Astrolabes

Instruments designed to observe the positions and measure the altitude of celestial bodies; used from the 2nd century BCE, until superseded by sextants.

An astrolabe is a two-dimensional representation of the celestial sphere. Its origin dates back to Hellenistic Antiquity, when it was devised as a precision instrument primarily for astronomical purposes. Its gradual development within Islamic culture increased its functions, mainly as a calendar and a clock, thus giving it an additional purpose as an instrument to be used by the Islamic cult.

The theoretical basis of the astrolabe is, essentially, a stereographic projection of the celestial sphere over the plane of the equator, formulated by Hipparchus of Nicaea in the 2nd century B.C. and subsequently brought together by Alexandrian astronomer Claudius Ptolemy (ca. 100 A.D.- ca. 170 A.D.) in his work Planispherium and his great astronomical treatise, Almagest.