The Astrolabe

The most famous instrument of medieval Islamic and European astronomy is the astrolabe, which uses the principle of stereographic projection to depict the heavens in the shape of a flat disk. The instrument has roots that go back to Greek antiquity, but would see its most fruitful period of development during the Islamic Middle Ages. It arrived in Christian Europe in the late tenth century. Its three main components are (a) a container unit known as the ‘mater’; (b) one or more horizon plates with coordinate markings relating to a particular local horizon; and (c) a ‘spider’s web’ or ‘rete’, which is a perforated disc containing star pointers and a circle representing the ecliptic (which is the Sun’s apparent annual path around the earth). By rotating the rete above a given plate, it is possible to simulate the revolution of the firmament relative to a given horizon and, on this basis, perform a number of astronomical and astrological computations. One of the chief applications of the astrolabe was to find the time of day or night, for which purpose the altitude of the Sun or stars was measured using the sighting tool mounted on the instrument’s back, the so-called alidade.

Commentary. Philipp Nothaft (May-June 2019)