An astrolabe is an instrument that can be used by astronomers to observe the relative positions of the stars. It can also be used for timekeeping, navigation, and surveying. The most common type of astronomical astrolabe, called the planispheric astrolabe, was a star map engraved on a round sheet of metal. Around the circumference were markings for hours and minutes. Attached to the metal sheet was an inner ring that moved across the map, representing the horizon, and an outer ring that could be adjusted to account for the apparent rotation of the sky.

To use an astrolabe, observers would hang it from a metal ring attached to the top of the round star map. They could then aim it toward a specific star through a sighting device on the back of the astrolabe, called an adilade. By moving the adilade in the direction of the star, the outer ring would pivot along the circumference of the ring to indicate the time of day or night. The adilade could also be adjusted to measure the observer’s latitude and elevation on Earth.