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| Space on Nov 25, 2012

What is retrograde motion?

Sometimes the planets appear to change direction in the sky. This retrograde motion is entirely an illusion caused by the Earth passing the slower moving outer planets.

Sometimes, 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. This is “retrograde motion”. Though it baffled ancient astronomers, we know now that retrograde planets are an illusion caused by the motion of Earth.

Animation of Mars in retrograde

An animation showing the retrograde motion of Mars in summer of 2003. Credit: Eugene Alvin Villar (via Wikipedia)

You can test this the next time you pass a car on the highway. As you approach the slower car, it is clearly moving in the same direction you are. Right as you pull along side and pass it, however, the car appears to move backwards for just a moment. As you continue pulling away, the car resumes its forward motion.

The same thing happens as Earth passes the slower moving outer planets. When we pass Jupiter, for example, the gas giant appears to reverse course in the sky for a couple of months.

Illustration of retrograde motion

A schematic of how retrograde motion works when Earth (T) passes an outer planet (P) as they both orbit the sun (S). The changing viewing angle from Earth makes the projection of the planet against the celestial sphere (A) move backwards (A2-A4) as we pass the slower planet. Credit: Wikipedia user Rursus

This makes the planets appear to move very strangely. Ancient astronomers went to complicated lengths to try and explain these motions. They envisioned each planet not only orbiting the center of the solar system (which to them was Earth) but also spinning 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 Copernicus and Kepler finally set us all straight when they realized  Earth orbited the sun.

Suddenly, the retrograde motion made a lot more sense!

The Ptolemaic solar system

A schematic of how astronomers envisioned the motion of the planets before Copernicus. The Earth sat near the center of the universe. The planets moved around a small circle (the epicycle) which in turn moved along a larger circle (the deferent). The deferent was centered on a point (X) midway between the Earth and another spot called the equant. This complicated setup was needed to explain the complex motions of the planets. Credit: Wikipedia user Fastfission.

Retrograde illusions on other planets can lead to very strange phenomena. On Mercury, for example, the sun sometimes moves in retrograde! As the planet speeds through its closest approach with 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 rotates 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 their planet spins. But not Triton, the largest moon of Neptune. And among the smaller asteroid-like moons that swarm about the giant planets, many have retrograde orbits.

Triton

A photomosaic from Voyager 2 of Neptune’s largest moon, Triton. The moon orbits Neptune opposite the direction that the planet rotates. Does this mean that Triton came from the Kuiper Belt and was eventually captured by the ice giant? Credit: NASA / Jet Propulsion Lab / U.S. Geological Survey

A retrograde orbit most likely means the moon was captured after the planet formed. Triton probably came 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 the past decade, astronomers have also discovered planets in other 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.

Retrograde motion refers to the occasional backwards motion of the planets. It is entirely an illusion caused by the moving Earth passing the outer planets in their orbits. Real retrograde motions—of planet rotation, orbiting moons, and planets in other solar systems—are a sign of long forgotten collisions and captures. They are one way that astronomers piece together the history of our solar system, and the systems of other stars in our galaxy!