What does "Retrograde" mean?
You've probably heard the term "retrograde", likely in conjunction with the planet Mercury. But have you ever wondered what it really means when a celestial object, or anything for that matter, is in retrograde?
Firstly, retrograde refers to anything, although especially planets, moving backwards. You or I could technically be in retrograde if we moon walk! But when someone says, for example, "Mercury is in retrograde", they're describing the visual phenomenon of Mercury appearing to "moonwalk" or move backwards throughout the night sky. Crazy right? Is Mercury, or any object, literally moving in reverse?
Before you start worrying your head about the implications and consequences of such odd motion, the answer to the previous question is no. Celestial objects (or planets at least) do not literally move backwards.
This visual effect is called apparent retrograde motion, meaning the planet only seems to move backwards from one's perspective on Earth. In reality, it continues its orbit in a normal forwards fashion. This illusion is caused by differing and relative speeds of Earth and other celestial bodies as they orbit the Sun.
Earth takes 365 days to orbit the Sun, but Mercury circumnavigates the Sun in 88 days. As Earth catches up to Mercury (who you can think of as having "lapped" Earth in a track race) and looks back at Mercury, it creates the illusion that it is moving backwards in the sky.
Here is a helpful visual explanation!
