The plane astrolabe may have been invented in Alexandria of Egypt as early as the second century B.C.E. It was successfully disseminated in the medieval Muslim world and, via Muslim Spain, penetrated into Europe in the tenth century.

The astrolabe parts are fitted onto a central axis. The body of the instrument is called the mater, a thick plate with a graduated limb and a ring from which it can be suspended. Inserted into the mater is the tympanum or latitude plate, which carries the projections of the observer's altazimuth coordinates with the main vertical circles and the main altitude parallels. The rete is a disk showing the Sun's annual path and the positions of selected bright stars.

With an alidade, or straight rule with sights, placed on the back of the astrolabe, the observer aims at one of those stars to measure its altitude. The rete is then rotated until the star indicator overlaps the altitude parallel corresponding to the measurement. On a twenty-four hour scale, the time of night is given by the angle between the position of the Sun on the observation day, recorded with a marker, and a fixed direction.

If, instead, the day and hour are known, one can predict the star's altitude. And also, for any given day, one can determine the time of a heavenly body's rising, culmination, and setting.