What is retrograde motion and why does a planet like Mars appear to change directions in the night sky? Is it playfully looping about in its orbit around the sun or is something less strange going on? From episode 2 of Wonders of the Solar System, Professor Brian Cox explains why Mars Loops The Loop.
Though it baffled ancient astronomers, we know now that retrograde planets are an illusion caused by the motion of Earth… You can test this the next time you pass a car on the highway. As you approach the slower car, it is clearly moving in the same direction you are. Right as you pull along side and pass it, however, the car appears to move backwards for just a moment. As you continue pulling away, the car resumes its forward motion.
The same thing happens as Earth passes the slower moving outer planets. When we pass Jupiter, for example, the gas giant appears to reverse course in the sky for a couple of months.