. But as the Moon departs only 24,000 from this radius, she must describe a path which is concave toward the Sun, as represented by the middle line, which is concave toward the Sun. 4. This subject may be further illustrated by the following cut, representing THE MOON'S PATH DURING A COMPLETE LUNATION. Here the plain line represents the Earth's orbit, and the dotted one that of the Moon. At A The Moon crosses the Earth's track 24,000 miles behind her. She passes on the right side of her own track, and then continues on her way until B. At B she has crossed her orbit at 24,000 miles ahead of her, being then at her Third Quarter. At C she has crossed her orbit again at 24,000 miles behind her, and has overtaken her at D at New Moon. From D to E the Earth continues its gain, till at F it has gained 24,000 miles over its starting point. At G it has reached its highest point behind her at A. Then the Moon winds her way along, first within and then without the Earth, A diagram showing a lunar orbit that is concave toward the Sun. 100 PRIMARY ASTRONOMY. always gaining upon us when outside of our orbit, and falling behind us when within it. The moon circles the earth representing the Moon's orbit with respect to the Earth, which is so nearly as to make as if the Earth had no revolution around the Sun.) **459. Does the Moon ever actually retrograde on the Ecliptic?** She does not. **460. What is her absolute velocity in space, in accompanying the Earth around the Sun?** It is never less than 65,700 miles per hour. (To this velocity we must add that with respect to the Earth, about 8300 miles per hour (451), which outside of the Earth, as in the above figures, adds nothing to her absolute velocity. The Earth's velocity (615), would give 73,500 miles as the absolute velocity of the Moon. But since the Earth's velocity at its axis leaves 9300 miles per hour, while its velocity at its equator leaves 63,700 miles per hour, her hourly velocity, while leaving 63,700 miles as the absolute velocity of the Moon, will leave 54,800 miles as she is falling behind the Earth. We may also consider that she is always exactly opposite the ecliptic, and see the paths of the Earth and Moon, as represented by the dotted lines in the annexed figure. Her path, however, would be conceived toward the Sun, as shown by the dotted line D D', but not contrary, as we were obliged to represent it here as so small a diagram.) **461. How is the Moon's orbit situated with respect to the ECLIPtic?** It departs only about $5^{\circ}$ from that plane ($5^{\circ} \text{ to } 48^{\circ}$). INCLINATION OF THE MOON'S ORBIT TO THE PLANE OF THE ECLIPTIC A circle with two smaller circles inside it labeled "MOON'S PATH" and "ECLIPTIC". The larger circle represents the Moon's orbit around the Earth, and the smaller circle represents the Ecliptic plane. (Let the line A B represent the place of the Earth's orbit, and the line joining the Moon at A and D, would represent the inclination of the Moon's orbit with that of the Earth's. The inclination being $5^{\circ}$ to $48^{\circ}$, it would be so small as to be neglected if it would be fixed at New A and New Moon C.) **462. Does the Line of the Moon's nodes remain stationary on the Ecliptic?** It does not; but retrogrades or revolves westward around the ecliptic every 19 years. (The amount of this motion is 107' 32" per annum, which would require 18 years and 219 days for a complete revolution.) PRIMARY ASTRONOMY. 101 LESSON XXIV. THE MOON'S CHANGES. **463. Is the Moon self-luminous, or opaque?** She is opake, like all the rest of the planets, and shines only by reflection. (This is obvious from the fact that only as much of the Moon is bright as is eclipsed by the Sun.) **464. What is the cause of the various Phases of the Moon, as New Moon, Full Moon, etc.?** It is caused by her revolution around the Earth, which enables us to see more of her enlightened side at one time than at another. CIRCUIT OF THE MOON'S PHASES. A diagram showing the phases of the Moon. The diagram shows a crescent Moon on the left, a half Moon in the center, and a full Moon on the right. The diagram also shows a circle with a line drawn through it, indicating that the Moon's circuit is complete after one revolution around the Earth. MOON'S CIRCUIT (ONE REVOLUTION) (1) The above circ represents the Moon revolving outward around the Earth. In this outside circle she is represented as she would appear, if viewed from a direction at right angles to her circuit. At A she appears as a crescent, because she is turned away from us; at B she appears like a half Moon at every point. At C she appears as a full Moon, because she is turned toward us. At D she appears like a dark globe. At B she would appear like a crescent, turned toward the east. At C more of her face would be seen than at B, because she has turned toward us. At D more of her face would be seen than at C, because she has turned toward the west. We then call her a Full Moon. From E around to A again the dark portion of her face will be seen to increase until we reach F. Then we call her a New Moon. When she has completed her circuit, she will be in exactly the same place as when she began. 2. To observe this phenomenon, take his book bottom upward, and hold it north of him; he will see why the crescent of the Old Moon at H is crescent on the west, instead of the east, p2 102 PRIMARY ASTRONOMY. like the New Moon; and why she is seen before sunset instead of just after sunset. But these points will be called up and more fully illustrated hereafter.] **465. What is meant by the Change of the Moon?** It is when she is in conjunction with the Sun, and changes from what is called an Old Moon to a New Moon. [It is only on the 13th day of the lunar month that the Moon will be in conjunction with the Sun at the dipole point, and by observing her carefully, we find that she is rapidly approaching that point, and that she will be in conjunction with the Sun about the 14th day after sunset, east of the Sun, just after sundown, or a New Moon. This change, as it is called, takes place when the phases (the Sun's motion) are **466. What is meant by the New Moon?** It is when she appears in the west like a slender crescent, and during the first seven days after her change. [1. It is New Moon from 7 to 8 Cn in proceeding cut.] NEW MOON IN THE WEST JUST AFTER SUNDOWN. 2. Here is a picture of what you have often seen—the New Moon in the west just after sunset. She has been moving eastward all day long, and now she is in the west. She will also very soon, and be out of sight. A professor is pointing her out to you, and he says, "This is the New Moon." Now, this professor has come to the schoolhouse near by, to a "spelling-school," or to hear a Lecture on Astronomy." **467. What are the Cursus of the Moon?** The extremities of the crescent. **468. What are the Moon's Cursus?** Two points in her orbit $180^\circ$ apart, where she is New and Full Moon. (See A and E in the cut, page 101.) A diagram showing two points in the lunar orbit labeled A and E, representing New and Full Moon positions respectively. PRIMARY ASTRONOMY. 103 **469. What are her Quadratures?** Four points in her orbit, 90° apart. (See positions 1, 2, 3, and 4, page 101.) **470. What are the Octants?** Divisions of 45° each part. (See A, B, C, D, page 101.) **471. What is the First Quarter?** It is when the Moon has performed one-quarter of her journey eastward around the Earth, and appears just one-half of her enlightened side. [L. At this time the Moon is south of us on the Sun goes down, or 90° from him, and we see one-half of her enlightened side.] THE MOON AT HER FIRST QUARTER. Here is a diagram showing the Moon's position at different times during her journey round the Sun. Half a month after the new Moon she is in quadrature, and showing herself to be Half-Moon, or the Moon at her First Quarter. A week ago she was close to the Sun and went down in the west; and now she is in a similar condition; but now she is in the West of the Sun, and will go up in the east. This is called the First Quarter. The Moon appears as half-sun, she is at the point No. 2 in her orbit, as shown in the diagram. **472. What is meant by the Full Moon?** It is when the Moon appears round, and reflects the greatest amount of light upon the Earth. **473. How is the Moon then, situated with respect to the Sun?** They are in opposition, or 180° apart. **474. How is the Full Moon situated with respect to the Earth's orbit?** 104 PRIMARY ASTRONOMY. She is outside of it. **475. How with respect to the Sun?** She is at her greatest distance from him. (The student will understand that the Moon, by revolving around the Earth almost in the plane of the ecliptic, must vary very her distance from the Sun, not only in her journey round the Earth, but also in her journey round the Sun. The distance between them varies from 350,000 miles from New to Full Moon. See illustration, page 101, where the Full Moon is shown.) **476. How does the Full Moon rise with respect to the Sun?** She rises in the east just as the Sun goes down in the west. THE FULL MOON RISING AS THE SUN SETS [In this picture we see the Full Moon rising at the west, as the Sun goes out of sight behind the horizon. The Moon rises when she is on the same side of the Earth as the Sun, and so she rises in the east. She rises when she is on the opposite side of the Earth from the Sun, and so she rises in the west. This is why we say that she rises with respect to the Sun.] **477. What is the Moon at her THIRD QUARTER?** When she has passed three-quarters of her journey, from New Moon around the Earth. (See cut, page 101.) **478. Is she east or west of the Sun at this time?** West of the Sun, and going toward him. **479. What is her distance?** Ninety degrees west. (At this time she will rise six hours before the Sun; will be south at midday; and will set twelve o'clock.) PRIMARY ASTRONOMY. 105 **480. How does the Moon appear when at her Third Quarter?** Just as she does at her First Quarter, except that her eastern side is enlightened instead of the western. (At A, in the figure, we have a view of the Moon at her First Quarter, when she is seen on the Sun's axis, and her western side is illuminated. At B, we see the same Moon at her Third Quarter, when she is on the Sun's axis, but her eastern side is en- lighened.) **481. What is meant by the Old Moon?** She is called an Old Moon from the Full to the New, and especially from the Third Quarter to the change. **482. How does the Old Moon appear?** Like a slender crescent, much like the New Moon. **483. When do we see the Old Moon?** The crescent of the New Moon point east, and those of the Old Moon west. **484. When and where can we best see the Old Moon?** In the east, just before sunrise. THE OLD MOON BEFORE SUNRISE. (1) Here the Old Moon is seen in the east just before sunrise. It looks just like this New Moon shown on page 191, except that the crescent is inverted, the crescent side being toward us. This is because the Sun is rising behind us, and he makes no difference between her present appearance and that of the New Moon, and the crescent is seen in the east. 2. The crescent of the New Moon appears in the west after sunrise, and then it is turned upside down, dressed before sunrise, to see the Old Moon. 3. The Moon is at this time very near her change. At noon she may be seen just
First and Third AUSTERN
