An astrolabe is an ancient astronomical instrument that was used for a variety of purposes, including telling time, determining the length of day and night, simulating the movement of heavenly bodies, surveying, and astrological purposes. It was invented in the Hellenistic civilization by Apollonius of Perga between 220 and 150 BC and was a marriage of the planisphere and dioptra, effectively an analog calculator capable of working out several different kinds of problems in astronomy. The astrolabe was widely used in the medieval Islamic world and Europe, and by the mid-15th century, it was adopted by mariners and used in celestial navigation.

The astrolabe consists of several parts, including a circular disk, a sighting tube, an alidade, and arms that hold graduated scales. The circular disk is the main part of the astrolabe, and it was mainly used to view figures in the sky. The sighting tube is placed on top of the circular disk and was used to observe stars or planets. The alidade had vertical and horizontal cross-hairs which plots locations on an azimuthal ring called an almucantar (altitude-distance circle) . The astrolabe provided astronomers with a plane image of the celestial sphere and the principal circles, including those representing the ecliptic, celestial equator, and tropics of Cancer and Capricorn.

Astrolabes have been traced back to the 6th century and were made of metal, usually brass or iron. They were widely used in the medieval Islamic world and Europe, and by the mid-15th century, they were adopted by mariners and used in celestial navigation. Today, astrolabes are no longer commonly used, but some still exist and are used as teaching tools.