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Retrograde motion can be real, or illusory

Retrograde motion: Mars making a retrograde loop in front of the stars.
An animation showing the retrograde motion of Mars in the summer of 2003. Planets typically move toward the east in front of the stars. When they move west, they’re said to be undergoing retrograde motion. The illusion is caused by our perspective, as seen from Earth. Image via Wikipedia.

Some retrograde motion is an illusion

As measured against the fixed stars, planets typically appear to move eastward. But, sometimes, they seem to pause briefly in this eastward motion and, for some months, to move westwards (backwards) in front of the stars. Then they pause again. And then they start moving eastward again. Astronomers (and astrologers) call their change in direction retrograde motion.

Though it baffled ancient stargazers, we know now that this type of retrograde motion is an illusion.

You can experience this illusion in an earthbound way, 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. But, as you pull alongside and pass it – from your vantage point in the faster car – the slower car may appear to move backwards for 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.

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, outer planet. Image via Wikipedia.

It baffled early astronomers

Early astronomers believed Earth lay at the center of the universe. And so they went to complicated lengths to attempt to explain retrograde motion in that Earth-centered universe. They theorized each planet not only orbited Earth, but also spun around a moving point on their orbit known as an epicycle.

Imagine whipping a ball on a length of string around your hand while you turned in place. That’s similar to the ancient view of retrograde motion.

When it became generally accepted that Earth and the other planets orbited the sun, suddenly 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. Image via Wikipedia.

Retrograde motion on other worlds

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 appears to move 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. The result is that, once a year, Mercury gets two sunrises on the same day!

Other retrograde motion is real

Astronomers also use the word retrograde to describe true backwards motion among planets and moons.

Venus, for example, rotates or spins on its axis in the opposite direction from every other planet in the Solar System. If the clouds ever parted, hypothetical Venusians would see the sun rise in the west and set in the east. Astronomers would say that Venus rotates in a retrograde manner.

Some moons also have retrograde orbits around their planets. In other words, most of the large moons orbit in the same direction that their planet spins. But that’s not true of Triton, for example, the largest moon of Neptune. It orbits opposite the direction of Neptune’s spin.

Among the smaller asteroid-like moons that swarm about the giant planets, many have retrograde orbits.

It’s the same word: retrograde. But now there’s no illusion. Whether speaking of a planet’s rotation – or its orbit – if it’s opposite what you’d expect, astronomers call it retrograde.

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? Image via NASA / Jet Propulsion Lab / U.S. Geological Survey.

How does it happen?

According to modern astronomers, a true retrograde orbit for an orbiting moon most likely stems from a capture. Triton, for example, might have come out of the Kuiper Belt, the region of icy debris beyond Neptune. Perhaps a collision in the belt sent Triton careening inward toward the sun. A close encounter with Neptune could have slowed it down, forcing it to settle into a backwards orbit.

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.

It’s puzzling, because planets form out of debris disks that orbit young stars. And those orbiting disks share the star’s rotation. So how does a planet end up with a true backwards orbit? The only way – according to today’s astronomy – is either by a near-collision with another planet, or if another star once passed too close to the system.

Either way, close encounters can disrupt the orbits of planets and set them on a backwards path!

Bottom line: Retrograde motion of Jupiter or Mars or Saturn in our sky is an illusion, a trick of perspective. But there’s a real retrograde motion, too.

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