106 PRIMARY ASTRONOMY. most of the Sun, and in a few days will pass him scattered, when she will be a New Moon again, soon to be seen at the Sun goes down, as represented on page 90. The Moon's motion is therefore very slow, and requires many months to make one revolution. 3. We earnestly recommend to both teacher and student to observe the process precisely, and to note the time of each new moon, and the time of each full moon, at least once a year. A little time spent in this way will do more to fix correct ideas in the mind than months of leisure study. What is meant by the Moon's Gnomons? When between a Half and a Full Moon. (See "Gibbous," cut, page 101.) **486. What is meant by the Moon's Waxing and Waning?** She waxes larger from the change to the full ; and wanes, or grows smaller, from the full to the change again. LESSON XXXV. DAY AND NIGHT, SEASONS, AND TELESCOPIC APPEARANCE OF THE MOON. **487. What is the line called which separates the dark from the enlightened portions of the Moon's disk?** The Terminator. [As] each one-half of the Moon is always enlightened by the Sun, whether it appears so or not, it follows that the terminator must extend quite around the Moon, and that it must be perpendicular to the plane of her orbit, which is called the Circle of Illumination. At New and Full Moon this circle is edgewise to us; but at all other times it inclines toward us. The terminator is thus a line which separates from the Earth into view the shadowed disk twice during every revolution.] **488. Has the Moon a Diurnal Revolution?** She has. **489. In what time does she revolve on her Axis?** In 29½ days, or once during every revolution around the Earth. **490. How is this known?** From the fact that the same side of the Moon is always toward the Earth. PRIMARY ASTRONOMY. 107 (1. By watching the Moon carefully with the naked eye it will be seen that the moon's spot occupies nearly the same place upon her disk from night to night, and that this spot on the moon side is always toward us.) The Moon's revolution. A diagram showing the Moon's orbit around Earth, with arrows indicating its apparent motion from left to right. The Moon is shown at various positions in its orbit, labeled A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z. A diagram showing the Moon's position relative to Earth and Sun. The Sun is shown at the top center of the diagram. The Moon is shown in various positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at different positions in its orbit around Earth. The Sun is shown at different positions in its orbit around Earth. The Moon is shown at不同位置的在它的轨道上。太阳是位于地球的中心。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。月亮是在地球的轨道上。太阳是位于地球的轨道上。 (2.) What are her Librations? 108 PRIMARY ASTRONOMY. A slight rolling motion, by which the parts about her poles alternately appear and disappear. **496. What is the cause of this?** The inclination of her axis to the plane of her orbit, and her revolution around the Earth. **497. What is the inclination of her Axis to her Orbit?** About a degree and a half (1° 30' 10"'). (If the inclination of the Earth's axis brings first one pole and then the other toward the sun, it will be seen that the same effect would be brought about if the Earth's axis would be first one pole and then the other away from the view of the Earth. But as her inclination is only 1°, the illusion in every sight.) **498. What is the Length of a Natural Day upon the Moon?** Twenty-nine and a half of our days. (An eclipse of the Sun takes place once in nineteen Days ; and if that requires 29½ of our days, her day must be equal to 29½ of ours.) **499. What is the Length of her Year ?** Only 29½ days. (For two years of a planet, we mean the time required for a complete revolution in its orbit.) The length of her year is therefore equal to twice the length of her period of Revolution as equal to 84 of our years; and thus of Neptune as equal to 60. Her period of revolution being 165 years, she completes one year by her periodic revolution, so that it is comprehended within the narrow limits of 29½ days.) **500. What curious facts does this establish?** That she has only one day in her year; or, in other words, that her days and her years are precisely of a length. **501. Can the Earth be seen from all parts of the Moon?** She can only be seen from the one side, which is always turned towards us. **502. How would the Earth appear from the Moon?** Like a bright stationary planet, thirteen times larger than the Moon, and exhibiting all her varying phases. (The Moon being at its perigee, or nearest point to the Earth, on either side of the moon, and the Earth would appear like a magnified Far-Nea. At B she would be seen as a bright spot, at C as a crescent, at D as a half-moon, at E as a full moon, at F as a crescent, at G as a half-moon, at H as a crescent, at I as a half-moon, at J as a crescent, at K as a half-moon, at L as a crescent, at M as a half-moon, at N as a crescent, at O as a half-moon, at P as a crescent, at Q as a half-moon, at R as a crescent, at S as a half-moon, at T as a crescent, at U as a half-moon, at V as a crescent, at W as a half-moon, at X as a crescent, at Y as a half-moon, at Z as a crescent.) PRIMARY ASTRONOMY. 109 would appear directly overhead, while 90° from that point she would always appear in the horizon.) **503. How does the surface of the Moon appear to the naked eye?** It exhibits a variety of dark lines and spots. **504. What do they resemble?** There is a dark figure on her western limb, resembling the head of a man with his hand to the north, and his body inclined to the east. Just east of him, and opposite his shoulders, is an irregular object resembling a huge bundle or pack. (1. Both these objects are represented in the ad- jacent plate, which was taken by Mr. John G. author on the evening of December 18th, 1851. It represents the Moon as she appears when about six hours after sunset, and at the time of the sixteenth taken during the same evening. Let the reader observe carefully how this figure agrees so far as drawing is correct. So long as we have a legend by which they explain this singular appearance of the Moon, we may suppose that such a figure is a beautiful Ogivale model, who was translated to the Moon "many ages ago" for some reason unknown to us. But let us refuse to many any of the "young brutes" who say that "the Moon is made up of coarse and mani- culate, and that the rest of the figures mean, the motion does not inform us what they mean." The reader will remember that in the first edition of this work I said that "the people who have been told that it is a man with a bundle of wood upon his back, who was sent to the Moon." This is not true. The people who have been told that it is a woman with a bundle of wood upon her back, are those who have seen them cross the Ohio river, as it often happens with the subject of diabolism to God, and with the sanctity of the holy Sabbath." **505. What are these objects supposed to be?** The outlines of her great natural divisions, as Mount- ains, Valleys, and Continents. I purposely omitted these objects from my plates upon the face of the Moon, and many others, because they were thought we knew very little about how few have supposed so they were not worth representing. But I am now convinced that these facts are true, and that their outlines were a natural map of its most recent history! Having thus proved this fact, I am now ready to proceed to prove that subject any further, and here their investigations have ended.] **506. How does the Moon appear through a Telescope?** A plate showing various features of the Moon's surface. NATURAL APPEARANCE OF THE FULL MOON 10 110 **PRIMARY ASTRONOMY.** Her surface is very rough and uneven, covered with deep valleys and lofty mountains. The profile of the Moon is remarkably characterized by inequalities like our moonlit nights. This indeed, is its most striking feature. There are a great number of particulars which may be noticed in this respect. The mountains are high and flat, and also several magnificent chasms or ravines, some of whose peaks are 25,000 feet high, and others 30,000 feet high. These mountains are not all on one side of the globe, but any of the regions of the globe. They are most prominent on the side toward the polar region, and gradually slope off through thousands of miles upon the oppo- site side. The valleys are also numerous, and they are generally long and narrow; viz: they are steep, almost perpendicular, on one side, and their other is a long distant plain. The valleys are often filled with water, and thus form lakes. **507. What proof have we that the Moon's surface is mountainous?** The line of the terminator is very rough and uneven, which would not be the case if her surface was level or smooth. [This is a reference to the line dividing the illuminated from the dark portion of the Moon.] **508. Have we any other proof?** The projection of long shadows, in a direction opposite the Sun, shows the existence of mountains that intercept the Sun's light. [This is a reference to the long shadows cast by mountains projected from right to left.] **509. Can you mention any further evidence of the existence of Mountains in the Moon?** From New to Full Moon bright spots break out from time to time, just east of the terminator, in the dark portion, and grow larger and larger, till they join the illuminated portion, showing them to be the tops of mountains, which reflect the sunlight before it reaches the intervening valleys. PRIMARY ASTRONOMY. 111 Depictions of these bright points may be seen in the cut. The writer has often watched them, and seen them change size and move, in the Sun's disc and enlightened the sides of the mountains. **510. When is the best time to examine the Moon with a Telescope?** Near the First or Third Quarter. **511. Why is this?** Because the shadows of objects are then seen at right angles with the line of vision, and to the best advantage; while at Full Moon objects cast no shadows visible to us. **512. How are the Shadows projected from the Full to the New Moon?** From east to west. OLD MOON. FULL MOON. NEW MOON. [1. The shadows are always projected in a direction opposite the Sun, or toward the illuminated side of the Moon; and on her eastern limb they come from the dark part to the full moon, and her western limb they come from the light part to the new moon. This law of projection must be reversed.] In the morning station at a distance directly over the Andes. Before the Sun rises, he would see the tallest peaks enlightened, and he knows the long shadow which they cast. In the evening station, he would see their long shadow would be visible; but at sunset they would again be seen stretching away to the west, and his telescope would show them in darkness. Thus we have two periods, except that the time required is a lunar day, equal to three-fourths of our days, instead of a solar day. **513. What is the Form of the Lesser Mountains?** Some of them are in extensive ranges, while others are of a circular form. Great numbers of these circular mountains may be seen with a telescope of moderate power. Through such an instrument the Moon will appear as a yellowish A circular mountain on the Moon. A circular mountain on the Moon. A circular mountain on the Moon. 112 **PRIMARY ASTRONOMY.** him, and the circular mountains like deeps of thick oil on the surface of water. Two extensive ranges and several of the circular elevations are shown in the cut, page 116. **514. What is their extremity?** From these considerations we see the proper rules for determining the height of objects by the length of their shadows; and by applying these rules to the shadows seen upon the Moon's surface, astronomers ascertain the height of the lunar mountains. **515. What is inferred from the Shape of the Circular Mount- ains?** That they are craters of immense volcanoes. **516. Is the Moon surrounded by an Atmosphere ?** It is not certain whether she is or not: if she is, it must be exceedingly thin or rare. The substance we call air or atmosphere is subject to the general law of gravitation. The force which draws it down towards the centre of the Earth is greater at the Moon's surface than at its centre; therefore, as the general density of the substances of any planet is dependent upon the attracting force of that planet; and the Moon has only about $\frac{1}{6}$ part as much matter as our Earth, it follows that her atmosphere, if she has one, ought to be much less dense than ours. **517. Has she any Water upon her surface?** It is thought not, from the fact that it would be converted into steam or vapor, during her long and hot days, and from the fact that no clouds are ever seen floating around her. **518. Was it ever supposed that she had Seas upon her sur- face?** It was; but the portions once supposed to be seas, are now found to be only precipices or plains. **519. How does the Moon appear through Lord Ross's great Tele- scope?** Dr. Scoreby, who examined her through this instru- ment, states that she appears "like one vast ruin of nature," with numerous volcanoes, and fragments of rock scattered about them in every direction. **520. What does Dr. Herschel say of the Lunar Volcanoes?" PRIMARY ASTRONOMY. 118 He believed that he actually saw the *fires* of several that were in active eruption. [This has not been confirmed by any recent observer, and is therefore somewhat doubtful.] LESSON XXVI. ECLIPSES OF THE SUN. 521. What is an Eclipse? It is a partial or total obscuration or darkening of the Sun or Moon, by the intervention of some opaque body. 522. How many kinds of Eclipses are there? The first general division is into Solar and Lunar. 523. What is a Solar Eclipse? An Eclipse of the Sun. 524. What is a Lunar Eclipse? An Eclipse of the Moon. 525. What is the first point to be observed in consider- ing the subject of Eclipses? The fact that all the planets, whether primary and secondary, cast shadows in a direction opposite the Sun. 526. Upon what does the form and length of these shadows depend? Upon the compar- ative magnitude of the Sun and planet, and their distance from each other. A diagram showing a solar eclipse with a crescent-shaped shadow on the Sun. SHADOWS OF THE PLANETS. 10* 114 PRIMARY ASTRONOMY. **527.** What would be the form of the Shadow if the Sun and planet were of a size? The shadow would be in the form of a cylinder. CYLINDRICAL SHADOW. A circular diagram showing a cylinder-like shadow. **528.** What would be the form of the Shadow if a planet were larger than the Sun? It would diverge or expand from the planet outward. DIVERTING SHADOW. A circular diagram showing a diverging shadow. **529.** As the planets are much smaller than the Sun, what must be the form of their Shadows ? They must converge to a point, taking the shape of a cone. CONVERGING SHADOW. A circular diagram showing a converging shadow. (By observing the cut on the preceding page, the student will see the shadows of the planets all running to a point, in accordance with this principle.) **530.** What effect has the distance of a planet upon the form and length of its Shadow ? The more distant, the longer its shadow, and the more slender the point of the cone. SHADOW MODIFIED BY DISTANCE. A circular diagram showing a modified shadow due to distance. (1.) In this cut, the Sun and Earth are of the same size as in the one immediately PRIMARY ASTRONOMY. 