The equatorium was a medieval
instrument designed to give a graphic solution to the Ptolemaic equations for finding planetary
positions. It was first described by Ibn al-Samh about ¥ 1025. Variations and improvements were noted through the 14th century by Abu al-Salt, Campanus of Novara, Richard of Wallingford, John of Linires, and other astronomers. The form of the
instrument was generally a circular brass or paper disk with a rotatable volvelle (a movable circle with pointers) and a circle of degree marks. Some equatoriums were
designed for one planet only, whereas others had several volvelles, each for a different planet.