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 essentially 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 lead to retrograde motion as:

(a retrograde animation). To further model the nonsteady speeds of planets in their orbits, equants were introduced (see above).

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.