115 preceding, and yet this shadow is shorter than the one more remote than in the other case, simply because the two bodies are placed nearer each other. The reason why the shadow of the Moon is so much longer when the Sun is near the Sun have comparatively short shadows, and the ones terminate quite abruptly; so that they do not extend to the horizon, but only reach to the primary, how- ever, cast a shadow long enough to reach the next extreme point.) **531. What is the cause of a Solar Eclipse?** It is caused by the Moon passing between the Earth and the Sun, and casting her shadow upon the Earth. **532. At what time of the Moon must she always occur?** At New Moon. **533. Why is this?** Because she never between the Earth and the Sun except at the time of her change. **534. Why do we not have a Solar Eclipse at every New Moon?** Because the plane of the Moon's orbit is not in the plane of the ecliptic ; so that she sometimes passes above the Sun, and sometimes below him. NEW MOONS WITHOUT ECLIPSES. About the Sun. A diagram showing a new moon without an eclipse. The Earth is shown with its axis tilted slightly away from the Sun. The Moon is shown moving from left to right, passing first above the Sun (a solar eclipse) and then below it (no eclipse). The Sun is shown as a circle with rays extending outward. The Earth is shown with its axis tilted slightly away from the Sun. Solar Eclipses Before the Sun. [Let the line joining the Earth and the Sun represent the plane of the ecliptic. Now as the orbit of the Moon departs from this plane 5° 7', as shown and illustrated on 60], she will pass through it at two points, which are represented in this figure ; and her shadow fall above the North Pole or below the South. At each thereof, there, there can be no eclipse.] SOLAR ECLIPTIC LIMITS. A B C D 116 PEIDARY ASTRONOMY. 535. Why do Eclipses of the Sun always come on from the West and pass over Eastward? Because the Moon, which causes them, revolves from west to east. 536. In the adjoining cut, the Moon is seen revolving counterclockwise, that is, from west to east, around the Sun at the centre of the Sun's disk. In some instances, however, when the eclipse is total, the Moon may appear to pass directly through the centre of the Sun; that is, she may be either above or below it; or even then her shadow may be so large as to cover the whole of the Sun's surface. 5. It will be obvious from this figure, that the shadow of the Moon upon the Earth must also traverse her surface from west to east, and that it will be visible only as long as it is within the west than in the east. 537. Why must the Moon be in her orbit at the time of its character in order to eclipse the Sun? At or near one of her Nodes. 538. What is the greatest distance at which she may be from either Node, and yet eclipse the Sun? About seventeen degrees (16° 59'). 539. What is this distance called? The Solar Ecliptic Limit. [This point is illustrated by the cut set up on the preceding page. The Moon is seen revolving counterclockwise around the Sun, so that in passing her ascending node at C, the Moon passes centrally between the Sun and Earth, and therefore eclipses them both; but if she would pass the Earth's shadow centrally, and would be totally eclipsed. At D, she is seen to have passed beyond the descending node of the Sun, and reaches her conjunction at H where seven degrees depart from her descending node. At I, she has reached her ascending node again; but now she is so far southwardly too high above the ecliptic. The distance from I to H represents her Eclipse Limit, within which she must be at her conjunction in order to eclipse the Sun.] THE MOON CHANGING AT DIFFERENT DISTANCES FROM HER NODES. A B C D E F G H I 5. Let us line A R represent the plane of the Earth's orbit; and C D that of the Moon. Now let us suppose that we are looking down on these planes from above, and see Earth on the line of her nodes (as shown at L) where the Moon is supposed to be in conjunction with it. Then if we look along AR towards C D we shall see that she would appear to pass directly over the Sun's center. Such an eclipse would, therefore, be central, and either total or annular. A circular diagram showing a view from above looking down on two planes representing Earth's orbit (A R) and Moon's orbit (C D). The Moon is positioned at L on Earth's orbit, implying conjunction with Earth's nodes. The diagram illustrates different positions of the Moon relative to Earth's nodes and how they affect eclipses. **PRIMARY ASTRONOMY.** 117 4. If the change took place when the Moon was either side of her node as at F, F', but before she passed that point, the Sun would appear to cross the upper or lower part of the disc. The eclipse, therefore, would be partial. 5. At G the eclipse would still be half, at H the two disks (Sun and Moon) would be equal, and at I the Moon would be entirely covered by the Sun. In these cases there would be no eclipse whatever, as the Moon would seem to pass entirely beneath the Sun. 539. When, therefore, may we expect Eclipses of the Sun ? Whenever the Moon is within 17° of either Node at the time of her change. 540. What is meant by the Umbra of the Earth and Moon? It is the dark shadow cast in a direction opposite the Sun. [Umbra is Latin word, signifying a shade or shadow.] 541. What is the Penumbra ? It is a partial shadow outside the Umbra. [Penumbra is Latin from Latin peno, shroud, and umbra, a shadow; and signifies about a shadow.] UMBRA AND PENUMBLA OF THE EARTH AND MOON. A diagram showing the umbra and penumbra of the Earth and Moon. 542. What is the average LENGTH of the Moon's Umbra ? About 280,000 miles. (To find this, take 280,000 multiplied by 60 miles, according to the Moon's distance from the Sun. See §95 on cut.) 543. What is its greatest DIAMETER at the distance of the Earth's About 170 miles. (This is what it means for the Earth exactly as perpendicularly to its surface. When it strikes obliquely, it covers a much larger surface.) 118 PEIMABY ASTRONOMY. **544. How large a portion of the Earth's surface may be covered by the Moon's Penumbra?** About 4,303 miles in diameter. **545. What is a Partial Eclipse?** One in which only part of the Sun or Moon is obscured. **546. When is an Eclipse total?** When the whole disk of the Sun or Moon is darkened. **547. What is an Annular?** It is when the Sun and Moon, or the Moon and the Earth's shadow, seem just to touch, without producing an actual eclipse. (See H II in the cut, page 116.) **548. At what places on the Earth are Solar Eclipses generally total?** At all places within the Moon's Umbra. **549. Where are they partial?** At all places beyond the Umbra and within the Penumbra. **550. What is meant by a Central Eclipse of the Sun?** It is when the Moon changes when exactly at one of her Nodes, and appears to pass centrally over the Sun's disk. **551. Are all Central Eclipses of the Sun necessarily total?** They are not. **552. If they are central, but not total, how must the Sun appear?** Totally obscured, with the exception of a bright ring, apparently around a dark body in the center. **553. What are such Eclipses called?** Annular Eclipses. **554. Why are they called Annular?** From annulus, a ring, because the Moon only hides A diagram showing a partial solar eclipse. PRIMARY ASTRONOMY. 119 the center of the Sun, and leaves a bright ring unob- scured. PROGRESS OF A CENTRAL ECLIPSE. Going off. Annular. Coming on. **555. When will the next Annular Eclipse visible in the United States occur?** May 26, 1854. **556. Why are some Central Eclipses of the Sun TOTAL and others ANNUULAR?** Because the apparent magnitude of the Sun and Moon varies as their distances vary. (See 178, and Ill- ustrations.)
Total TOTAL AND ANGULAR ECLIPSES.
A Annular.
B
1. At A the Earth is at her aphelion, and the Sun, being at his most distant point, will have his least apparent magnitude. At this same time the Moon is in perigee, and appears larger than usual. It therefore, she passes entirely over the Sun's disk, the eclipse being total. 2. At B this order is reversed. The Earth is at her perihelion, and the Moon in exact opposition to the Sun. At this time, when the Sun is at its greatest apparent size, central eclipses occur under these circumstances, the Moon will not be large enough to eclipse the Sun completely, but will leave a ring, apparently around herself, unob- scured. Such eclipses will be annular. **557. What are the effects of a Total Eclipse of the Sun?** The heavens are shrouded in darkness, so that stars and planets are visible; the temperature declines; the 120 PRIMARY ASTRONOMY. animal tribes become agitated; and a general gloom overpreads the landscape. (Thus was the effect of the great eclipse of June, 1866.) 558. When will the next Total Eclipse of the Sun, visible in the United States, occur ? August 7th, 1869. LESSON XXVII. ECLIPSES OF THE MOON. 559. What is the cause of Lunar Eclipses? The Moon's passing through a portion of the Earth's shadow. 560. In what time of the Moon do they always occur ? At Full Moon. 561. Why is this ? Because the Moon is always full when in opposition to the Sun; and the Earth's shadow being in opposition, the Moon must pass through it when full, if at all. 562. Why do all Eclipses of the Moon begin on the East, and pass over Westward, contrary to the course of the Sun's Eclipses ? Because as the Moon revolves eastward, her eastern limb first comes in contact with the Earth's shadow. (By looking the book up south of him, his student will see at once why the eastern limb of the Moon must be first eclipsed, and why she passes over Westward.) 563. Why have we not a Lunar Eclipse at every Full Moon ? PEIARYM ASTRONOMY. 121 For the same reason that we have not a Solar Eclipse at every New Moon; namely, because the Moon's orbit is not in the plane of the ecliptic, where the Earth's shadow lies. NEW AND FULL MOONS WITHOUT ECLIPSES. Shadow below the Earth. Above the Earth's shadow. ECLIPSE Shadow below the Earth. Above the Earth's shadow. (Here it will be seen that the Moon's orbit departs 20° from the plane of the ecliptic, she may pass either above or below the Earth's shadow, and therefore not be eclipsed.) **564. Where must the Moon be in her orbit, at the time of her op-position, in order to be eclipsed?** At or near one of her Nodes. **565. What are her Ecliptic Limits?** About twelve degrees (12° 25' 40"). **566. What is the average Luminosity of the Earth's Shadow ?** About 800,000 miles. (See cut, page 116.) (But this luminosity is only 7,038 miles, according to the Earth's distance from the Sun, see 113, and on.) **567. What is the average breadth at the distance of the Moon?** About 6,000 miles. (This breadth also varies from 3,929 to 6,365 miles.) **568. How long, then, may any Eclipse of the Moon last?** If central, it may last four hours. **569. What is the natural progress of a Lunar Eclipse?** As the Moon enters the Earth's Penumbra, she loses a portion of the Sun's light, and begins to grow pale or dusky, till at length she enters the Umbra, and is wholly eclipsed. **570. Can an Eclipse of the Moon ever be Annular?** A diagram showing a lunar eclipse with a crescent-shaped moon partially covered by a dark ring around it. 11 122 PRIMARY ASTRONOMY. It cannot. 571. Why not? Because the diameter of the Earth's shadow, where the Moon passes it, is always greater than the diameter of the Moon. 572. What is the greatest and least number of Eclipses that can ever occur in one year? There can never be less than two, nor more than seven. 573. What is the most common number? Four. 574. How do astronomers record the EXTENT of Solar and Lunar Eclipses ? By dividing the diameters of the Sun and Moon into twelve equal parts, called Digits, and observing how many of these parts are eclipsed. FIVE DIGITS ECLIPSES. TWELVE DIGITS. 575. How were Eclipses regarded by the Ancients? With amazement and fear; as supernatural events, indicating the displeasure of the gods. 576. What did Columbus make of this Supposition? When the inhabitants of St. Domingo refused to allow him to anchor, in 1502, or to furnish him supplies, he told them the Great Spirit was offended at their conduct, and was about to punish them. In proof, he said the Moon would be darkened that very PRIMARY ASTRONOMY. 123 night, for he knew an eclipse was to occur. The arti- fice led to a speedy supply of his wants. LESSON XXVIII. SATELLITES OF JUPITER, ECLIPSES, KTC. 577. How many Moons has Jupiter? Four. 578. Are they easily seen? They are with a spy-glass or telescope, but not with the naked eye. (See note after 405.) 579. Can we see them at any time? They occasionally be seen with the naked eye; but such occasions and such eyes will rarely be met with.) 579. Who first discovered them ? Galileo, the inventor of the telescope. 580. How are they distinguished? As first, second, third, and fourth, according to their distances from their primary. 581. What is their NAME? AMPLITUDE! They are all little larger than our Moon, except the second, which is a little less. 582. How are they situated as to distance from Jupiter? The first is about the distance of our Moon, and the others respectively about two, three, and five times as far off. COMPARATIVE DISTANCES OF JUPITER'S MOONS.
6th. 6th. 6th.
6th. 6th. 6th.
