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

You are watching: The apparent westward movement of a planet

Sometimes, as checked out in Earth’s sky, the planets it seems ~ to relocate backwards!

Typically, the planets change slightly eastward native night to night, drifting slowly versus the backdrop of stars. Native time come time, however, they readjust direction. For a few months, they’ll head west before turning back around and resuming your easterly course. Their westward activity is called retrograde motion through astronomers. Despite it baffled old stargazers, we know now the retrograde movement is an illusion led to by the movement of Earth and these planets around the sun.

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

How does this illusion work? You can test it because that yourself, the following time you happen a automobile on the highway. As you strategy a slow car, it’s plainly moving in the exact same direction girlfriend are. Together you pull alongside and also pass it, however, from her vantage point the car shows up to move backwards for simply a moment. Then, together you traction ahead the it, the car shows up to resume its front motion.

The exact same thing wake up as planet passes the slower-moving outer planets. Once we happen Jupiter or Mars or Saturn, for example, these more outward planets in orbit – i beg your pardon move an ext slowly than planet in orbit – appear to reverse course in our skies for a couple of months.

A schematic of exactly how retrograde movement works when earth (T) overcome an external planet (P) as they both orbit the sunlight (S). The changing viewing edge from planet makes the estimate of the planet against the celestial sphere (A) move backwards (A2-A4) together we pass the slower planet. Credit: Wikipedia user Rursus

Ancient astronomers – who thought Earth lay in ~ the facility of the world – go to facility lengths to attempt to define retrograde motion. Theirs was a facility cosmology in which each earth not just orbited Earth, but additionally spun around a moving point on their orbit. Imagine whipping a ball on a length of string about your hand while girlfriend rotated in place. Astronomers choose Nicolaus Copernicus and Johannes Kepler finally collection us every straight once they realized planet orbited the sun.

Suddenly, the retrograde movement made a lot more sense!

A schematic of just how astronomers envisioned the activity of the planets before Copernicus. The earth sat close to the center of the universe. The planets moved approximately a little circle (the epicycle) which consequently moved along a larger circle (the deferent). The deferent was centered on a allude (X) midway between the Earth and another spot dubbed the equant. This complex setup was required to define the facility motions of the planets. Credit: Wikipedia user Fastfission.

If you might see the skies from an additional planet as well as Earth, retrograde illusions would result in your see some an extremely strange phenomena. Top top Mercury, for example, the sun periodically moves in retrograde. As Mercury speeds through its closest strategy to the sun, its orbital rate overtakes its rotational speed. An astronaut on the surface ar would watch the sun partially rise, climate dip ago below the horizon, then rise again prior to resuming that is east-to-west trek across the sky. When a year, Mercury gets 2 sunrises top top the same day!

But retrograde activity isn’t constantly an illusion.

There are actual retrograde movements in the solar system. Venus, because that example, rotates or spins top top its axis in the contrary direction native every various other planet. If the clouds ever parted, the Venusians would see the sun climb in the west and collection in the east.

Some moons additionally have retrograde orbits roughly their planets. Many of the big moons orbit in the same direction the their earth spins. But not Triton, the biggest moon the Neptune. That orbits the contrary the direction of Neptune’s spin. And amongst the smaller sized asteroid-like moons that swarm around the large planets, numerous have retrograde orbits.

A photomosaic indigenous Voyager 2 the Neptune’s biggest moon, Triton. The moon orbits Neptune the contrary the direction that the planet rotates. Does this median that Triton came from the Kuiper Belt and also was eventually recorded by the ice cream giant? Credit: NASA / Jet Propulsion lab / U.S. Geology Survey.

A retrograde orbit for an orbiting moon many likely method that moon was recorded after the world formed. Triton could have come out of the Kuiper Belt, the region of icy debris past Neptune wherein Pluto lives. Possibly a collision in the belt sent out Triton careening inward towards the sun. A close encounter v Neptune could have slowed the down and forced it to settle right into a backwards orbit approximately the far-off planet.

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In previous decades, astronomers have also discovered planets in remote solar systems with retrograde orbits. These exoplanets orbit their suns in the contrary direction from just how the star rotates. This is puzzling because planets type out that debris disks that orbit young stars, disks i beg your pardon share the star’s rotation. The only way to obtain a earth orbiting backwards is either by a near-collision with an additional planet or if one more star as soon as passed as well close to the system. Close encounters tend to disrupt orbits.

So the is retrograde motion. Astronomers usage the hatchet to refer to the occasional backwards movement of the planets as checked out in Earth’s sky. When used in this way, retrograde activity is totally an illusion led to by the moving planet passing the outer planets in their orbits. Meanwhile, real retrograde motions – of a planet’s spin on the axis, of moons orbiting planets, and even that planets in remote solar equipment – space a authorize of long-forgotten collisions and captures. Real retrograde motion is among the clues that astronomers usage to piece together the history of ours solar system, and also the systems of other stars in our galaxy!