What is Retrograde Motion of the Planets?

For centuries during the ancient times, mankind grew puzzled by the bizarre polka-dot pattern across the night sky. Every spec and dot appeared to be the situated in the same position night after night. Eventually, changes were noticed when some of the dots altered their positions within the fixed starry background of the night sky. You can almost imagine what kind of thoughts and assumptions passed through the minds of the ancient Greeks at the time. The rambling behavior of these moving dots caused the Greeks to dub them as the wanderers. It was from this Greek term that we get the present term we have all become familiar with - the Planets.

But the Greeks noticed the wanderers behaved strangely. Normally they moved across the sky just as the Sun and moon did--east to west. But once in awhile, they seemed to actually stop and then go backwards.This was what led to the discovery of Retrograde Motion of the planets.

From our perspective on Earth, from time to time, the planets seemingly switch direction as they move across the sky. Normally they move from east to west every night due to the rotation of the Earth. This motion is called direct or prograde.

As the Earth catches up with a planet in its orbit, the planet will appear to halt its journey to the east and then rotate back to the west. Then as Earth moves past the planet, the west to east motion appears to resume as normal. This is particularly apparent with planets beyond Earth, as Earth moves faster in its orbit than they do.

Mercury and Venus also have retrograde motions but not as often. The planets that are further from the Sun retrograde more often. For instance, Mars goes into retrograde for a period of 72 days every 25.6 months and Jupiter goes into retrograde for 121 days every 13.1 months.

This first image below is that of a time lapse composite photo of Mars illustrating its apparent retrograde motion.

This second photo below is a drawing of Jupiter's apparent meanderings in 2002 to 2003.

There is not a single planet in the solar system that has true retrograde orbital motion. But some outer moons of Jupiter and Saturn orbit the planets in retrograde. These are thought to be captured asteroids. Once a small celestial body is captured by a giant planet it will in most cases go into a retrograde orbit.

Neptune's largest moon, Triton, orbits retrograde. It may also be a captured body having come from the Kupier belt, a region of icy asteroids and rocks similar to the asteroid belt but on the outskirts of the solar system.

Mars composite: University of Edinburgh Royal Observatory http://www.roe.ac.uk/~rsc/me.html

Jupiter Retrograde motion: University of Massachusetts http://www.astro.umass.edu/~myun/teaching/a100/images/retrograde.motion.jpg