583. In what time do they revolve about their Primary? From 1 day 18 hours to 17 days; according to their respective distances. 124 PELIMARY ASTRONOMY. Through the fourth satellite is 184,000 miles from Jupiter, or about five times the distance of our Moon, she revolves around her primary in seventeen days. **584. Why do the Moons of Jupiter revolve so rapidly?** In order to counterbalance his powerful centripetal force or attraction, and keep the satellites from falling to his surface. **585. How are the Orbits of these Moons situated?** They are all in or near the plane of his equator. (See representation in the last cut.) **586. How would this place them with respect to the Ecliptic?) As Jupiter's orbit and axis are but slightly inclined, his equator nearly coincides with the ecliptic; and if his satellites revolve near the plane of his equator, they must necessarily be in the ecliptic. **587. How do these Moons usually appear to be situated?** At different distances ; some on one side of the primary, and some on the other. **588. What is their apparent motion?** They seem to oscillate like a pendulum, from their greatest elongation on one side, to their greatest elon- gation on the other. (If we could look down perpendicularly upon the ecliptic, we should see these satellites revolving round Jupiter in a circle, parallel to the ecliptic, which is the plane of their orbit; they merely seem to pass in and out from the plane.) PRIMARY ASTRONOMY. 125 **500. In what directions do they revolve?** From east to west, or in the direction their primary revolves, both upon his axis and in his orbit. (See also the question by the answer in the next cut.) **501. Have they a revolution about their own Axes?** They are supposed to revolve once upon their axes during every revolution around their primary, as is the case with our own satellite. **502. Are Jupiter's Moons always visible?** They are not. Sometimes only one or two can be seen. (See the lower figure in the opposite cut.) **503. Why is this?** Because, as their orbits lie near the plane of Jupiter's orbit, they have to pass his broad shadow, and be totally eclipsed at every revolution. A diagram showing the positions of Jupiter and its moons relative to each other and to Jupiter's shadow. The diagram on the left shows Jupiter's orbit (D) with three moons (A, B, C) in different positions relative to Jupiter's shadow. The diagram on the right shows a closer view of Jupiter's shadow (C) and the three moons (A, B, C) passing through it. The text "ECLIPSES OF JUPITER'S MOONS" is written below the diagrams. (1. In this cut we have a perpendicular view of the orbits of Jupiter's satellites, and the appearance of their shadows on Jupiter's face.) 2. It is proper on the Earth A, the fourth Moon would seem to pass and repeat from time to time, but never more than three times in their respective intervals.) **504. Are there no exceptions to the total eclipse of these Satellites at every revolution?** There is one exception. As the fourth satellite departs about 3° from the plane of Jupiter's orbit, and is quite distant, it sometimes passes above or below the shadow, and escapes eclipse. But such escapes are not frequent. 11* 136 PRIMARY ASTRONOMY. **595. Do these Satellites ever eclipse Jupiter?** Their shadows are often thrown upon his bright disk, and may be seen like small round ink-spots, traversing it from side to side. [For this is called a Solar Eclipse in the preceding note.] **596. What kind of an Eclipse is it upon Jupiter, when the shadow of a Satellite falls upon the primary?** An Eclipse of the Sun, or Solar Eclipse. **597. How many Eclipses, visible from Jupiter, take place every month?** About forty. **598. When a Satellite goes into Jupiter's shadow, what is it called?** The Immersion ; because it is immersed, hid, or buried in the shadow. **599. What is its coming out of the shadow called?** Its Emergence, because it emerges or comes out. **600. Can these Immersions and Emergences always be seen?** They cannot; as the position of the Earth in her orbit is sometimes unfavorable for such observations. A diagram showing the positions of a satellite (A) entering and exiting Jupiter's shadow. (If the Earth were at A in the cut, the immersion, represented at C, would be invisible; and if at B, the emergence at D could not be seen. So also, if the Earth were at Y, neither could be seen; as Saturn and all his satellites would be directly beyond any part of Jupiter's disc.) Immersion and emergence of Jupiter's moon. **601. How does the system of Jupiter and his Satellites be regarded?** As a miniature representation of the Solar System; As a miniature representation of the Solar System;Diagram showing a miniature representation of the Solar System with Jupiter and its moons. PEIARYM ASTRONOMY. 127 and as furnishing triumphant evidence of the truth of the Copernican theory. **602. In what other light may it be viewed?** As a great NATURAL CLOCK, keeping absolute time for the whole world. **603. How is time marked by this system?** By the immersions and emersions of the satellites. **604. What use can we make of the time thus denoted?** We may ascertain the longitude of any place upon the Earth's surface. (1. By long and careful observations upon these satellites, astronomers have been able to determine their distances from the Earth, and to ascertain how long it will take place, at Greenwich Observatory, near London, that they will be seen at their greatest immersion or emergence.) (2. The first satellite was discovered by Halley, and the second satellite to be observed at 1 o'clock at Greenwich, when we find it is occur at 9 o'clock; for instance, for our time. This discovery was made in 1687. The third satellite was discovered in 1750.) In other words, we thus were three hours, or 437 earths, to one time. This was calculated by Sir Isaac Newton, who found that the distance of the moon from us is about 300,000 miles per second. We must therefore allow, then, the amount of 157 for every hour of variation. See Question 27 to 274, page 13. **605. What great discovery was made by observations upon the Eclipses of Jupiter's Moons?** The progressive motion and velocity of light. (The light may be illustrated by again referring to the opposite end. In the year 1671 it was shown that the light from a star which had just risen from behind Jupiter to Jupiter at all, the eclipses took place 8 minutes and 32 seconds later than the same star would have been seen if it had risen directly over Jupiter. This is sufficient evidence of the motion of the progressive motions of light, which he concluded re- quired a velocity of about 186,000 miles per second (see Question 27 to 274).) **606. What is the estimated Velocity of Light?** About 200,000 miles per second (192,987). (98. A.M., W.A., 98 a. f.) The radius of the Earth's orbit be 65 millions of miles, the diameter must be twice that, or 190 millions. Divide 190 millions by 987 million miles per second = 1.93 million miles per second. At this rate, light will pass nearly eight times around the globe at every tick of the clock in less than one minute.) **607. Is there any thing more rapid than Light?** Electricity is supposed to move about one-third faster; or near 300,000,000 miles per second. 128 PRIMARY ASTRONOMY. LESSON XXIX. SATELLITES OF SATURN, HEESCHIL, AND NEPTUNE. 609. How many Moons has Saturn? Eight. (See 429, and cut, page 9.) 609a. Are they similar to the naked eye? They are not ; and can only be seen with telescopes of considerable power. 610. When is the best time to observe them ? When Saturn is at his equinoxes, and his rings nearly invisible. (On January, 1849, the author saw six satellites of Saturn, seated in the adjacent cut. The rings appeared only as a line of light between the two outermost planets, and the middle one was in the plane of the rings.) Satellites of Saturn. 611. What is the Force and Position of their Orbits? They are nearly circular; and all except the eighth revolve in the plane of the rings. (See the above cut.) 612. In what direction do they revolve? From west to east, or with the planet and his rings. 613. What are their Distances? From 125,000 to 3,565,000 miles. COMPARATIVE DISTANCE OF THE MOONS OF SATURN. 614. What are their Periods of Revolution? From 22 hours to 79 days, according to their respective distances. 615. Is any thing known of the Magnitude of Saturn's Moons? A small circular image showing six moons orbiting Saturn. PRIMARY ASTRONOMY. 129 The most distant is the largest, supposed to be about the size of Mars; and the remainder seem to grow smaller, according to their respective distances. **616. Do they suffer any Eclipses?** Very seldom, except when the rings are seen edge-wise. [I. Let the Bee A represent the planet Saturn, and B its ring. D his axis, and R T his plane of his rings. The Sun's rays, which pass through the rings, will revolve around the shadow of the primary, instead of passing through it.] A. At the time of his equinox (see A and B, cuts, page 81), they have passed through each other, and revolved around the Sun (see A and B, cuts, page 81), they must lie in the center of the shadow on the opposite side; and the Moon, receiving them at this time, would appear to be eclipsed by Saturn. But in the eighth month, however, may sometimes escape, on account of his departure from the plane of the ecliptic. **617. Do these Satellites revolve upon their respective Axes?** The eighth, which has been studied more than all the rest, is known to revolve once upon its axis during every periodic revolution; from which it is inferred that they all revolve on their respective axes in the same manner. SATELLITES OF HERSCHEL **618. How many Satellites has the planet HERSCHEL?** Six. (He is supposed allowed to have six, upon the authority of Sir Wm. Herschel, and his son Sir John Herschel.) Only times, however, have ever been seen by any other observer than Sir William Herschel himself. **619. What are their DISTANCES?** From 224,000 to 1,500,000 miles. **620. Their PERIODS TIMES!** From 1 day, 21 hours, to 117 days, according to their respective distances. 130 PRIMARY ASTRONOMY. **621. What remarkable peculiarities do these Satellites exhibit?** Their orbits are nearly perpendicular to the ecliptic, and they revolve **backwards**, or from east to west, contrary to all the other motions of our planetary system. **622. Have we any other information respecting these Moons?** None; except that they revolve in orbits nearly circular, and are described by Dr. Herschel as "the most difficult objects to obtain a sight of of any in our system." SATELLITE OF NEPTUNE **623. When and by whom was this Satellite discovered?** By Mr. Lassell, of Liverpool, England, October 10, 1846. **624. What is its Distance from Neptune?** About 250,000 miles, or near the distance of our Moon. **625. What is its Periodic Time?** Five days and twenty-one hours. (We have here another illustration of the great law of planetary motion explained by Newton. It is evident that no body can move with such a velocity as to keep a satellite at the distance of our Moon, from falling to its surface, must resume some five times its velocity before it reaches the Earth.) The conjectural and conjectural forces must be balanced in all cases.) **626. Are there any suspicions that Neptune has other Satellites?** Professor Bond, of Cambridge, Mass., states that he has at times been quite confident of seeing a second satellite. **627. What general fact has been arrived at with respect to the Secondary Planets?** That the laws of gravitation and planetary motion, discovered by Newton and Kepler, extend to and prevail among all the secondaries. PRIMARY ASTRONOMY. 131 LESSON XXX. of Tides. **628. What are Tides?** The alternate rising and falling of the waters of the ocean, at regular intervals. **629. What is meant by High and Low Tides?** When the waters are rising, it is called *Flood Tide*; and when they are falling, *Ebb Tide*. **630. What is meant by High and Low Tide?** When the waters have reached the highest and lowest points to which they usually go. **631. Is this elevation and depression uniform as to its amount?** It is not; some high tides are much higher and others much lower than the average elevation. **632. What are these extraordinary High Tides called?** Spring Tides. **633. What are the remarkably low Flood Tides called?** Neap Tides. **634. How often does the Tide ebb and flow?** Twice every day. (Thus in, we have one ebb and two flood tides every twenty-four hours, nearly.) **635. What is the cause of the Tides?** The attraction of the Sun and Moon upon the waters of the ocean. (In this figure, the Earth is represented as surrounded by water, in a state of rest or equilibrium as it would be if it were not acted upon by any force.) 132 **636. What should we suppose would be the natural effect of the Moon's attraction?** To produce a single tide-wave on the side of the Earth toward the Moon. 132 PRIMARY ASTEONOMY. In this case, the Moon is shown at a distance above the Earth, one tide-wave. not interfering with the other, and revolving round the Earth in the same time as it does at A. But as the Moon makes her apparent westward revolution, according to the motion of the Earth, she will appear to move on the side of the Earth towards the Moon, giving us but one half of her surface visible; and then, when she has made her eastward revolution, we shall have two halves of her surface visible; wherefore it is known that we have two curves. The third idea says Dr. Hume, "that a subject on which men differ so much, as they do upon this question, can never be right." Now, by the motion of the Earth, the Moon appears to move on the side of the Earth towards the Moon, giving us but one half of her surface visible; and then, when she has made her eastward revolution, we shall have two halves of her surface visible; wherefore it is known that we have two curves. 637. Does the Moon cause only one Tide-Wave upon the globe? She produces two at the same time; one nearly under her, and the other on the opposite side of the Earth. (For this tide we have a representation of the tide-waves as they would appear to an observer standing on the surface of the Earth; but this is only a representation; for it is impossible for any observer to see both sides of the Earth at once.) The reason why Dr. Hume's principle is not universally accepted, while it is admitted by all who know anything about this subject, is because he does not consider how great is the difference between the height of tide at one place and another. For instance, if we look at a map of England, we shall find that at some places there is a high tide at 5 o'clock in the morning, and at others there is none at all; and yet these places are very near together. This difference arises from the fact that at some places there is a high tide at 5 o'clock in the morning, and at others there is none at all; and yet these places are very near together. This difference arises from the fact that at some places there is a high tide at 5 o'clock in the morning, and at others there is none at all; and yet these places are very near together. 