coordinates of the church Satint-Sulpice are:
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2011, June 15:
transit: 13:51 CEST, altitude 64.46°
photo: 13:35 CEST, altitude 64.26°, azimuth 171.5°
1743, June 15:
transit: 13:50:26 CEST, altitude 64.49°
1743, June 22:
transit: 13:51:56 CEST, altitude 64.63°
x=11.144 m (H=23.5 m)
1743, Dec 22:
transit: 13:49:41 CEST, altitude 17.72°
x=73.547 m (H=23.5 m)
h = 5.08
2011, June 21:
transit: 13:52:22 CEST, altitude 64.59°
x=11.164 m (h=23.5 m)
The present church Saint-Sulpice, erected over a church of the 13th century, was founded in 1646 and mostly completed in 1732. It is second largest church in Paris after Notre-Dame de Paris.
The French astronomer Pierre Charles Claude Le Monnier (1715 – 1799), member of the Academy of Sciences, constructed the meridian Saint-Sulpice in 1743 "Ad Certam Paschalis Aequinoctii Explorationem", to explore the date of spring equinox on which depends the Easter date, and to determine the nutation and the obliquity of the Earth's axis.
The sun shines through a small hole in the southern wall to cast its light on the meridian line each day (aperture gnomon).
The aperture gnomon.
The mean transit time (solar noon) is 11:51 UT, which varies according to the equation of time from around 11:34 UT to 12:05 UT.
The distance d of the disk of light along the meridian line and the size (horizontal and vertical) are increasing from summer solstice to winter solstice, whereas the time t to cross the meridian line remains nearly constant.
The diameter S of the disk of light is mainly determined by the apparent diameter α of the Sun:
S = D·tan α
The duration t (in seconds) of the meridian transit depends on the Sun's diameter α (arc seconds) and the declination δ:
The diameter a of the aperture and an amount due to diffraction (2·D·1.22·λ/a ≈4 mm) has to be added to S.
The diameter of the aperture is supposed to be 6.6 mm
The image is of greatest sharpness if the distance D from the aperture (diameter a) to the surface is given) by (wavelength λ):
D = 19.8 m (for a=6.6 mm, λ=550 nm)
In the past centuries the obliquity of the ecliptic decreased by 0.013° = 46.8'' per century. The result is a shift of the sun spot on the meridian line:
Rougé: Le gnomon de l'église
Saint-Sulpice, Paroisse Saint-Sulpice, Paris 2006.
Saint-Sulpice, Paroisse Saint-Sulpice, Paris 2003.
J. L. Heilbron: The Sun in the Cathedral, Cathedrals as Solar Observatories,
Harvard University Press, 1999.