The following animation shows a simplified geocentric model with three objects: the Earth (blue) in the centre with the Sun (yellow) and Mars (red) in circular orbits around it. Mars travels on an epicycle which is centred on its deferent. The phenomenon of ``retrograde motion'' occurs while Mars is at opposition. (Note how this model successfully predicts that Mars should be brighter during retrograde motion than at other times due to its smaller distance to the Earth at that time.) Mars orbits on its epicycle at a rate such that a line drawn from the deferent to Mars is always parallel to the Earth-Sun line.
You can stop the planet motion by clicking on the image at any time, and then press the mouse again to activate it.
This animation was created by Ming Zhu . Send questions or comments to : firstname.lastname@example.org