638. In what direction do these Tide-Waves move? From east to west, or as the Moon appears daily to revolve. (These four tides, two high and two low, increase the ocean from coast to coast every twenty-four hours; and decrease every twenty-four hours.) 639. How do you account for the Tide-Waves on the side of the Earth opposite the Moon? It is due principally to the difference between the Moon's attraction on different sides of the Earth. (1) The student may do well to review the subject of gravitation, 103, 200, and 397. (2) The student may also do well to review Newton's Principia Mathematica Philosophiae Naturalis, Book I., Prop. 1., where he will find by the rule, Question 296, the difference in her attraction on the side of the Earth towards her, and on the opposite side will be about 1%. The attention being thus drawn to this point, it will be seen that if we suppose that there were no other causes than those mentioned above (the attraction of the Sun and Moon), it would result in separating these three portions of the globe giving us an ocean equal to that which exists now. 640. What other influence helps to produce the Tide-Waves oppo- site the Moon? The revolution of the Earth around the common cen- PRIMARY ASTRONOMY. 133 ter of gravity between her and the Moon during every lunation. **641. What is meant by the Center of Gravity?** The point between them where they would exactly balance each other if connected by a rod, and poised upon a fulcrum. A diagram showing the center of gravity between the Earth and Moon. Earth Moon **642. Where is the Center of Gravity between the Earth and Moon situated?** About 6000 miles from the Earth's center. (This point is represented as B in the above cut, and also in the line following. We give this name as the answer to the question, on the authority of Farguson, see his Note on the "Principles of Mechanics".) **643. How does the Revolution of the Earth and Moon around the common Center of Gravity between them, help to produce a Tide-Wave opposite the Moon?) By generating an increased centrifugal force on that side of the Earth. CAUSES OF EACH TIDE OPPOSITE THE MOON. [1. The point A represents the center of gravity between the Earth and Moon; and so is it in this point which makes the regular course of the Earth's orbit, it is represented by C. This point is situated at about 6000 miles from the center of E. Now the law of gravitation requires that while both the Moon and Earth revolve round their common center of gravity, they will always be in such a position between them, or around the point A. This would give the Earth a third resistance, in addition to its own weight, to any motion towards or away from A, and round around the center of gravity, and thus answers her direction.] In this case we have two centers of gravity, one at B, representing the Earth's center of gravity, and then here to make the tide-wave opposite the Moon. But as this motion is slow, contrary to what we find actually occurring round around the Earth, the centrifugal force could not be greatly augmented by such a cause.] 134 PRIMARY ASTRONOMY. **644. Which attracts the Earth most powerfully, as a whole, the Sun or the Moon?** The Sun. **645. Which contributes most to the production of the Tides?** The Moon. **646. Why is this?** On account of her nearness, which makes a great difference in her attraction on different sides of the Earth. (1. It must be remembered that tides are the result, not of the attraction of the Sun and Moon upon our globe as a whole, but of that difference in their attracting forces, existing between the two hemispheres of the Earth.) 2. The difference in the distance of two sides of the Earth from the Moon is $\frac{1}{3}$th of its diameter, while that between two opposite points on the same side is only 72,000 miles, or 59,000,000 = 8,600 = 1.175. **647. What is the comparative influence of the Sun and Moon in causing Tides?** As one to three ; the Moon contributing three times as much as the Sun. **648. Does Flood Tide occur at the same hour each successive day?** It does not. **649. What variation is there in?** It happens about 50 minutes later. **650. Why is this?** Because the Moon, which causes the tides, is revolving eastward, and comes to the meridian 50 minutes later each successive day. (As the two tide-waves are opposite each other, as if one next the Moon is later, an hour after it has passed over any point on the Earth's surface.) **651. What is the Time between two successive High Tides?** Twelve hours and twenty-five minutes. **652. Is it Flood Tide when the Moon is on the Meridian?** It is not. **653. Why is it not?** **PRIMARY ASTRONOMY.** 135 Because the waters do not at once yield to the impulse of the Moon's attraction, but continue to rise after she has passed over. **654. How far is the Flood Tide behind the Moon?** In the open sea it is generally about three hours, or 45° behind. (On the coast, however, the highest point of the wave is at A, or 45° east of the meridian; and the corresponding wave on the opposite shore is at B, or 45° west of the meridian.) **655. Do any other causes affect the time of High Tide?** It is affected by winds, and by the situation of different places. (If a place is situated on a large bay, with both a narrow opening into the sea, and the tide running in and out, such a bay has its tide up through a narrow gate. Hence it is not usually high tide at New York till eight or nine hours after the Moon has passed the meridian.) **LESSON XXXI.** OF SPENDING AND NEAR TIDES. **656. What is the cause of the Spring Tides?** The combined influence of the Sun and Moon. A diagram showing two circles representing the Earth and Moon orbiting around each other. The inner circle represents the Earth with a line indicating the position of the Sun (A) and Moon (B). The outer circle represents the Earth's surface with a line indicating the position of a point on the surface (C). The diagram shows that when the Sun and Moon are on opposite sides of the Earth, they exert their combined gravitational pull on the oceans, causing a high tide (D). When they are on the same side of the Earth, they exert their combined gravitational pull on the oceans in opposite directions, causing low tides (E). The diagram also shows that when the Sun and Moon are aligned with respect to each other, they exert their combined gravitational pull on the oceans in a straight line, causing a spring tide (F). The diagram also shows that when the Sun and Moon are aligned with respect to each other, they exert their combined gravitational pull on the oceans in a straight line, causing a neap tide (G). CAUSE OF SPRING TIDES. Here are some notes about this lesson: [1] Here are some notes about this lesson: - The Sun and Moon being in conjunction, unite their forces to produce an ex- 136 PRIMARY ASTRONOMY. tracendency tide. The same effect follows when they are in opposition; so that we have two tides in every day, one at the rising and another at the setting of the Sun. 2. If the tide-waves A and C are one-corded alike at the Moon's quadrature than usual, the wave D will be very low. **657. What is the cause of the New Tides ?** The Sun and Moon acting against each other. 1. On the right side of the cut the Sun and Moon are in conjunction, and therefore have no action on each other. 2. At the First Quarter they are opposite, and therefore have their greatest action on each other. 3. At the Full Moon they are in opposition, and therefore have their least action on each other. 4. At the Third Quarter they are in quadrature, and therefore have their least action on each other. 5. At the Last Quarter they are in quadrature, and therefore have their least action on each other. **658. Are all Spring Tides alike as to their elevation?** They are not : some are much higher than others. **659. What is the cause of this variation?** The variation in the distances of the Sun and Moon. VARIATIONS IN THE SPRING TIDES. A diagram showing variations in spring tides. 1. A The Earth is in aphelion, and the Moon is in egress, and so both the Sun and Moon are at their greatest distance; the Earth is least affected by their attraction, and the spring tides are proportionally low. A circular diagram with text "SPRING AND FULL TIDES" around its edge. A circular diagram with text "CONDUCTIVE TIDE" around its edge. A circular diagram with text "EFFECTIVE TIDE" around its edge. PRIMARY ASTRONOMY. 137 S. At B the Earth is in perigee, and the Moon in perigee; so that both the Sun and Moon exert their greatest influence upon our globe, and the spring tides are highest, as shown in the figures. In this case the Sun and Moon are in conjunction, and the visibility at the distance of the Sun and Moon shows variation in the spring tides. 660. What other general variation of the Tides has been noticed? That in Winter and Summer every alternate tide is higher than the intermediate one. 661. What is the cause of this? It is owing to the greater direct- line inclination of Sun and Moon. At the time of the equinoxes, the Sun being over the equator, and the Moon within 5° of it, the crescent tides will be very high; but as the Sun and Moon descend north of A, they will be inclined to each other at a greater angle than when they are opposite one in the north, as at C. If the declination of the Sun and Moon were equal, no such effect would be reversed. This analogy may be still further illustrated by the following diagram: A diagram showing different positions of the Sun (A) and Moon (B) relative to each other during various times of year. ALTERTATE HIGH AND LOW TIDES. 1. Let us take A represent the plane of the ecliptic, and B the equatorial plane. The Sun and Moon are then in opposition with each other, so that the tide following about three hours after the Sun and Moon, will be higher than those preceding them. 2. On the 04th of December, the Sun and Moon being over the meridian tropic, the tide following about three hours after them will be lower than those preceding them. This is shown by D. 3. On the 04th of January, the Sun and Moon being over the equator, we have shown about three clock A.M.; while at this north this order will be reversed. It is on this account that in high latitudes, where there is little difference between the inter- val of tides in Summer exceeding the morning tides and late morning tides in Winter exceeding those of evening. 662. To what other variations are these Tides subject? They are often hastened or retarded, and increased or diminished, by strong winds. 663. What is the cause of the great variation of the same Tides in different places? In some places the tide-wave is pressed into nar- 138 PRIMARY ASTRONOMY. row bays or channels, which makes it rise much higher than at other places. [The average elevation of the tide at several points on our coast is as follows: Cumberland, head of the Bay of Fundy. 71 fath. Boston. 60 New Haven. 5 New York. 4 Charleston, S. C. 3] 664. Have Inland Seas and Lakes any Tides ? None that are perceptible. 665. Why is this ? Because they are too small, compared with the whole surface of the globe, to be sensibly affected by the attraction of the Sun and Moon. 666. How is the subject of the Tides generally regarded ? It is difficult one to be fully understood and explained. [La Place, the great French mathematician and astronomer, pronounced it one of the most difficult problems in the whole range of celestial mechanics.] 667. Is it likely that the Atmosphere has Tides as well as the Waters? It is probable that it has, though we have no means as yet for definitely ascertaining the fact. LESSON XXXII. OF COEKS. 668. What are Coeks ? They are a singular class of bodies, belonging to the Solar System, distinguished for their long trains of light, their various shapes, and the great eccentricity of their orbits. 669. From what is the term Coek derived ? From the Greek word coma, which signifies hair; on PRIMARY ASTRONOMY. 139 account of the bearded or hairy appearance of some comets. **670. Are Comets Self-Luminous, or Opake?** They are known to be opake, from the fact that they sometimes exhibit phases, which show that they shine only on one side. (See N.) **671. How do Comets usually distinguished one from another ?** By the date of their appearance, or by a specific name given to them. (Thus we have the comets of 1583, 1680, 1811, &c.; and also Halley's Comet, Eucte's Comet, Bode's Comet, &c.) **672. What is the principle pulparum of a Comet?** The nucleus, the envelope, the tail. **673. What is the Nucleus?** It is the most dense or solid portion, sometimes called the head. (See N in the cut.) **674. What is the Envelope?** A thin misty wrapper or covering surrounding the nucleus. (See E in the cut.) **675. What is the Tail of a Comet?** An expansion or elongation of the envelope, extending off in one direction from the nucleus. (See T in the cut.) **676. Have all Comets these three parts ?** No--not even the head or tail; the central part of any body about which matter is collected. The plural of this term is enucleate. † Enucleate, a wrapper or husking covering. A diagram showing a comet with its nucleus, envelope, and tail. 140 PRIMAEY ASTRONOMY. They have not. Some have a nucleus and no envelope; some no perceptible nucleus; and others a nucleus and envel- ope, but no tail. [A comet that appeared in 1765 had simply an envelope, without any nucleus. The comet of 1835 was a comet in another of this kind. See cut, page 141.] The comets of 1765 and 1835 are as yet unknown, and hence we cannot say whether they are rare exceptions to the general character of comets.] **677. How are the Tails of Comets usually situated?** They extend in a direct opposite direction to the Sun. [This is true, not only when going toward the Sun, but also when going from him. See cut upon the opposite page.] **678. What is its usual course?** They assume a great va- riety of shapes: some ap- pearing like an enormous fan; others like a long sword or saber; but all curved more or less, and conceal toward the regions from which they come. The comets of 1714 represented in this illustration show how the tails may be formed by the inclination of the orbit of Mars from the Sun; but this is not always the case. In the comets of 1765 and 1835, the tails were seen to curve in the same direction. Each of these tails was about 60° wide, and from 360° to 450° long. In the comets of 1765 and 1835, the suns were seen in the vening sectors as dark as the rest of the firmament, the stars shining in them. When comets were seen to be at the end of their visible body, they were seen as shining and magnificent, after beyond description. Milton's "Paradise Lost" contains this passage: **679. What is the Form of their Orbits?