The pattern depends on where Earth and Mars happen to be in their tilted racetrack orbits. Connect the dots, and you'll draw either a loop or an open zigzag. 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. It's as if the two planets were on separate tracks that are a little tilted with respect to each other. Just to make things a little more odd, the orbits that Earth and Mars follow don't quite lie in the same plane. This apparent erratic movement is called 'retrograde motion.' The illusion also happens with Jupiter and the other planets that orbit farther from the sun. 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. In both cases, retrograde motion occurs when the Earth and planet are lined. While we're passing by the red planet this year, it will look to us as though Mars is moving up and down. Venus prograde motion is caused by the Earths motion around the Sun. 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.Ībout every 26 months, Earth comes up from behind and overtakes Mars.
The two planets are like race cars on an oval track.
