In the Planisphaerium, Ptolemy develops the mathematical method for projecting circles of the celestial sphere onto a plane while preserving circular properties — today known as stereographic projection. This work is the theoretical foundation of the planispheric astrolabe, the most widespread astronomical instrument of the medieval Islamic world and later Latin Europe. The original Greek text is lost save a fragment; it survives via a 9th-century anonymous Arabic translation and later Latin translations, chiefly by Hermann of Carinthia (Toulouse, 1143).