Astrolabe

A mechanical device, predecessor to the sextant, whereby mariners determined the time of day by the Sun, of the night by the stars, and the height and depth of mountains and valleys. The astrolabe of Christopher Columbus was on display at the Philadelphia Sesquicentennial. The oldest known example, called “The Mathematical Jewel,” is of Persian origin. It was made by Ahmad and Mahud, sons of Ibraham (q.v.) the Astrologer of Isfahan, and is in the Lewis Evans collection in the Old Ashmolean Museum at Oxford, England. The invention is attributed by some to Hipparchus (q.v.) but others credit it to the Arabs, some 400 years prior. 

DeVore, Nicholas. Encyclopedia of Astrology. New York: Philosophical Library, 1947.