What is Mercury Retrograde? Weird Planetary Movement Explained

If you’re into astrology, you might know that Mercury, the smallest planet in our solar system, is "in retrograde." Some people link this celestial event to weird goings-on here on Earth—whether its communication breakdowns, technological troubles or even presidential decision-making.

But what does Mercury retrograde actually mean?

A planet is “in retrograde” when it appears to change direction as it passes across our skies, NASA reports. Planets normally slowly pass from west to east, but every now and then their motion appears to snap back like an elastic band—east to west.

But retrograde planets aren’t actually flipping direction—it’s just an illusion created by our solar system’s orbital gymnastics. The phenomenon happens when a speedier planet overtakes a slower one in their orbits around the sun, NASA explained. This makes it appear to change direction in the sky.

EarthSky suggests you think of the planets like cars on a highway. As you overtake a slower car, it briefly appears to move backward from your viewpoint. As you pull in front of the car, it will look like it’s traveling forward again.

This false color image of Mercury enhances colors that indicate the chemical, mineralogical, and physical differences between the rocks that make up Mercury's surface. Carnegie Institution of Washington/Johns Hopkins University Applied Physics Laboratory/NASA

Mercury spins around the sun quite a bit faster than Earth. When its orbit takes it past our planet, it appears to change its direction of travel across the sky.

See all of the best photos of the week in these slideshows

It’s unclear how these astronomical motions are supposed to wreak emotional—and technological—havoc on Earth. As NASA explains on its Space Place website: “Astronomers and other scientists know that stars many light years away have no effect on the ordinary activities of humans on Earth.

“Astrology is something else. It's not science. No one has shown that astrology can be used to predict the future or describe what people are like based only on their birth date.”

But if you’re not convinced, this handy website will tell you whether or not it’s time to blame the ills of the world on the movement of its smaller, speedier cousin.

Beyond the weird motion of Mercury, this week is particularly exciting for astronomers. Friday evening and Saturday morning—depending on where you live—will see the longest lunar eclipse of the century. At least some of the blood moon will be visible from Asia, Africa, Europe, South America, Oceania and Antarctica. Most North American skywatchers won’t be able to see the blushing orb in person, but they can track the spectacle online. The U.S. will catch its next total lunar eclipse in January 2019.

Just in case a red moon isn’t special enough, the Red Planet will sparkle much brighter than usual as it creeps towards our planet. Mars will make its closest approach to Earth since 2003 on July 31.

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