Early astronomers and navigators, from about the seventh century A.D., used the astrolabe to find or define the positions of stars. The original instrument was designed by the Arabs, using ingenious geometry at a time when spherical trigonometry was unknown. Star positions were given in terms of altitude (angle above the horizon) and azimuth (distance around a horizon circle). The astrolabe could also be used to tell the time by the stars or the sun. It gave devout Muslims the prescribed times for prayers and provided a reliable sun or star compass for travellers and navigators.
The medieval astrolabe was, in effect, a sort of calculator for transforming the coordinates of right ascension and declination of any star at a specific time into its corresponding altitude and azimuth. This capability was of great convenience in finding out just where to look in the sky for a particular star. The circular plate representing the ecliptic was divided into 12 segments, often represented by the signs of the zodiac, which served also as calendar dates.Read More Star positions
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