Astronomy Fun Fact #37

You've probably heard of planetary retrograde motion, which is when a planet appears to move backwards from our perspective here on Earth. Well, it's totally an optical illusion.

All of the planets in the solar system are orbiting the Sun in the same direction, but at different speeds. That means that planets sometimes lap other planets, or move ahead of them in their orbit.

When the Earth laps another planet or gets lapped itself, it can look like that other planet is moving backwards. This is kind of like how a car looks like it's moving backwards if you speed past it on the highway. Of course it's not moving backwards, but your relative speeds create the illusion.