The Retrograde Motion of Mars

 

Each year, the progress of Mars is interrupted in its normal west to east journey as the planet performs a peculiar loop among the stars. The loop is called the retrograde or backward motion of Mars.

This happens as our planet Earth moves between Mars and the Sun. Our diagram is not to scale and includes only the Sun, Earth and Mars, but it will do as a small model of what happens each year.

First off, Mars doesn’t actually slam on the brakes, throw itself into reverse, and so on. The retrograde motion of the planet is actually an optical illusion that’s caused by a change in our line-of-sight view of Mars against a background of visible stars.

Because Earth is closer to the Sun and completes its orbit faster (our year is 365 days while a year on Mars would be 687 Earth days long), we get a change in perspective that places Mars against different background stars as our planet approaches, comes alongside (so to speak), and passes Mars in its orbit. You can see this effect in our diagram as the view of Mars from the Earth places it against different background stars depending on where the Earth is in its orbit.

You might compare the apparent backward motion of Mars as well as the accompanying change of background stars to the views a jockey riding a fast horse in a race at the track might have. Our jockey (we’ll call him Willie) is going to have to come from behind in the closing stretch to win the race, but Willie is confident because he’s got the faster horse and an inside track. As Willie comes past the grandstand in the final stretch, he can see the lead horse out in front and, in the distance, the far end of the grandstand will be in his line of sight. When he is just nosing out his opponent at the finish line, Willie can look across at the other horse and jockey and will also see the center of the grandstand beyond them. After Willie is well past and looks over his shoulder at the other horse and rider, he will have the other end of the grandstand as a backdrop.

And there you have it! If all this still seems a bit confusing, try setting up objects on a table and try different eye-lines to get a feel for it. And if it’s still a riddle, don’t worry too much; back in the Dark Ages when the Earth, not the Sun, was placed at the center of our solar system they had a really tough time figuring out the cause of the retrograde motion of Mars.