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.