Developed in the Islamic world by the 9th century, the astrolabe was introduced to Europe from Spain three centuries later and became the predominant astronomical instrument until the 17th century. A multifunctional object mainly used to show the position of the sun and the stars in the sky at a given time, the astrolabe was also used to solve astronomy problems, to find time during day and night, as a way-finding device at sea, to track the sun's position, and to educate future astronomers. The main brass disc in the astrolabe is called mater; it is hollowed in the center to accommodate two plates that provide astronomical information about the sun, the position of the stars, etc. Over them, a perforated disc with a circle showing the sun's ecliptic, called rete, rotates to simulate the daily rotation of the stars in the sky. All four discs are held together by a pin. The back of the astrolabe has engraved mathematical scales for astronomical calculations, and a brass segment, the alidade, for measuring the altitude of celestial objects. For taking measurements of the Sun or the stars, the astrolabe is suspended by a cord connected to a ring. The top of the astrolabe, where the ring is connected, is called throne. The Fusoris astrolabe in the picture (15th century) is 16.3 cm (6.4 inches) in diameter, with an overall height of 20.6 cm (8.1 inches). The thickness of the mater plate is 0.8 cm (0.31 inches).
