Sometimes we hear that Mercury or Mars is in retrograde. But what does this actually mean?
Firstly, retrograde is a description of motion, where the orbital (or rotational) motion of an object is in the opposite direction of the central object (like our Sun). There is also prograde motion where the motion of the object is in the same direction.
In our Solar System, all the planets move around the Sun in the same direction (prograde), and at a constant speed. Most of the planets also spin on their axis (their day) in the same direction. There are two planetary exceptions - Venus which spins backwards (retrograde) and Uranus, which spins on its side.
It is no coincidence that the planets orbit and spin (mostly) in the same direction. The Sun as well as the planets and other objects in our Solar System, were formed from the same spinning ball of gas. As it started to collapse and have gravity pull it together into objects, they conserve angular momentum (like a figure skater spinning and moving their arms in and out), and end up going in the same direction.
However, as viewed from Earth (this can also happen on other planets), at certain times, the planets when compared to the background stars, can look like they are moving backwards - even though the planet has not changed its direction or speed. This is often what is referred to as retrograde motion.
Ultimately, this is an optical illusion of the apparent motion of the planets.
Apparent retrograde motion also isn't unique to planets. It can happen when any two objects move at different speeds in the same direction.
Next time you are in a car (and you are not driving), look how other cars on the road either passing you or you passing them look compared to the distant background trees or mountains. This optical illusion will not work for something really nearby, so you'll need the horizon or distant trees. What you'll see is that the cars moving at different speeds than you can look like they are moving backwards at times.
Retrograde motion is important and critical to our understanding of the Solar System. Thousands of years ago, one of the leading models of the Universe was that the Earth was at the center of the Solar System or Universe, not the Sun. However, there was a problem with this model in that people noticed stars sometimes moved forwards, and sometimes moved backwards. It was hard to reconcile this motion with this model.
In fact, that's where the name for planets comes from - wandering star, as these objects were wanderers.
Eventually, Copernicus showed that if we put the Sun at the center of the Solar System, not the Earth, this could explain how we can get planets that move forward and occasionally move backwards.
Retrograde motion is important to understanding how our Solar System works, however the way we see it, it is all an optical illusion. This means that while there is no effect on the Earth or us because of it, it changed our understanding of our place in the Universe.
- Brad E. Tucker is an astrophysicist and cosmologist at Mt Stromlo Observatory and the National Centre for the Public Awareness of Science at the ANU.
