The astrolabe, an invention of Hellenistic Alexandria, was the principal Islamic instrument for telling the time, surveying and determining latitude. By the later ninth century it was used throughout the Islamic world from Spain to India and later reached Christian Europe. By modelling the apparent rotation of the stars about the celestial pole, it solves a number of astronomical, and astrological, problems. Notably, the provision of a simple sighting device made it possible to calculate the elevation of a particular star, or of the sun, and thus both determine the time and indicate the direction of the North Pole. Moreover, although an astrolabe does not show the movement of the planets, planetary tables enabled the astrologer to know the position of a planet relative to the constellations of the zodiac, which are generally engraved on the plates.