Problem of Time I
Astrolabes were used extensively in the Islamic world from the 8th century, and Abd al-Rahman al-Sufi listed many uses for it, from astrology to navigation and surveying. A typical astrolabe has a stereographic projection of the night sky on the base plate, and a dial marked with the main constellations, which can be turned to the setting or rising position. The outer edge of the base is marked with the hours of the day. On the reverse side of the plate is usually a rod with sights, which may be used to determine, for instance, declinations of stars (albeit not very accurately).