An astrolabe is a projection of the three-dimensional celestial sphere on to two dimensions - in the same way as a map represents the projection of the earth on a flat piece of paper or other material. The concept of the astrolabe was known to the ancient Greeks at least from the time of the birth of Christ onwards, but its origin is shrouded in mystery. The word 'astrolabe' is also of Greek origin and means essentially 'star-holder'. It is a multi-functional instrument which can be used to tell the time during the day and at night, to determine the time of sunrise and sunset, to simulate the movements of the heavenly bodies, to serve as an accurate star-map, and for surveying and astrological purposes.
With the introduction of Greek science to the Islamic civilisations through translations starting in the ninth century, the astrolabe became one of the most popular astronomical instruments and was developed further with great sophistication and extraordinary craftsmanship. It was widely used in Europe from its reintroduction in the tenth century until the seventeenth century.