You may have heard the phrase "Mercury is in retrograde" in popular culture, but what does that actually mean in an astronomical sense?
What are planets doing when they are "in retrograde"?
In ancient Greece, the planets were called "planete" which translates to "wanderers" since they moved across the sky in a way that was different to the stars.
Ancient peoples also noticed that these "wanderers" sometimes appeared to be moving backwards with respect to the rest of the stars - and this apparent backwards motion is what we call retrograde motion.
All the planets of our Solar System orbit the sun, and these orbits, or the year on a planet, are different lengths, and from our vantage point on Earth we see the other planets in different positions in their orbits around the sun.
You can think of the paths of the planets' orbits as a circular racetrack, and each planet stays in its own lane.
The planets closer to the sun finish an orbit (or a year) faster because they have a shorter distance to travel, whereas the planets further from the sun take longer because they have further to go.
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Since the Earth is on one of these racetracks, sometimes we will be "chasing" other planets, sometimes we will be equal with them and sometimes we will be ahead of them. Much like when you overtake a car on the highway.
This is how planets can appear to move backwards in the sky - that part where we catch up and overtake them can make it look like they are so slow they are moving backwards, and this is what we call retrograde motion.
The phenomena of retrograde motion caused all kinds of problems when people were trying to figure out the place of the Earth in the universe. If the Earth was at the centre of the Solar System, then not only did the theory have to explain how the sun orbited us, but how the planets orbiting us could appear to be going backwards.
Eventually the theory that the Earth was the centre of the Solar System was thrown out in favour of the sun being at the centre, which made it much easier to explain the motions of the other planets. Although there are still some oddities in the Solar System to be explained.
So far we have been talking about the appearance that things are moving backwards, but in the case of Venus, it's not just about appearances. Venus spins backwards on its axis, so that the sun rises from the west to the east, in contradiction with all of the other planets. This is a case of actual retrograde motion - probably caused by a large collision more than a billion years ago.
Understanding the retrograde motion of the planets was an important step in helping us to better understand the universe around us, and our place in it. So when a planet like Mercury is in retrograde, it simply means that it appears to be moving backwards on the night sky.
- Eloise Birchall has a Masters of Astronomy and Astrophysics (Advanced) from the Australian National University.