Astrolabe is an astronomical tool that let's us determine the time and the position of stars in the sky. It was a form of a mechanical map of the celestial sphere adjustable to the observer's position on the Earth and a particular date. Used during the Middle Ages it was by far the most efficient astronomical instrument in these times.

The astrolabe was usually made of brass and was about 6 inches in diameter. There were many uses of the astrolabe, described in detail by Geoffrey Chaucer in his "A Treatise on the Astrolabe". They included determining the time during the day or night, specifying the time of a celestial event (sunrise, sunset, etc.) and as a reference of celestial positions.