The astrolabe appears to have been an invention from the Alexandria school, perhaps by Hipparchus or Ptolemaeus. The word astrolabe means an understanding of the stars. It is an instrument of precision which was used as an observatory and abacus for astronomical calculations. It has a hollowed metallic circle, or mater, in which graduated circles and quadrants are engraved on both sides, the sun deferent with its 365 days and 12 months, in addition to the zodiac which has its origin in the first point of Aries, which coincided with the vernal equinox. The plates were positioned in the centre, and were different for each latitude, and within these, a stereographic projection was outlined on the equator of the zenith of the corresponding latitude, which is 37º in the Granada astrolabe, the almucantarats or circles parallel to the horizon of this latitude, the inclined horizon, the equator and the tropics of Cancer and Capricorn. The rete is located on this set, made up of a grille with arrows the points of which point out the location of some of the brightest stars in the sky. The rete is free to rotate around the centre of the astrolabe, which represents the north-south axis, and on this turn, the apparent movement of the canopy of heaven. The Muslims carried out their astronomical and astrological calculations with this instrument. The astrolabe of Granada is one of forty preserved in the world.