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Sorry, astrologers -- this is what it really means when a planet is in retrograde

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You usually hear about a planet being "in retrograde" from astrologers, which might give you the impression that the term is mystical jargon. In fact, it does have an astronomical meaning. That meaning just happens to come from a rather outdated concept, when the Earth was still the center of the solar system. Today, it explains why planets seem to go backwards in the sky, and why one planet can have two sunrises a day.

Astronomy during antiquity and pre-history was, in some ways, much like astronomy today. People looked up at the sky and recorded what they saw. For the most part they saw stars that zoomed past in the same configuration every night. Then they noticed a few exceptions, which moved independently across the night sky on a specific course. These special stars eventually came to be understood as planets, specifically Mercury, Venus, Mars, Jupiter, and Saturn, but originally they were just seen as moving stars. Astronomers tracked their paths across the sky, until they assumed a regularity that was almost as certain as the fixed stars.


Then, one night, some astronomer looked at the sky, noticed that one of these moving stars had started moving backwards, and thought, "This can't be good." That set the precedent for the astrological concept of retrograde astronomical motion as an omen of bad times for people on Earth. Although this odd motion was eventually fit into just another pattern, no one knew exactly why it happened. As long as the Earth stayed unmoving at the center of the solar system, no possible explanation made sense.

When people finally understood that Earth and the traveling stars were actually all planets moving around a central sun, things became a bit more clear. The planets are essentially lined up on a race track with widely-spaced lanes. In the case of Mercury, the lane it's on is shaped a bit differently from Earth's course around the sun. Mercury has the kind of extreme elliptical orbit that makes its forward progress around the sun seem to slow considerably as it enters the "pointed" end of the ellipse. The planet's speed doesn't actually alter, but if you are driving in a straight light and a car to the side of you begins to turn, it seems to slow down, even if it's keeping the same pace.


As the Earth keeps going on its rounder orbit and Mercury turns into its ellipse, it seems to move backwards in the sky. The other planets don't have the same extreme orbit as Mercury, but they occasionally seem to go backwards as the Earth zooms by them, too. It's similar to the way cars on a motorway appear to be moving backwards briefly from the point of view of the faster car that overtakes them.

The Earth isn't the only planet to see other planets in retrograde. And planets aren't the only things that can suddenly seem to reverse their motion. If you want to see a real show-stopper, go to Mercury. The planet's elliptical orbit means its progress past the sun speeds and slows. Taking a sharp curve can make one leg of the journey round the sun take forever, while during the shallow stretches of the elliptical orbit it can zoom past a section of sun. Meanwhile, its rotation is very slow. This doesn't make a difference when its orbit around the sun is slow, too, but when it reaches the relatively straight section of its orbit, it can have spectacular results. Mercury rotates enough to get the sun over the horizon and up into the sky, just like it does on Earth. Then the planet's fast orbit moves it far enough along its path around the sun that the sun seems to dip back down below the horizon, until the rotation can make the sun rise again. Mercury can put the sun in retrograde. That's got to be bad luck.

Image: NASA, Messenger

Via Earthsky and Universe Today.