Retrograde Motion

This animation shows how the planet Mars appears to stop going eastward, goes back towards the west, and then resumes its original eastward journey (relative to the "fixed" stars). Such motion is referred to as retrograde. When a planet is going in the normal or eastward direction, the motion is called prograde. All planets exhibit retrograde motion.This animation shows the planet Mercury moving in a retrograde fashion. Here the blue sky is turned off, and you can clearly see Mercury circling the Sun even as it makes a retrograde loop relative to the "fixed" stars.

The image to the left shows how the Earth, moving faster in its orbit than Mars, makes Mars seem to move retrograde, relative to the "fixed" stars. Click the image to enlarge it.

Please note that the word "retrograde" applies both 1) to a planet's apparent movement relative to the "fixed" stars, as viewed from Earth, and 2) to a planet's non-standard rotation around its own axis. Because of the way planets form as their home star forms, we expect planets to rotate in the same way as they orbit their home star.

For our Solar System, viewed from above the Ecliptic Plane, i.e., from above the Earth's North Celestial Pole, all the planets orbit the Sun in a counterclockwise (CCW) fashion. From the same perspective, all of the planets also rotate CCW around their axes of rotation, except Venus. Venus rotates very slowly in the opposite, or clockwise (CW), fashion. We call such planetary rotation retrograde rotation. Because of this retrograde rotation, a Venusian sidereal day is longer than a Venusian solar day (213 Earth days vs 117 Earth days).