Have you ever heard someone describe a planet as moving in retrograde, or in other words, as moving in reverse?
Since the planets all orbit the Sun in an unchanging direction, how can this be possible?
When we talk about a retrograde motion of a planet, we're talking about its apparent movement relative to the stars, from our perspective on Earth.
As astronomers, we're used to watching objects move across the sky. But much of this is due to the apparent movement of the celestial sphere when seen from Earth.
Take the stars, for example. You can see them moving across the night sky in a matter of hours, but this is of course down to the fact that Earth is spinning while it orbits the Sun.
The stars aren't really speeding across the sky. They just appear to be, from our perspective, and photographers are found of capturing this in long-exposure images known as star trails.
This same apparent motion can be said for the planets, too.
And once in a while, if you observe the movements of the planets along the ecliptic night after night, you might notice one suddenly appearing to stop, then beginning to move backwards.
We call this apparent backwards movement 'retrograde motion'.
Movement of the planets
The planets normally move from west to east relative to the stars.
If you look skywards at the same time of night on two different nights, you’ll notice that the planets have moved to the east.
This is because all the planets, as viewed from above, orbit the Sun in an anticlockwise direction. This is known as normal or direct motion.
It’s generally only comets, with eccentric orbits, that travel around the Sun in a clockwise, or retrograde, direction.
Why retrograde happens
Every once in a while a planet will also appear to move in a retrograde direction.
Mars retrograde is a well-known and well-observed example.
Its motion will first appear to slow down, then stop for a short while, and then start up again, but in the opposite, westward direction.
Eventually it will stop again and resume its eastward movement.
Nicolaus Copernicus was an early astronomer known for arguing that the Sun and not the Earth was at the centre of the Solar System (heliocentrism vs geocentrism).
He reasoned that planets further away from the Sun would move more slowly than closer ones.
As a faster planet overtakes Earth, its motion against the stars, as seen from Earth, reverses.
The same explanation works for the outer planets too, only now it is the Earth that overtakes the other planet.
Before Copernicus, astronomers had difficulty in marrying their belief that the Earth was the centre of the Solar System with the retrograde motion of the planets.
Their solution involved the planets each moving in mini-orbits called epicycles, while they travelled around the Earth on their main orbits.
The combined effect of these two motions would occasionally produce a westward motion of the planet when viewed from Earth.
This article originally appeared in the March 2006 issue of BBC Sky at Night Magazine.
