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How did Ptolemy explain the way planets seemed to move at different speeds and even backwards?

The Ptolemic Model of the planets relied on the notion that the planets' movements were determined by two different spheres: the deferent and the epicycle. 

The deferent is just the sphere whose center point, called the eccentric, is an imaginary point offset a little from earth. The planets each have their own deferent, and revolve about it. 


Of course, this alone would not be enough to fully explain the seeming motion of the planets. So the epicycle was posited, which was a second sphere that the planet moved around. Or, more accurately, the epicycle revolved around the deferent, and the planet itself revolved around the epicycle. 


If that's confusing, then it is probably most easily understood by linking each term with its analogous counterpart in the modern model:


  • The eccentric is the sun, and is the point about which all else revolves.

  • The deferent is earth's orbit around the sun. It is not an object in itself, just a path the planet follows.

  • The epicycle is the moon's orbit about the earth. It revolves around the earth, while the earth revolves around the sun, giving it two spheres of revolution. 

In this analogy, the planets' motions most closely mimic that of earth's moon as viewed from the sun; revolving around two spheres. Except that the earth, in the Ptolemic model, is not the same as the eccentric. So, really, it would be like viewing earth's moon from a point a little bit away from the sun.


The idea that planets move around two spheres whose center points are not earth is what accounts for the seeming different speeds and directions in which the planets moved. Even though we now know this to be wrong, this model was impressively accurate at predicting the movements of celestial bodies.

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