An Islamic Astrolabe
The front (above) and back (below) of an Islamic Astrolabe in the Whipple Museum.
This astrolabe is signed "Husain b. Ali" and dated 1309/10 AD. It is probably North African in origin, and is made of brass. It has four plates (for the front of the astrolabe, representing the projection of the celestial sphere and marked with lines for calculation), each for a specific latitude, and 21 stars marked on the rete (the star map, with pointers, fitting over the plate). I have chosen this astrolabe since it is right in the middle of the time frame for Islamic astrolabe use (ca. 600 to ca. 1800) and because it demonstrates many of the features common to Islamic astrolabes.
On the back is a shadow square for measuring the heights of inaccessible things and other similar calculations (shadow squares are quite common, but not on all astrolabes), and scales for calendrical calculations and calculation of the qibla (the direction to face during prayers).
A typical text on the astrolabe describes more than forty uses of the astrolabe, indicating its versatility as an astronomical calculating device. Some of its principal uses to the Islamic astronomers were to provide answers to astrological, calendrical, and meteorological questions. Although less accurate than direct mathematical calculations (the astrolabe is only as accurate as the positioning of the rete and so on) it provided an easy and quick way to calculate values.