astrolabe

An astrolabe is an ancient astronomical instrument in the form of a compact disk that can be used to solve problems related to time and the positions of celestial bodies. The central feature is the rete, a movable lattice that depicts bright stars and their projected paths on the sky. Beneath it lies the mater, a circular plate or disk engraved with a stereographic projection of the celestial sphere, often accompanied by a tympan or plate that provides a fixed coordinate grid. A rule or alidade is used to sight objects and read measurements from the scales.

Historically, the astrolabe originated in the Hellenistic world and was further developed in the Islamic world from the 9th century onward, later spreading to medieval Europe. It exists in several forms, most commonly the planispheric astrolabe, which represents the sky at a given latitude, and the universal astrolabe, which uses multiple plates for different latitudes. It was a versatile tool used by astronomers, navigators, and scholars for centuries.

Functions and uses include determining local time and geographical latitude by measuring the altitude of celestial objects, solving problems of spherical astronomy such as rise and set times and the length of day, and calculating celestial coordinates. In the Islamic world it was employed to determine prayer times and the Qibla direction. In navigation, it aided sailors in estimating position through celestial sightings before the advent of more modern instruments.