The following animation shows a simplified helicentric model with three objects: the Sun (yellow) in the centre with the Earth (blue) and Mars (red) in circular orbits around it. The phenomenon of ``retrograde motion'' occurs while Mars is at opposition and is overtaken by the Earth. This model also 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.
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