The Ptolemaic universe (a) with a central Earth and an epicycle-deferent system for each planet. To account for as many observations as possible (b), the Earth is displaced from a central point C. This device, known as an eccentric, has a planet on an epicycle revolving around a central point on a deferent, which in turn revolves around C, which in turn revolves around the Earth. Also, the planet revolves uniformly in relation to an equant point, rather than in relation to the Earth. Further complicated variations of this scheme could include epicycles on epicycles and eccentrics on eccentrics.