Caption: Apparent retrograde motion is easily explained in a heliocentric solar system model as illustrated in the diagram.

Further explication:

1. Most of the time, planets as seen from the Earth move east (i.e., counterclockwise as seen from north celestial pole (NCP)) on the celestial sphere. However, inferior planets at inferior conjunction and superior planets at opposition move west (i.e., clockwise as seen from north celestial pole (NCP)) on the celestial sphere. These westward motions in angle are called apparent retrograde motions.

2. The ancient Greek astronomers found apparent retrograde motions a great puzzle since they interpreted them as westward motions relative to 3-dimensional outer space. This led to the Aristotelian cosmology of nested celestial spheres (based on Eudoxon models with hippopedes) and the Ptolemaic system (based on the epicycle theory). But let's NOT go there.

3. However, as aforesaid, apparent retrograde motion is easily explained in a heliocentric solar system model.

4. As you can see by reading the diagram, the apparent retrograde motion of a superior planet (in the diagram Mars) is just caused by the Earth passing it on the inside track at the time of the planetary configuration opposition.

5. For inferior planets (which are lowly degraded planets), you can understand their apparent retrograde motion from the diagram too. Just note that Earth is an inferior planet relative to Mars and it exhibits apparent retrograde motion relative to Mars when it is at inferior conjunction relative to Mars: i.e., Earth moves west (i.e., clockwise) as seen from Mars at inferior conjunction.

6. Finally, we emphasize that apparent retrograde motion is a real reversal of a planet's motion in angle as seen from the Earth. It's NOT an illusion. But it's NOT a reversal of motion relative to 3-dimensional outer space.

   Recall that in astro-jargon, APPARENT does NOT mean "seeming" or "illusionary": it means as seen from the Earth.