Sometimes, as seen in Earth’s sky, the planets seem to move backwards!
Typically, the planets shift slightly eastward from night to night, drifting slowly against the backdrop of stars. From time to time, however, they change direction. For a few months, they’ll head west before turning back around and resuming their easterly course. Their westward motion is called retrograde motion by astronomers. Though it baffled ancient stargazers, we know now that retrograde motion is an illusion caused by the motion of Earth and these planets around the sun.
How does this illusion work? You can test it for yourself, the next time you pass a car on the highway. As you approach a slower car, it’s clearly moving in the same direction you are. As you pull alongside and pass it, however, from your vantage point the car appears to move backwards for just a moment. Then, as you pull ahead of it, the car appears to resume its forward motion.
The same thing happens as Earth passes the slower-moving outer planets. When we pass Jupiter or Mars or Saturn, for example, these more outward planets in orbit – which move more slowly than Earth in orbit – appear to reverse course in our sky for a couple of months.
Ancient astronomers – who believed Earth lay at the center of the universe – went to complicated lengths to attempt to explain retrograde motion. Theirs was a complex cosmology in which each planet not only orbited Earth, but also spun around a moving point on their orbit. Imagine whipping a ball on a length of string around your hand while you rotated in place. Astronomers like Nicolaus Copernicus and Johannes Kepler finally set us all straight when they realized Earth orbited the sun.
Suddenly, the retrograde motion made a lot more sense!
If you could see the sky from another planet besides Earth, retrograde illusions would lead to your seeing some very strange phenomena. On Mercury, for example, the sun sometimes moves in retrograde. As Mercury speeds through its closest approach to the sun, its orbital speed overtakes its rotational speed. An astronaut on the surface would see the sun partially rise, then dip back below the horizon, then rise again before resuming its east-to-west trek across the sky. Once a year, Mercury gets two sunrises on the same day!
But retrograde movement isn’t always an illusion.
There are real retrograde motions in the solar system. Venus, for example, rotates or spins on its axis in the opposite direction from every other planet. If the clouds ever parted, the Venusians would see the sun rise in the west and set in the east.
Some moons also have retrograde orbits around their planets. Most of the large moons orbit in the same direction that their planet spins. But not Triton, the largest moon of Neptune. It orbits opposite the direction of Neptune’s spin. And among the smaller asteroid-like moons that swarm about the giant planets, many have retrograde orbits.
A retrograde orbit for an orbiting moon most likely means that moon was captured after the planet formed. Triton might have come out of the Kuiper Belt, the region of icy debris beyond Neptune where Pluto lives. Perhaps a collision in the belt sent Triton careening inward toward the sun. A close encounter with Neptune could have slowed it down and forced it to settle into a backwards orbit around the distant planet.
In past decades, astronomers have also discovered planets in distant solar systems with retrograde orbits. These exoplanets orbit their suns in the opposite direction from how the star rotates. This is puzzling because planets form out of debris disks that orbit young stars, disks which share the star’s rotation. The only way to get a planet orbiting backwards is either by a near-collision with another planet or if another star once passed too close to the system. Close encounters tend to disrupt orbits.
So that is retrograde motion. Astronomers use the term to refer to the occasional backwards motion of the planets as seen in Earth’s sky. When used in this way, retrograde motion is entirely an illusion caused by the moving Earth passing the outer planets in their orbits. Meanwhile, real retrograde motions – of a planet’s spin on its axis, of moons orbiting planets, and even of planets in distant solar systems – are a sign of long-forgotten collisions and captures. Real retrograde motion is one of the clues that astronomers use to piece together the history of our solar system, and the systems of other stars in our galaxy!
Bottom line: An explanation of retrograde motion.
Christopher has a Ph.D. in astronomy from the University of California, Los Angeles. After eight years of searching for exoplanets, probing distant galaxies and exploring comets, Chris realized he enjoyed talking about astronomy a lot more than actually doing it. After being awarded a 2013 AAAS Mass Media Fellowship to write for Scientific American, he left a research career at the U.S. Naval Observatory to pursue a new life writing about anything and everything within the local cosmological horizon. Since 2014, he's been working with Science News.