Retrograde Motion

The motion of the Earth around the Sun has a major effect on the apparent motions of the other planets, especially those at a greater distance from the Sun. According to Kepler's Laws the greater the distance of the planet from the Sun, the slower its speed in its orbit. This results in the Earth overtaking the superior planets at regular intervals (the synodic period). When the Earth passes one of the superior planets the line of sight to the planet moves backwards with respect to the fixed stars in the background and the planet appears to move backwards. This motion was considered to be a real motion rather than an observed phenomena by ancient astronomers and was a complicating factor in geocentric models of the universe.

The image to the right shows the motion of Mars against the background stars. Move the mouse pointer over the numbers on the bar at the bottom of the image to view a stage in the orbits of the planets. The yellow line represents the line of sight from the Earth to Mars. The panel at the bottom of the image shows the view of Mars as seen from the Earth. As you view progressive stages of the two planets moving around their orbits Mars seems to change its direction af motion as the Earth overtakes it. Move the mouse pointer over 'Autorun' to watch the animation in full.
It is also worth noting that while Mars is undergoing retrograde motion it is at its closest point to the Earth and therefore appears at its brightest in the sky.