From Andreas Cellarius Harmonia Macrocosmica, 1660/61. Chart showing signs of the zodiac and the solar system with world at centre.
In the Ptolemaic system, each planet is moved by a system of two or more spheres: one called its deferent, the others, its epicycles. The deferent is a circle whose center point exists halfway between the equant and the earth, marked by the X in the picture to the right where the equant is the solid point opposite the earth. Another sphere, the epicycle, is embedded inside of the deferent and is represented by the smaller dotted line to the right. A given planet then moves along the epicycle at the same time the epicycle moves along the path marked by the deferent. These combined movements cause the given planet to move closer to and further away from the Earth at different points in its orbit, and caused observers to believe that the planet even slowed down, stopped, and moved backward (in retrograde motion). This apparent retrograde motion was one of the largest inconsistencies in Greek cosmological systems, and was one of Ptolemy's main reasons for creating the deferent, epicycle model. The apparent retrograde motion was eventually replaced by the heliocentric model, and dispelled as an observation that is made only from earth bound observers. However, this model of deferents and epicycles made observations and predictions much more accurate than all preceding systems. The epicycles of Venus and Mercury are always centered on a line between Earth and the Sun (Mercury being closer to Earth), which explained why they were always near it in the sky.

The Ptolemaic order of spheres from Earth outward is:

1. Moon
2. Mercury
3. Venus
4. Sun
5. Mars
6. Jupiter
7. Saturn
8. Fixed Stars
9. Sphere of Prime Mover

The deferent-and-epicycle model had been used by Greek astronomers for centuries, as had the idea of the eccentric (a deferent which is slightly off-center from the Earth). In the illustration, the center of the deferent is not the Earth but X, making it eccentric (from the Latin ex- or e- meaning "from," and centrum meaning "center"). Unfortunately, the system that was available in Ptolemy's time did not quite match observations, even though it was considerably improved over Aristotle's system. Sometimes the size of a planet's retrograde loop (most notably that of Mars) would be smaller, and sometimes larger. This prompted him to come up with the idea of an equant. The equant was a point near the center of a planet's orbit which, if you were to stand there and watch, the center of the planet's epicycle would always appear to move at the same speed. Therefore, the planet actually moved at different speeds when the epicycle was at different points on its deferent. By using an equant, Ptolemy claimed to keep motion which was uniform and circular, but many people did not like it because they did not think it was true to Plato's dictum of "uniform circular motion." The resultant system which eventually came to be widely accepted in the west was an unwieldy one to modern eyes; each planet required an epicycle revolving on a deferent, offset by an equant which was different for each planet. But it predicted various celestial motions, including the beginnings and ends of retrograde motion, fairly well at the time it was developed.