The astrolabe is an ancient astronomical instrument used for reckoning time, for observational purposes, and for navigation. It was first used in ancient Greece and was extensively developed in the medieval Islamic world. The astrolabe is essentially a two-dimensional projection of the three-dimensional sky and has many uses, including observing the positions of the sun and stars as well as estimating ones latitude. 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 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 was later supplanted by sextants.