Retrograde motion for Mars starts December 7

The retrograde or “backward” motion of an outer planet – like Jupiter, Saturn or Mars – is an illusion, a trick of perspective. Mars will begin its retrograde motion on Saturday, December 7, 2024. This is a milestone in the mutual orbits of Earth and Mars around the sun. It means Mars is about to appear at its brightest in our sky!

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. They reach what astronomers call a stationary point. Then, for some months, the planet moves westward (backward) in front of the stars. That’s what’s happening with Mars today. It reaches a stationary point – for the first time in about two years – at 21 UTC on December 7, 2024. Soon, Mars will begin moving opposite its normal motion, westward in front of the stars. Astronomers (and astrologers) call a planet’s westward motion its 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 is happening as Earth prepares to pass the slower-moving planet Mars on January 15-16, 2025. When we go between the sun and Mars (or another outer planet), these planets – all of which move more slowly than Earth in orbit – appear to reverse course in our sky.

Retrograde motion for Mars will continue through mid-February 2025. Afterwards – as Earth pulls ahead of Mars in our smaller, faster orbit – we’ll see Mars resume its eastward motion again.

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Read more: Why is Mars sometimes bright and sometimes faint?

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.

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!

i am once again tapping the “this is how retrograde motion works” sign pic.twitter.com/ISDtoOyyiy

— Seven Machina Rasmussen (@toomanyspectra) April 22, 2024

Other retrograde motion is real

Astronomers also use the word retrograde to describe true backward 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.

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 backward 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 backward orbit? The only way – some astronomer’s believe – 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 backward path!

Bottom line: The red planet Mars will start its retrograde motion on December 7, 2024. It’ll pause briefly in front of the stars, and then begin moving westward.

Read more: Why is Mars sometimes bright and sometimes faint?