If you were to look up in the eastern sky at the same time each night and note where Mars appears to be compared to the constellations of stars, you would find the planet a little farther east with each viewing. That is, Mars appears to move from west to east from one night to the next.
Every two years or so, there are a couple of months when Mars' position from night to night seems to change direction and move east to west. This strange behavior was very puzzling to early skywatchers. Did the planet really stop, back up, change its mind, and then continue to move forward? Did it have some weird, mystical meaning?
Today we know what's going on. It's an illusion, caused by the ways that Earth and Mars orbit the sun.
Animation of Mars Retrograde using 2003 as an example.
Mars Retrograde Happens Every Two Years
The two planets are like race cars on an oval track. Earth has the inside lane and moves faster than Mars -- so much faster, in fact, that it makes two laps around the course in about as much time as it takes Mars to go around once.
About every 26 months, Earth comes up from behind and overtakes Mars. While we're passing by the red planet this year, it will look to us as though Mars is moving up and down. Then, as we move farther along our curved orbit and see the planet from a different angle, the illusion will disappear and we will once again see Mars move in a straight line.
This apparent erratic movement is called "retrograde motion." The illusion also happens with Jupiter and the other planets that orbit farther from the sun.
Just to make things a little more odd, the orbits that Earth and Mars follow don't quite lie in the same plane. It's as if the two planets were on separate tracks that are a little tilted with respect to each other. This causes another strange illusion.
Suppose you were to draw a dot on a sky map each night to show where Mars appears as it moves forward, goes through retrograde, and then resumes its forward motion. Connect the dots, and you'll draw either a loop or an open zigzag. The pattern depends on where Earth and Mars happen to be in their tilted racetrack orbits.
Two Patterns In Retrograde Motion: Examples
These apparent patterns caused by retrograde motion do not occur each evening.
The patterns would appear if you charted Mars' position in our night sky over several
months' time (during retrograde).
Image Credit: NASA/JPL-Caltech