Ptolemy’s system was purely geometric, like Eudoxus, with combinations of circular motions. The Ptolemaic system is a geocentric cosmology; that is, it starts by assuming that the Earth is stationary and at the centre of the universe. Ptolemy’s system for each planet involves a large (imaginary) circle around the Earth, called the deferent, on which revolves a smaller circle, the epicycle. The visible planet sits on the edge of the epicycle. Both deferent and epicycle revolve in the same direction.