What is the retrograde motion of Mars?
When observing planetary motion with respect to the stars that are located very far away and can be considered stationary, a few unusual things are observed. One of them is retrograde motion.
The observance of retrograde motion is best explained by an example where you are travelling in a car and another car is travelling beside you. If the speed of both the cars is the same, the other car appears stationary with respect to a very distant object, if the car is moving faster than you, you see it moving ahead. On the other hand, if you are moving faster you would observe the car moving in the reverse direction.
The same phenomenon occurs with the planet Mars. When observations of the planet are taken over extended periods of time, it is observed that the planet does not appear to move in a constant circular orbit but instead moves backwards and creates an s-shape or a loop.
There is no particular month of the year when the planet is seen to be in retrograde motion. The time varies and this occurs approximately once every two years.
The links below show the retrograde motion of the planet in the years 2003 and 2010.