The astrolabe, an astronomical instrument used for observing planetary movements, was indispensable for navigation. A type of analog calculator, brass astrolabes were developed in the medieval Islamic world, and were also used to determine the location of the Kaaba in Mecca, in which direction all Muslims face during prayer. Planispheric, or flat, astrolabes, were more common than the linear or spherical types. In planispheric astrolabes the celestial sphere was drawn on a flat surface and represented on one plate.
The earliest known dated astrolabe is of the planispheric type. Made of cast bronze, it bears the name of its maker. The inscription at the back of the kursi, or throne, is written in Kufic , the oldest calligraphic form of the various Arabic scripts, and states that the astrolabe was made by Nastulus (or Bastulus) and gives the date, which corresponds to 927/28. The date is rendered in Arabic letters, whose numerical values total 315, signifying the year in the Islamic calendar in which the astrolabe was made. It is preserved in the School of Oriental and African Studies at the University of London.