Why planets such as Mars undergo retrograde motion?

Answer 1

The outer planets appear to have retrograde motion -- they look as if they're going backwards -- because of the relative orbital periods of the Earth and the outer planets. This was one of the aspects of observational astronomy that led Copernicus and others to propose that the Earth orbited the Sun, rather than the other way around, and further led Kepler to propose elliptical orbits for the planets instead of circular orbits.

In fact, Mars does not move backwards; it just APPEARS to move backwards against the background of the stars as we observe it from earth, and here's why.

The Earth orbits the sun much more rapidly than the outer planets. The Earth orbits the sun in one earth year -- 365.25 earth days -- but Mars orbits the sun in 689 earth days -- 1.886 years. So if you start with the Earth, Sun and Mars all in a straight line (opposition -- the point of closest approach of the two planets each year), then let them "go", the Earth speeds ahead. This makes Mars appear to move backwards -- retrograde -- against the stellar background, until the Earth - Sun -Mars geometry reaches a point that forms an angle of about 90°. At that point, Mars appears to stop, and then go forward again. As the earth passes behind the sun (as seen from Mars) and starts to come around from behind, Mars continues to plod forward, but once again we'll reach a point where it will appear that Mars starts moving backwards, because we are "overtaking" the more distant planet.

Answer 2

It is an illusion similar to what happens when a fast car passes a slow car. The slow car may seem to be going backward even though it isn't, but in the case of a car the trees, road and other fixed surroundings give away the illusion. IN space, where the only reference points are the moving planets themselves, it