Astronomy Essentials

What is retrograde motion?

Retrograde motion of Mars in 2005. Astrophotographer Tunc Tezel created this composite by superimposing images taken on 35 different dates, separated from each other by about a week. See Tunc’s video of the Mars and Saturn in retrograde in 2016.

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.

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

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.

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

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!

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.

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 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 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.

February 6, 2017
Astronomy Essentials

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