The Coperican system was not as simple as is often advertised. To account for the observations of the Sun (a), the Earth revolved around a central point Ce, which in turn revolved around a point CCe, which in turn revolved around the Sun. In (b) Mars is on an epicycle revolving on a deferent. The deferent's center, Cm, keeps a fixed geometric relation to the point Ce, the central point of the Earth's orbit, which moves as in (a). The Coperican explanation of retrograde motion is shown in (c). The relative motions of the Earth and Mars around the Sun cause the appearence of retrograde motion against the backdrop of the stars.