Ptolemy was not the originator of the geo-centric model, rather, he was the one who developed the most sophisticated version of the geo-centric model. His model was one of wheels turning on wheels with the Earth sitting inside of the smallest wheel:

Each planet appeared to move on an circle known as an epicycle whose center connected to a larger circle known as the deferent. This convoluted construction led to retrograde motion by adjusting the rate at which the planet moved along its epicycle and the rate at which the center of the epicycle moved along the deferent.

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However, the motions of the planets in the sky are not modeled well by simple circular motions such as above. To further model nonsteady speeds of planets in their orbits, equants and the eccentric were introduced (see above). The center of the Solar System was the eccentric (the cross) and the planets moved at uniform speed around the equant . The breaking of the elegant notion of uniform motion along a circle was a major sticking point of the Ptolemaic model.

The model although cumbersome (and silly by today's standards), served its purpose in that it was consistent with the observations and physics of the day.