The front side of a typical astrolabe displays a stereographic projection of the northern portion of the celestial sphere, with the North Pole at the center and the Tropic of Capricorn at or near the edge. Also present are altitude circles for the terrestrial latitude to which the instrument was designed. The altitude circles converge on the zenith, which appears slightly above the central pin of either of the astrolabes shown here. Most astrolabes came with a set of plates bearing altitude circles for different latitudes.

A fretwork "rete" bearing pointers for prominent stars can be rotated to align those pointers to the coordinates of the stars at any particular time. Once this is done the coordinates of other stars can be read directly off the plate, and the time of day can be read from a scale on the rim of the astrolabe. The rete also contains a prominent ecliptic circle, allowing the position of the sun to be used along with those of the stars.

The back of the instrument bears a sighting device or "alidade" that is normally used to measure the altitude of the sun or a star. Once this altitude is known, the rete can be rotated to the correct position.