The celestial globe is a three-dimensional map of the stars, and has been used since classical times. The stars were thought to sit on the surface of a giant sphere around the Earth, and the constant movement of the stars every night and throughout the year seemed to be caused by this giant sphere slowly turning overhead. Just like a terrestrial globe, the celestial sphere is mapped by a North and South Pole, an Equator, and lines of longitude and latitude. Celestial globes were produced first by Greek astronomers, and later also in the Islamic world, where the earliest known globes date from the late eleventh century. Islamic astronomers built upon many of the achievements of classical Greek science, further refining concepts and the design of astronomical instruments, such as the celestial globe and the astrolabe. This is why an Islamic globe depicts the classical constellations, such as the Great Bear, Pegasus, Orion and the twelve signs of the zodiac. At the South Pole of this globe, the craftsman has inscribed his signature: 'Made by the most humble in the supreme God, Muhammad ibn Hilal, the astronomer from Mosul, in the year 674' (AD 1275-76). Mosul is an important city in northern Iraq, famous in the first half of the thirteenth century for its skilled metalworkers. However, this globe may not have been manufactured in Mosul. In 1262, the city was sacked by the Mongols. The invading force was led by Hulagu Il-Khan (died 1265), who had recently founded an important observatory at his new capital of Maragha, in north-western Iran. The Mongols were known to deport skilled citizens from conquered lands, and Hulagu may have decided to send Muhammad ibn Hilal to the new observatory straight away. The globe, constructed some twelve years later, may therefore have been constructed at Maragha.