** They are generally very elliptical. [The form of a comet's orbit is represented on the opposite page.] A diagram showing a comet with a nucleus and an envelope, but no tail. cutout of 1765 A diagram showing a comet with a nucleus and an envelope, but no tail. cutout of 1765 A diagram showing a comet with a nucleus and an envelope, but no tail. cutout of 1765 A diagram showing a comet with a nucleus and an envelope, but no tail. cutout of 1765 PEIARYM ASTRONOMY. 141 ORBIT OF A COMET. [Here it will be seen that the orbit is very eccentric, that the perihelion point is very near the Sun, and the aphelion point very remote. See cols. pages 20 and 30.] 680. What effect has this Eccentricity upon the motion of Comets? It makes a great difference in their velocity in different parts of their orbits. (See first cut, page 70, and Note.) 681. What can you say of their Movements when near the Sun? When passing their perihelion their velocity is sometimes incredibly rapid. (1.) The comet which appeared in 1473 described an arc of 180° to the heavens in a single day. Thus of 1800 moved at the rate of 18000 miles an hour! (2.) The comet which appeared in 1531 describes its orbit with a velocity inconsiderable; but we should observe that the speed therein, which they pass is not slower than the speed of our gales, but very nearly zero (or nil). 682. What effect has the change of position upon their appearance? Their tails usually increase both in length and breadth as they approach the Sun, and contract as they recede from him. This elongation and expansion, however, may be merely apparent. As the comet approaches the Sun, it draws nearer to us, and therefore appears larger to us than it does when it is away from us. This is known to be true of the Moon; but when within the orbit of Jupiter, or about the Sun, we often have a false impression that it is much larger than it really is. This is owing to the following cause. By observing the tail end, we shall find only one or two nights constant extension of its appearance; but on other occasions it may appear much larger than its neighborhood, then when five stars at a distance. 683. What other peculiarities has been noticed! The tail of the comet of 1511 is said to have expanded suddenly to a great distance. 142 PRIMARY ASTEONOMY. QUART COAST OF 1811. **Q64. How are the Orbits of Comets situated with respect to the Ecliptic?** They approach the Sun from every point of the heavens, all around and on both sides of the ecliptic, and having passed their perihelions, plunge off again into space, and see last their ways up to the Sun, and then return to the same place in the heavens, and round about the universe, and having passed around the Sun, resseded for above all human visio. **Q65. Is any thing known of their Temperatures?** Only that some approach very near the Sun, and must therefore become very hot. (The comet of 1600 came within 128,000 miles of the Sun's surface, and must have received from it a heat equal to that which would be given off from the Sun--a total tone than 3000 times greater than that of red-hot iron.) **Q66. What can be said of the Size of Comets?** Their nuclei or heads are comparatively small, being only from 33 to 2,000 miles in diameter. Their tails are often of enormous length. (1. The comet of 371 B.C. (pago 139) had a tail 485 kgs, covering one-third part of the sky. The comet of 1811 was 250 million miles long. 2. The comet of 1680 was 700 in length, estimated at 100 millions of miles. Though this is a very large number, it is not so great as that of the comet of 1811. 3. In 341 B.C. comet appeared which was 167 kgs. Its tail next not all risen when its head first reached the middle of the sky. 4. The comet of 242 A.D. was 605 or 130,000,000 miles in length.) PRIMARY ASTRONOMY. 143 GREAT COURT OF 1843. **Q87. Is any thing known of the Physical Nature of Comets?** They are known to be exceedingly light vapor or gas. **Q88. How is this known?** From the fact that the fixed stars have been seen through their denser portions. **Q89. Are they subject to the Law of Gravitation?** They are; but are so light as to have no sensible effect upon the planets. (1) While Jupiter and Saturn revolved and delayed comets for months in their periodic course, the Sun did not power its turn to hasten the time of the planets for a single hour. The comet of 1719 got entangled, by attraction, round the Moon of Jupiter, and was kept in that position until the Sun's turn had passed, and not scarcely affect Jupiter as did the Moon. This shows conclusively that the relative motion of the planets is not affected by their mutual gravitation. 2. That the laws that revolve about the Sun, is a sufficient proof of their being sub- ject to the law of gravitation. **Q90. What is known of the Periodic Times of Comets?** Only four or five have been ascertained; and these vary from 24 to 570 years. (The following periodic revolutions have been fully determined :) Eckel's Comet ..... 31 years. Rudolph's Comet ..... 6 years. Haley's comet ..... 76 years. Cassini's Comet ..... 12 years. The next return of this last will be in the year 259.] A diagram showing a comet with a tail extending away from the Sun. 144 PRIMARY ASTRONOMY. **601. Are all Comets supposed to revolve continually around the Sun?** Professor Nichol and Sir John Herschel are of opinion that the greater number visit our system but once, and then fly off in nearly straight lines, till they pass the center of attraction between the Solar System and the Fixed Stars, and go to revolve around other Suns in the far-distant heavens. **602. What can we say of the Distant to which many Comets go?** In some cases it must be immense, from the time they are gone, and the rapidity of their motions. (L. The orbit of Kepler's comet is wide enough to contain the whole solar system, while that of Halley's extends but a short distance from the Sun. The diameter of Halley's comet is 380 miles, and its orbit is 75 million miles beyond the orbit of Neptune. But this is only a fraction of its distance.) E Though the distance to which comets may travel must be very great, still their bounds are not yet known. For were they to pass the point where gravitation turns the other way, they would return to the Sun, and so all do return, after their "long travel of years." It is impossible to conceive this effect of so almost infinite a distance from the Sun (and the Fixed Sun). T Therefore we find either orbit of a comet represented on the second run on page 30. The aphelion point is represented as only half way to the Fixed Sun. **603. How were Comets regarded by the ancients?** As harbingers of famine, pestilence, war, and other dire calamities. (The comet of 1611 was regarded by the ignorant as the precursor of the war that was beginning in the following year.) The same view was entertained by those who believed in astrology. The judgment of judgment was at hand; and thus they were appointed messengers of Divine wrath, hastening to burn up the world. This was the origin of superstition respecting comets. **604. What other fears have been entertained with regard to Comets?** A diagram showing orbits of several comets. HALLEY'S COMET HALLEY'S TEETH HALLEY'S TEETH HALLEY'S TEETH PRIMARY ASTRONOMY. 145 That they might come into collision with our globe, and either dash it to pieces, or burn every thing from its surface. 693. Is there really any danger of collision? Not at all! the thing is next to impossible. (1. It has been determined, upon mathematical principles, and after the most ex- tensive researches, that the earth is not likely to be struck by comets; there is only one en- viable case in which one can produce a collision between the two bodies.) 2. When we consider the order and harmony that prevail throughout the planetary system, it is evident that no such collision could ever take place. The planets move in paths the planets in their courses, marks also the pathing of every comet, it is not easy to imagine how a body so small as a comet could possibly strike a planet without causing collision with the planets. It would appear a waste of design and perfection in the mechanical arrangement of the universe, if it were possible for a comet to strike a planet. 694. Would it be destructive if a collision were actually to take place? Probably not. Comets are generally too light even to penetrate our atmosphere to the Earth's surface. (1. The sun is to us what the waters are to fish. Some fish swim around in the deeps of the ocean, while others swim near the surface. The sun is like this, while men and beasts are on the "shallow" that crowd around on the bottom. Now suppose that some one were to throw a stone into the water, it would cause ripples to spread over the earth, then there is a limit for how far waves driven down by wind will go before they reach the bottom.) 2. In former times, ancient men thought that, in the event of a collision, not a particle of the comet would reach the earth--that the portions encountered by her would be destroyed by friction, and that she would be left with nothing but a few fragments of hills; or a mere insignificant shell than was ever before observed; the phenomena of shooting stars, or meteoric showers.) LESSON XXXIII. THE SUN. 695. How is the Sun distinguished? As the great center of the Solar System, the fountain of light and heat. (See also 131.) 696. What names did the ancients give to the Sun? The Romans called him Sol, and the Greeks Helios. (See Notes to 126 and 305.) A page from a book about astronomy. 17 146 PRIMARY ASTRONOMY. **609. What did they suppose him to be?** A vast globe of fire. In my youth I entertained this opinion which obtains among the ancients with respect to the Sun. It has even been held by some modern astronomers, among whom it is maintained and professed by Newton. This opinion, however, is now almost uni- versally rejected, and the Sun is considered as a body of immense heat, which is com- mited from him but is produced by the centric of the rays with other substances; and as such he is called a "fire-body" (or "fire-star") by some philosophers. **700. What is his Magnitude?** He is 88,000 miles in diameter. (1. The vast magnitude of the Sun has been long known to the ancients, either by their observation of its rising or setting or by other obvious larger phenomena, such as eclipses, which are the longest terms. Now if we consider the distance of the Sun's diameter over 100 feet, it will be seen that it must be at the distance of 58 million of miles at least. For instance, if we take the Sun's center and extend a line of sun rays to the Earth, it will be found that the rate of suns in an hour, would be about 300 miles per second. Hence, to traverse one revolution of the Sun's diameter, at the same rate of speed, would require sixty-four years. (2. The circumference of the Sun would take 1280 miles each way. If laid side by side, to reach across the diameter of the Sun: 88,000 = 7,012 on E., 7,012 on W., would be 140,244 miles.) **701. What is his Magnitude or mass as compared with our globe?** He is equal to 1,400,000 such worlds. (1. For comparative magnitudes of the Sun and planets see, e.g., page 43. The Sun's diameter is about 93 times that of the Earth's diameter; and its mass is in proportion to the diameters, but not to the cubes of their diameters. See Note after §71.) **702. How does the Sun compare, as to size, with the rest of the system?** He is 500 times larger than all the other bodies of the system put together. (See note below) In fact he is more than 100 times larger than any planet in our solar system. **703. How does he compare with the size of the Moon's orbit?** If his center occupied the position of the Earth, he would fill the whole orbit of the Moon, and extend more than 200,000 miles beyond it in every direction. PRIMARY ASTRONOMY. 147 (The mean distance of the Moon from the Earth's center is 58,000 miles; consequently the diameter of its orbit is 116,000 miles; its radius is 49,000. Subtract this from 58,000, the distance of the Moon from the Earth's center, we have 8,000 miles left, or 32,000 miles on each side, be- yond which the Sun is situated.) **704. How does the Sun appear through a Telescope?** Like a vast globe of fire, with dark spots here and there upon its surface. **705. What is the NUMBER of these Spots?** It varies at different times from two or three to fifty. **706. How does the TELESCOPIC VIEW of the Sun differ from the ordinary view?** The power of the instrument through which we see the Sun is so great that we shall reveal much more than our eyes alone can show us. In September, 1766, during the expedition to America under the command of Captain Cook, who was then in charge of the ship "Endeavour," several spots were distinctly visible, and most of them were destroyed; but in the 7th of October, 1769, when the ship was again in sight of land, they were visible, though some of the telescope was not perfect. The difference between them is very favorable. A spot is a sun of different size and color from other parts of the Sun. It may be seen by looking through a telescope at a solar disk, which has been made to shine like a bright star. In some cases (as in 1843) spots have been observed by Mr. Bessel and Mr. Cawell, of Brown University; the heat being so great that it would destroy the instrument, and sometimes the eye of the observer is immediately injured! **706. Do these SPOTS appear stationary, or in motion?** They appear to pass over the Sun from left to right in about 134 days. **707. What has been inferred from this fact?** That the Sun revolves on his axis, from west to east, or in the direction of all the planets, every 254 days (25 d. h. 10). (1.) This is true of all observed or real revolutions. Its apparent or periodic revolu- tions require 27 days; 28 hours; but it is much more probable that no continuous revolution upon his axis as the Earth has advanced in her orbit in 254 days. Let us represent A diagram showing a circular diagram labeled "THE SUN AND THE MOON'S ORBIT." The diagram includes concentric circles representing different distances from the center to the edge of a circle. The innermost circle represents a point near the center of the diagram. The next circle outwards represents a distance slightly greater than this point. The third circle outwards represents a distance slightly greater than this second point. The fourth circle outwards represents a distance slightly greater than this third point. The fifth circle outwards represents a distance slightly greater than this fourth point. The sixth circle outwards represents a distance slightly greater than this fifth point. The seventh circle outwards represents a distance slightly greater than this sixth point. The eighth circle outwards represents a distance slightly greater than this seventh point. The ninth circle outwards represents a distance slightly greater than this eighth point. The tenth circle outwards represents a distance slightly greater than this ninth point. 