Racecars, Retrograde Motion, and the Red Planet
To our ancestors, the stars moved from East to West across the sky just like the sun, completing one crossing every night. It appeared that the heavens orbited the Earth, so it was only natural to suppose that we are the centre of the universe, and everything moves in perfect circles around us.
However, something threatened this perfect view: the planets. Although they looked like points of light just like the stars, the planets wandered in their own paths against the background sky. Sometimes, their motion even seemed to reverse direction for a while, and they’d loop around before continuing on their normal path. This is known as retrograde motion, and astronomers tried extremely hard to make it fit into their geocentric model of the universe.
Ptolemy proposed the concept of epicycles, where the planets completed orbits within their orbits, moving in smaller, looping paths on their way around the Earth. As you can imagine, this model got extremely complicated very quickly, creating layers upon layers of epicycles to explain the complication of the heavens. Astronomers realised that it wasn’t entirely accurate, because it often failed to predict the movements of the planets as precisely as it should. Nonetheless, it was the best model they had for centuries.
Cue Copernicus, who suggested a teeny adjustment to model: how about we put the Sun at the centre of the solar system, instead of the Earth?
After much grumbling, astronomers realised that this actually simplified all their complications. They could scrap the epicycles entirely, and with Kepler’s refinements—realising that planets orbit in ellipses, and not circles—we suddenly had a precise model that explained away retrograde motion.
So if planets aren’t moving in orbits within orbits, what’s happening when they loop around?
Simple physics tells us that planets further from the sun orbit more slowly than those closer to the sun. Because Mars is further out than Earth, it orbits slower. Imagine Earth and Mars are racecars, with Earth on the inside lane and Mars on the outside. Earth is travelling more quickly, so at regular intervals, it overtakes Mars on the inside and zooms on around the Sun, while Mars continues along behind, never to catch up.
The trick with retrograde motion is that it’s all in the perspective. So let’s think about what this overtaking motion looks like from Earth. At first, as Earth approaches to overtake, Mars looks like it’s moving the same way as Earth is. As a passenger in Earth’s car looks forward at Mars, they see—in a snapshot of time—Mars set against a particular part of the crowd. We’ll use a person as a landmark: say there’s a guy with a hotdog.
But Earth is moving so fast that it overtakes Mars before it reaches the hotdog guy itself. At the moment Earth overtakes, our passenger sees Mars against a different crowd—a lady draped in a flag, sitting a whole block of seats before hotdog guy. To the passenger, it seems like Mars has moved backwards a little bit, relative to the crowd.
Then, as Earth completely overtakes and zooms past, the passenger must look back at Mars. This time, they see Mars against yet another crowd: there’s a yelling toddler on her dad’s shoulders, who is sitting a whole block of seats before the flag lady. Mars has moved even further back again.
When Earth and Mars both speed on a little longer, things return to normal, because to our passenger, Mars seems to pass the yelling toddler, the flag lady and the hotdog guy, and then it’s moving forward relative to the crowd again.
Because we understand how cars move, we know that Earth and Mars are both moving forward the whole time, and it’s only the passenger in Earth’s car who thinks Mars is looping back on itself. The backward motion is only an illusion—a trick of perspective from the limited view of Earth’s passenger seat.
For the real Mars, the illusion is the same: we only think Mars is moving backwards. From Earth’s faster vantage point, we’re looking at Mars against not a different crowd, but a different set of background stars. This 2-minute video will help you visualise it.
One last thing: if Earth and Mars orbited on exactly the same plane, the retrograde motion of Mars would just be the red planet moving back and forth along a straight line. But because Mars is orbiting around the Sun at a slightly tilted angle relative to Earth’s orbit, we see this beautiful, curved looping motion that so fascinated those who came before us.