It was not until the year 1543 when the great Polish astronomer, Nicolaus Copernicus (1473-1543) had his lifelong work "De revolutionibus" published, that the secret of the odd retrograde loops were finally revealed. By demoting the Earth from its hallowed position at the center of the solar system and replacing it with the Sun, he was able to triumphantly explain the riddle of the apparent "backwards motion effect" of the planets.
In fact, it's the same effect obtained when passing another car on a highway: both cars are going in the same direction, but one is moving more slowly. As they pass, the slower car will appear to be moving backward in relation to the faster one.
Copernicus simply applied the same effect to the planets out in space.
In the upcoming situation, both Earth and Mars are moving in the same direction around the Sun, but the slower one - Mars - appears to move backwards compared to the faster one, the Earth. [Animation]
I would like to stress again, that the retrograde motion of Mars - like the apparent motion of the slower car on the highway - is nothing more than an illusion. The last time Mars underwent retrograde motion (during the summer of 2003), I received inquiries from a number of readers asking if the Sun, as seen from Mars, would appear to stop and move backwards across the sky. The answer is most definitely, no