148 **PRIMARY ASTRONOMY.** The Sun and the Earth in her sidereal and synodic revolutions of the Sun, are always at the same distance, when seen upon the disk of the Sun E. B. The Sun revolves in the diurnal circle, in 24 hours, or 360°. C. The Earth revolves in the diurnal circle, in 24 hours, or 360°. D. The Earth revolves in the sidereal revolu- tionary circle, in 365 days, or 360°. E. Thus the Sun appears to move in the diurnal circle, A. During these 24 days, 10 hours, but only during 12 hours, because he is in his orbit only 50%, or nearly so. F. The Earth moves in its orbit during two days for the spot at B to go directly toward the Earth, as shown at D. This line is a periodic revolution. It con- tinues until the spot at B has reached the Sun upon his axis, and about 177° east. **708. Where are these Spots situated ?** They are usually on each side of the Sun's equator, and within a zone of 60°. **709. How is the Sun's Axis situated with respect to the Ecliptic?** It is inclined toward it 7° 20'. **710. How near this inclination ascertained?** By observing changes in the direction of the solar spots, at different seasons of the year. A diagram showing various directions of solar spots. A. VARIOUS DIRECTIONS OF THE SOLAR SPOTS. B. C. D. E. F. **1. Let E F represent the axis of the ecliptic.** March the spot describes a curve, which is convex to the south, as shown at A. In June they cross the Sun's disk in nearly straight lines but inclined upward. In September they curve again through its center, and then describe a curve convex to the north, inclining down- ward. (See figures B and D show the inclination of the Sun's axis.) **711. How does this prove that the Sun's Axis is inclined?** If the Sun's axis were perpendicular to the ecliptic, the spots would revolve in circles parallel to the eclip- tical plane. PRIMARY ASTRONOMY. 149 tie, and apparently in straight lines; whereas the inclination of his axis would give the spots precisely these motions during the year. (This subject will be fully understood by consulting the following figure, in con- nection with the accompanying table.) SOLAR SPOTS OBSERVED FROM DIFFERENT POINTS. DEL TURE 1. If the Sun's axis were at right angles to the ecliptic, his equator, and the spots upon it, would all lie in straight lines parallel to the ecliptic, and would appear to move even their disk in straight lines, from all parts of the Earth's orbit, throughout the year. In fact, this is exactly what happens; but we have seen that the Sun is not so inclined, there being a difference of about 7° between the line of his North Pole and the line of his Equator; and the spots seem to pass over in straight lines, as represented in A. In the opposite case (B), they would appear to move in curves, the reverse of what they were six months before. The curve shown in B is drawn on a scale much reduced, and the spots again move in curve lines, the reverse of what they were six months before. The curve shown in C is drawn on a scale still further reduced; and the spots cross in straight lines, as indicated, or shown in the opposite figure (D). 712. What is the Size of the Solar Spots? They are of various sizes and forms, some having been estimated at 50,000 miles across. 713. What is their general appearance? They are darkest in the middle, and are shaded off at the edges by a sort of penumbra. 714. Do they appear of the same size throughout their whole course? They appear narrow when first seen on the Sun's eastern limb; expand gradually till they reach the center; and then contract till they pass off on the west. 715. What is the cause of this apparent expansion and contraction? 15* 150 PRIMARY ASTRONOMY. It is because the spots are on the surface of a globe, and are seen partly edgewise, except when near the Sun's center. **716. Is their rate of motion across the Sun uniform?** It is not; but is accelerated from the eastern limb to the center, and retarded from the center to the western edge. **717. What are these Spots supposed to be?** They are generally thought to be openings through the luminous atmosphere of the Sun. (Some have thought them to be the tops of mountains, laid bare by tides, or other fluctuations of the solar atmosphere.) **718. What is the prevailing opinion in regard to the nature of the Sun?** That his body is **opaque**, and that his light proceeds from a luminous atmosphere that surrounds him. THE ZODIACAL LIGHT. **719. What is the Zodiacal Light?** It is a faint nebulous light, resembling the tail of a comet, sometimes seen in the neighborhood of the Sun. **720. At what time may it be seen?** Just after sundown or before sunrise in March, April, October, and November. **721. What is its Appearance?** It is quite faint, hardly distinguishable from ordinary twilight. **722. What is its Force and Position?** It has the form of a pyramidal, with its base toward * Zo-m'ac-al.* **PRIMARY ASTRONOMY.** 151 the Sun, and inclines a little toward the horizon. **723. How far does it extend from the horizon?** From forty to ninety degrees. **724. What is its width at the base?** It varies from eight to thirty degrees. **725. How is this substance situated with respect to the Sun's equator?** Its major axis is at right angles with the axis of the Sun. [Let A represent the Sun, B B his axis, and C C his equator, and D D the thickness of this circular atmosphere.] **726. What is this Light supposed to be?** Some have thought it an extension of the Sun's atmosphere, while others have regarded it as nebulous vapor, of the nature of comets. **727. Is it thought to be at rest, or in revolution?** It is believed to revolve with the Sun on his axis, and to be flattened out as we see it, by that revo- lution. [See the effect of the revolution of rotating bodies upon their figures illustrated by this form supposed to be that of a cone, of which the above is an edge-on view.] **728. What other motion has the Sun besides that on its Axis?]** A diagram showing a cone-like shape representing the Sun's atmosphere, with a line extending from its center to its base, indicating rotation around an axis. The diagram also shows a smaller circle within the larger one, representing the Sun's equator. FORM, EXTENT, ETC., OF THE SOMATICAL LIGHT. 152 **PRIMARY ASTRONOMY.** A slight periodical revolution around the common center of gravity of the Solar System. (This notion resembles that of the Earth, illustrated on page 35, to which the student may refer.) The Sun revolves from his apparent fixed position to the amount of more than twice his diameter.) **729. Has he still another motion?** He is found to be revolving, with all his refines of planets and comets, in a very orbit, around some distant and unknown center. (One of these motions represented in the second or on page 36, in which the student will see well to turn.) The first of these motions represented in the second or on page 36, in which the student will see well to turn. This is called the revolution of the Sun, has recently assumed as a discovery that the star distant, one of the seven stars, is the center around which the Sun and Solar System are revolving. A second motion of the Sun. **Secundary revolution around primary: primaries around the Sun; and the Sun revolve round some other center; and so on, we come to the conclusion that there are three centers in this system.** (See page 38.) **730. What is the estimated Velocity of the Sun and Solar Sys- tem?** About 88,000 miles per hour, or 8 miles per second. **731. What is the supposed Period of Revolution ?** About 18,000,000 years. (If this be correct, he has only passed over one 3,000th part of his orbit or about twenty-four million years. This is a very long time; but it is small compared with the whole, as to be hardly distinguishable from a straight line.) LESSON XXXIV. GENERAL REMARKS UPON THE SOLAR SYSTEM. **732. How did the Solar System originate?** The Scriptures say that "in the beginning God created the heaven and the earth" (Gen. i. 1). **733. Describe the Neptunian Theory of Creation.** It teaches that the elements or matter, of which the system is composed, was originally a vast cloud of vapor or mist, which has been condensed and formed PRIMARY ASTRONOMY. 153 into Sun and planets, during a vast period of time, by the simple law of gravitation. **734. Is this theory correct, or even probable?** It is not. **735. What objections can be urged against it?** 1. It would make the creation, mentioned by Moses, a mere organization, instead of composition of pre-existing materials. The Bible says that "the worlds were framed by the word of God," so that things which are seen were not made of things which do appear"—Hob. xi. 3. 2. If it allows that God created the materials of the system at all, it throws the period of their creation back indefinitely into eternity, and substitutes gravitation for the direct agency of the Almighty. **736. What led to the discovery of the first Asteroid?** The suspicion that there was a large planet in the space between the orbits of Mars and Jupiter. **737. What singular opinion has been entertained in regard to these bodies?** Doctor Ohlers, of Bremen, Germany, was of opinion that they were originally one large planet, that had been broken into fragments by explosion, or by coming in collision with some other body. **738. What says Dr. Herschel of this theory?** He says it may serve as a specimen of the dreams in which astronomers, like other speculators, occasionally and harmlessly indulge. **739. Why is this theory improbable?** Because such an explosion or collision would be at variance with the harmony and order that every where prevail throughout the planetary regions. 154 PRIMARY ASTRONOMY. **740. Is it probable that the planets are inhabited by rational beings?** It is. **741. Have we any direct evidence of this fact?** We have not: no inhabitants have ever been seen, heard, or heard from, upon any of the planets. **742. Is this any proof that they do not exist there?** It is not. We must not conclude that a newly discovered island is uninhabited, because it is so far distant than first seen that we cannot see or hear the people. **743. Why is it believed that the planets are inhabited?** Because they are globes like our Earth; and have atmospheres, seasons, days and nights, satellites, &c., which would be unnecessary if they were uninhabited. (Our own primary world has only one small one inhabited, would be like a fisher's basket; but the other worlds are large enough to contain all dimensions, and after having furnished them all with resolutions, mirrors, lamps, &c., to make each into one of the worlds, and leave the remaining universe uncon- cipated.) **744. Do not the extremes of Cold and Heat upon the several planets furnish their being in evidence?) By no means. The Creator adapts every creature to the climate where he assigns it to dwell. (Fish have cold blood; and some animals may be driven off when they cannot find food; and when they cannot find food will come to life. The lion and polar bear are each suited to their respective abodes, and can live in climates much colder than those of the human race; but what of the planet? Cannot He, who adapted the three Hebrews to their fiery furnace (Dan. iii. 28), adapt the human race to any climate whatever? If He could do this, why does He not now give us a climate which would be more agreeable to us than that of the present day? What may exist even upon Neptune, to which a milder climate would be uncontradictible.) What part of the book have you now gone over? Part First and Second, including Preliminary Observations and Definitions; and what relates to the Solar SYSTEM. What yet remains to be examined? Part Third, which relates to the SIDERAL HEAVENS. PEIARY ASTRONOMY. 155 PART III. THE SIDEREAL HEAVENS. LESSON XXXV. THE FIXED STARS-THEIR NUMBER, DISTANCES, MAGNITUDES, ETC. **745. What is meant by the Sidereal Heavens (127)?** **746. Why are some Stars called Fixed Stars?** Because they occupy the same positions with respect to each other from age to age, while the planets are seen to be in motion. **747. In what other respect do the Fixed Stars differ from the planets?** They are self-luminous, and seem to twinkle or scintillate, while the planets appear tranquil and serene. **748. How may a Fixed Star be distinguished from a planet by the aid of a Telescope?** The planets exhibit a round, mild-looking disk, while the Fixed Stars appear only as a point of brilliant light. **749. How are the Fixed Stars situated with respect to the Solar System?** They are inconceivably distant, and surround our system in every direction. (When it not for the Sun, we should see the stars in the daytime as well as in the night. See §31.) **750. What is the estimated distance of the Fixed Stars?** The nearest are supposed to be 20,000,000,000 (recently billions) of miles from the Sun, or more than 7,000 times as far off as Neptune. 156 PRIMARY ASTRONOMY. (1. For light to pass over this space, at the rate of 200,000 miles per second, would require several of our years.) (2. The sun, being the nearest circle of light, it would not appear larger than a nail's fingering from the nearest Fixed Star.) **751. How would our Sun appear from such a distance?** Only like a bright star. **752. What do the Fixed Stars supposed to be?** Distant Suns, or centers of other planetary systems. **753. How then, should we regard our own Sun?** As one of them, should be regarded as our own Sun! **754. How do the Stars appear in the heavens?** They seem to be equally distant from us, and scattered at random over the concave firmament. **755. What can you say of their apparent Sizes?** They vary from the large and bright star to the smallest that the eye can discover. **756. What is the cause of this variation?** It is due, in a great measure, to the variation in their distances. **757. How are the Stars classified?** They are first arranged in groups, or patches, called constellations. **758. How are the Constellations distinguished?** They are named after some animal or object which the ancients imagined them to resemble. **759. Into how many constellations are the heavens divided?** Ninety-three. **760. How are they situated?** Twelve in the Zodiac, 35 north, and 46 south of it. **761. Can you name one of the most conspicuous in each of these divisions?** PRIMARY ASTRONOMY. 157 The Great Bear in the north; Taurus in the Zodiac; and Orion just south of the Zodiac. (If the student will look up these three in the heavens, it will form a good begin- ning for his study of the subject, when he comes to work up the study more, with a more advanced text-book.) **763. What is the second step in classifying the Stars?** They are divided into twelve classes, according to their apparent magnitudes.
