1. A type of star map developed by the ancient Greeks consisting of a horizon circle graduated in degrees, or azimuths, from the north point, and star altitude circles.

There is also a sighting bar to measure the altitudes of stars and other celestial bodies.

2. An ancient navigational instrument, forerunner of the sextant.

Astrolabes usually consisted of a flat disk with a sighting rod that could be pivoted to point at the sun or bright stars.

From the altitude of the sun or star above the horizon, the local time could be estimated.

A small instrument, used during ancient and medieval times, for calculating the positions of bodies in the heavens.