Ptolemy's geocentric system, taken in part from the earlier work of Heraclides and Apollonius, presented each planet as moving uniformly around a small circle called an epicycle. The center of the epicycle in turn revolved uniformly around the circumference of a large circle called a deferent. By means of proper combinations of sizes and rates of motion for the epicycle and deferent, planetary motions could be mostly direct and occasionally retrograde. Also, since a planet on an epicycle is sometimes nearer and sometimes farther from the Earth, this accounted for the observed variations in planetary brightness. To represent the irregular rates of motion of the planets, Ptolemy continued to employ the device attributed to Hipparchus of having the deferent off center from the Earth (to produce an eccentric deferent), so that a planet would appear to go fastest when it was closest to the Earth.
Having constructed orbits for the Sun, Moon and planets out of a combination of epicycles and eccentric deferents, Ptolemy found that the heavenly bodies were moving at an even more irregular rate than could be accounted for by these devices. His solution to this problem was to suppose that the planets' motions were uniform not as viewed from either the Earth or even the center of the eccentric deferent, but from a point on the other side of the center of the deferent from the Earth; this point was called the equant.