1. Stars of different magnitudes.
2. Stars of different magnitudes.
3. Stars of different magnitudes.
4. Stars of different magnitudes.
5. Stars of different magnitudes.
6. Stars of different magnitudes.
7. Stars of different magnitudes.
8. Stars of different magnitudes.
9. Stars of different magnitudes.
10. Stars of different magnitudes.
11. Stars of different magnitudes.
12. Stars of different magnitudes.
**763. How many of these can be seen by the naked eye?** Only the first six classes. The remaining six are seen only by the aid of telescope, and are called Tele- scopic Stars. **764. What is the third step in classifying the Stars?** To classify the stars in each constellation by the use of the Greek alphabet, calling the largest alpha (α), the next largest beta (β), etc. When the Greek alpha- bet is exhausted, the Roman is taken up; and when this fails, recourse is had to figures. **765. Is there any other method by which particular Stars are designated?** Many have specific names, as Arcturus, Sirius, Al- debaran, &c. **766. What is the estimated number of the Fixed Stars?** A finite mind can number them, but estimates have been made indicating nearly to about four hundred millions. The Christian asserts the infinite knowledge of God, by saying that "He taketh the number of the stars, and counteth all them all by themselves."—Psalm cvi. 4.) **767. In what proportion do the several Magnitudes occur?** There are but few of the first magnitude, and the number increases rapidly as the magnitudes diminish. 14 158 PRIMARY ASTROLOGY. [The number of stars down to the twelfth magnitude, has been estimated as follows :]
1st magnitude 13 7th magnitude 20,000
Visible to the naked eye 3d magnitude 177 8th magnitude 170,000
3d magnitude 177 9th magnitude 1,100,000
4th magnitude 2,600 10th magnitude 1,000,000
5th magnitude 46,000,000 11th magnitude 46,000,000
Total number. L.633 Total number. 354,501,897
**769. Why are there so many more of the small Stars than of the large ones ?** It is because we are in the midst of a great cluster, with but few stars near us, the number increasing as the circumference of our view is enlarged. See second cut, page 30. [I. We have here a representative of a great cluster of stars. Let A be the Sun; B and C two stars (a star only among the rest), with their respective distances from the Sun; D and E two stars between them and the first circle. The distance of these two stars from the Sun is greater than that of B and C; they are thus three times nearer than B and C; therefore they are three times more numerous than B and C. The space of this circle is one million times greater than that of the first circle; but as it contains more stars, they will be more numerous. Thus at each successive circle there will be more stars than at the last; and as we go on to infinity, the apparent magnitude of all these stars will become less and less; but their actual increase will be constantly increased. The large number of stars in the heavens is due to this cause.] NUMBER OF STARS OF EACH MAGNITUDE. | Magnitude | Number | |-----------|--------| | 1 | 13 | | 2 | 177 | | 3 | 177 | | 4 | 2,600 | | 5 | 46,000 | | Total | L.633 | This table fails to prove fully to the eye the cause of the rapid increase in numbers; for we can only show the surface of a cut section of our firmament of stars, which is very small compared with its whole extent. In reality, in order to include in the midst of a magnified cluster, the stars of which we view around us as every observer does. **769. Is any thing known of the actual Magnitude of the Stars ?** Nothing very definite, though many of them are estimated to be much larger than our Sun. (The diameter of the First Star has been estimated at 999,006,998 miles, or more than 999 times the diameter of our Sun.) PRIMARY ASTRONOMY. 159 LESSON XXXVI. OF DOUBLE, VARIABLE, AND TEMPORARY STARS. **770. What are Double Stars?** Such as appear single to the naked eye, but when ex- amined by the telescope are found to be double. (1.) The stars which appear like a single star to the naked eye, but with a telescope is found to consist of two; (2.) The stars what appear to a single star is found to consist of three to six, and even seven. **771. Is it likely that all Stars that appear double are actually near each other?** It is not. Probably many appear near each other simply because they are near the same line of vision. WHY STARS MAY APPEAR DOUBLE. Apparent position. True position. A diagram showing two stars (A and B) appearing close together in the sky, with arrows indicating their apparent positions and true positions. [There the observer on the left sees a large and small star at A, apparently very together; the lower star being much the smallest. But instead of their being distant from each other, they are really very near together, and this is shown by the fact that one star may be at B instead of A; and so the difference in their apparent magnitudes may be wholly due to their being near each other.] **772. In what sense are Stars said to be double when one is far behind the other?** They are said to be optically double. **773. How many Double Stars are to be met with in the Heavens?** It is supposed there are not less than six thousand. **774. What suspicion did the great number of Double Stars awaken in the minds of astronomers?** That such stars were specially connected by gravitation. **775. What surprising fact has been ascertained in regard to some of the Double Stars?** 160 PRIMARY ASTRONOMY. That they are actually revolving one around another, or both around the center of gravity between them. **776. How do we distinguish Double Stars that are thus con- nected ?** As being physically double. **777. What are these Systems called?** Binary Systems. (There, it must be remembered, consist of one or more stars revolving as described, but not to appear only at once.) **778. Are there any such Binary Systems ?** Sir William Herschel noticed about fifty instances of changes in the relative position of double stars, and the revolution of some sixteen he considered certain. **779. In what time do they revolve?** From forty to twelve hundred years. **780. What are Variable Stars?** Such as undergo a regular periodical increase and diminution of light. **781. What are the causes of these variations?** They are not known. It is thought they may be less luminous on one side than the other, and by turning on their axis, they become more brilliant on that account; or that planets revolving near them may cut off a portion of their light at regular intervals. **782. What are Temporary Stars?** Such as have disappeared from the heavens, and such as shine out suddenly, in a place previously void, as though just created. (Those writers clearly those under the head of New and Last Stars; **783. Are these sudden appearances and disappearances fre- quent?** Ten new stars have appeared, and thirteen old ones seem to have perished, during the last hundred years.) PRIMARY ASTRONOMY. 161 **784. How have some Christian writers regarded these sudden dis- appearances of Stars?** As the terminations of probationary periods, like the conflagration that is to take place upon our own globe at the end of time. (See 2 Peter iii. 7, 8.) LESSON XXXVII. CLUSTERS OF STARS AND NEBULE. **785. What are Clusters of Stars?** They are patches in the heavens where the stars are unusually thick or near together. **786. Have you seen any spectacles of Clusters?** The Seven Stars, or Ptolemaic, and the Hyades just east of them. **787. Are these Clusters numerous?** With a telescope many hundreds may be seen. **788. How do they appear through a Telescope?** They are found to consist, in many instances, of thousands of stars, as if constituting a separate universe by themselves. **789. What are Nebulae?** Clusters of stars so remote as to appear through common telescopes like a faint haze of light. **790. How are the Nebulae distinguished?** Into Resolvable, Irresolvable, Planetary, Stellar, and Annular. * * * A CLUSTER OF STARS. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 14 162 PRIMARY ASTRONOMY. **791. What are Resolvable Nebulae?** Clusters, the light of whose individual stars appears blended through ordinary instruments, but which can be resolved into distinct stars by telescopes of higher power. **792. What are Irresolvable Nebulae?** Paints patches of light, formerly supposed to be vast fields of unorganized matter, in a high state of rarefaction. **793. What has Lord Rosse announced in regard to these bodies?** That nearly 200 nebulae, hitherto considered irresolvable, were easily separated into stars by his magnificent telescope. **794. What does this seem to prove as to this class of Nebula?** That they could all be resolved into distinct stars, if we had telescopes of sufficient power. **795. What are Planetary Nebulae?** Clusters so nearly round as to resemble planets through ordinary telescopes. **796. What are Stellar Nebulae?** Such as seem to have a bright star at or near their center. **797. Where are these Stars probably situated?** In the direction of the nebula, but between them and the observer. **798. What are Annular Nebulae?** Clusters that have the appearance of a ring, the stars being much thicker around the edge than in the center. A circular nebula with a bright central star. ANULAR NEBULA. PRIMARY ASTRONOMY. 163 **799. What is the Galaxy or Milky Way of our own Firmament?** It is a zone of light surrounding the heavens, which is found by the telescope to consist of countless myriads of stars. **800. How do astronomers account for the vast number of Small Stars in this Belt?** They suppose our cluster to be in the form of a lens or oblate spheroid very much flattened; and that the Milky Way is an edgewise view from a position near the centre. [1. The second cut is a representa- tion of the great stellar cluster, called the "Milky Way," in our Solar System. It may be regarded as an oblate spheroid, with its axis of rotation parallel to the plane of the sun, and inclined at an angle and contents to revolve between 8 and the sun. Then if a person stood on the surface of the earth to look out of the diameter toward the sun, he would see all the stars of the belt, the stars would appear bunched up together, and would looked off in the direction of the sun. But if he stood at a distance, much more numerous, constituting a great small star around the heavens.] **801. Where are the Nebulae supposed to be situated?** Entirely beyond the great cluster composing our own immediate firmament. **802. How do they appear through the most powerful Telescopes?** A diagram showing a representation of the "Milky Way" in our Solar System, described as an oblate spheroid with its axis of rotation parallel to the plane of the sun, and inclined at an angle and contents to revolve between 8 and the sun. The diagram also shows a person standing on the surface of the earth looking out of the diameter toward the sun, seeing all the stars of the belt, and if he stood at a distance, much more numerous, constituting a great small star around the heavens. A side view of the "Milky Way" cluster, showing its oblate spheroidal shape and its inclination relative to the plane of the sun. 164 PRIMARY ASTRONOMY. They are found to be vast collections of glowing stars. **s02. What are they supposed to be?** Clusters like that in the midst of which the Solar System is found, but so remote as to appear like faint points of light. **s03. What, then, is the supposed structure of the Universe?** It is supposed to consist of vast distinct clusters, at immense distances from each other, and composed of stars, each of which is a Sun, surrounded by his own retinue of revolving worlds. A diagram showing the structure of the universe, with a central cluster of stars and galaxies surrounding it. (Let a regiment of our own cluster, with its sun, be placed in the center of this diagram, where it is bottom. Then the nearest groups would appear in succession, and their distance would increase; and the more remote like irreducible nebulae, all without any visible field of magnitude; the next cluster being at a distance equal to that of the first, and so on. At last we reach the outermost limits of the universe, and find no more clusters than those already described. The reason why these clusters are not seen in the sky observations upon the outside in the heavens beyond them.) **s04. How would things appear if we were to pass out of our own cluster, and go to one of those Nebula?** As we passed star after star, on our way to the borders of our cluster, they would swell to the magnitude of Suns, and again diminish to stars; while our own Sun would gradually dwindle to a star, and finally disappear. As we left our cluster it also would contract while we continued our journey; and when we approached and entered the till at length we should find ourselves surrounded by a new firmament of constellations, and our own cluster would appear only as a distant nebula. SUPPOSED STRUCTURE OF THE UNIVERSE. PRIMARY ASTRONOMY. 165 LESSON XXXVIII. OF THE ATMOSPHERE, WINDS, CLOUDS, STORMS, ETC. **§06. What is the Atmosphere?** It is the air we breathe, an elastic gas which surrounds the globe on every side. **§07. To what height does it extend above the Earth?** Its precise height is not known, but it is supposed to extend from 40 to 60 miles. **§08. What keeps it so closely wrapped around the Globe?** The same power that keeps the waters in their place; namely, gravity, prevents it. **§09. How does not the Air get swept off from the Globe in its rapid motion around the Sun?** Because there is no substance to sweep it off, the region through which the Earth passes being entirely empty. **§10. Does the Air, then, revolve with the Earth?** It does, both around the Sun and the Earth's axis. **§11. Is the Density of the Air the same at all elevations?** It is not; but grows more rare as we ascend from the Earth's surface. **§12. What is Wind?** It is a great motion. **§13. What is the Velocity of the Air in a gentle, pleasant Wind?** From four to five miles an hour. **§14. What of Brisk or High Winds?** From fifteen to fifty miles an hour. **§15. What is the Velocity of the Air in a Storm?** From fifty to sixty miles an hour. **§16. What is the Velocity in a Hurricane?** From eighty to one hundred miles an hour. 166 PRIMARY ASTRONOMY. **§17. What is the cause of Winds, Storms, and Hurricanes?** The influence of heat, causing bodies of air to rise, and other air to rush in to supply its place. (Whenever air is heated it expands, and becomes lighter than cold air so that this temperature difference causes the warm air to rise and the cold air to sink back. On this account, so if a large fire take place when the air appears perfectly still, the wind will blow from the direction of the fire.) **§18. What are Clouds?** A collection of misty vapors suspended in the air. **§19. How high are the Clouds?** They range from two miles to half a mile, according to their density and weight. **§20. Of what benefit are Clouds to us?** They often screen us from the oppressive heat of the Sun, and convey water from the rivers and oceans, and pour it down in showers upon the Earth. **§21. What is Rain?** Water condensed, or collected into drops by attraction, and falling from the clouds. **§22. What is Heat?** Drops of rain frozen on their way from the clouds to the Earth. **§23. What is Snow?** Particles of clouds frozen before being condensed into drops, and falling to the Earth. **§24. What is Lightning?** The passage of a fluid called electricity, from one cloud to another, or from the clouds to the Earth. **§25. What is Thunder?** A sudden shock given to the atmosphere by the passage of electricity through it. **§26. Why do we generally see the Lightning before we hear the Thunder?** PRIMARY ASTRONOMY. 167 Because the velocity of light is much greater than that of sound. **827. Is there any danger after the flash of Lightning is past, though we have not heard the Thunder?** There is not. It is the lightning that does the harm, and not the thunder. **828. What is the Aurora Borealis, or Northern Lights?** A reddish, unsteady light, that is sometimes seen in the North. **829. Is the cause of this Light known?** It is not ; but it is generally supposed to be produced by electricity. **830. What is meant by the Occulta- tion of a Star?** It is when the Moon passes be- tween the Earth and a star, and for a time hides it from view. (See also No. 50.) The Moon just about to occlude the star on the left.) **831. What are "Smoothing Stars"?** Meteors that shoot from the sky downward toward the Earth, like stars falling from their spheres. **832. Where are they generally seen?** One at a time, and only in the night. **833. Do they fall in great numbers?** They do. From two o'clock in the morning till day- light, on the 13th of November, 1833, the whole heav- ens were filled with fiery particles, and streaks of light darting downward from the sky. **834. Is it known what these Meteors are?** It is not. **835. Where are they supposed to come from?** From the regions beyond our atmosphere. A crescent moon. OCCLUSION OF A STAR 168 PRIMARY ASTRONOMY. A METEORIC SHOWER. A large meteor. **§36. How are they supposed to be set on fire?** By friction, in passing with great velocity through the atmosphere. **§37. Do they always ap- pear small, as represented in the picture?** They do not. Me- teors of great size have been known to traverse the atmosphere, and to explode with a loud re- port. **§38. Is any substance ever found belonging to Meteors?** What are called Meteoric Stones, and masses of iron, have fallen from the sky at various periods, and on almost every part of the globe. 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