The path of Mars, as viewed from the Earth.
Retrograde motion is the apparent backward motion of a planet caused by its being lapped by another planet, or vice-versa.1 These two planets are usually on two different orbits, a larger one and a smaller one, and they move around the sun in the same direction (eastward). The planet on the smaller orbit moves faster than the planet on the larger orbit. When the two planets on different orbits do overlap, and align with the sun, either one of the planets sees the other planet as moving in the opposite direction. However, this is only an apparent motion, as the planets are truly moving in the same direction. This apparent motion is what we call “retrograde motion.” The period between two retrogradations is the synodic period of the object/planet.2 A practical example of retrograde motion is the motion that when you pass a car on the freeway. The car being passed appears to move backwards relative to you. Alternatively, you can experience this by standing side by side with a friend, and having them. walk forward slowly. Now walk forward at a faster speed and watching your friend, think about how they are moving relative to you. At first, they move away, then as you pass them, they appear to be moving backward relative to you – even though they are still walking forward. Ancient Greek astronomer Ptolemy in 150 AD believed that the Earth was the center of the Solar System and therefore used the terms retrograde and prograde to describe the movement of the planets in relation to the stars, and the names have remained since then.3