Is your coffeepot clogging up when you brew your morning cup of java? Did someone just ghost you after a great first date? Is your smartphone inexplicably bricking whenever you try to open your favorite app?
What can be causing all these malicious misfortunes?
If you believe in astrology, you might blame Mercury. Starting on August 23 and extending through the next three weeks, you can bet that astrologers and their adherents, when confronted with bad luck, will shake their fists at the sky and snarl the phrase “Mercury is in retrograde!” like a curse.
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If you find this situation confusing, you’re not alone. What does the innermost planet have to do with events on Earth, and what does Mercury being in retrograde actually mean anyway?
Because I hate rhetorical questions, the answers are, in order, “nothing at all” and “cool astronomy stuff.”
To us, stars don’t appear to move relative to one another; they’re so far away that any real motion they have across the sky is reduced to a crawl that is so mind-numbingly slow that it takes years to discern, even with telescopes. This is why the familiar constellations we see today are scarcely changed from the ones people saw millennia ago.
But those bygone stargazers noticed something odd: some of those “stars” did move against the background of “fixed” stars night after night. The ancient Greeks called these moving lights planetes, meaning “wanderers.” We now call them planets and know them to be other worlds orbiting our sun.
If you were high above the solar system, looking down toward the sun’s north pole, you’d see the planets circling around in a counterclockwise direction, and each would take a different amount of time to complete one orbit. Mercury is closest to the sun and most in thrall to the star’s gravity. Its year lasts just 88 Earth days.
Earth orbits the sun farther out, so our view of Mercury is similar to that of someone watching a race car vrooming around a track. Imagine you’re just such a spectator cheering for your favorite driver. As their car starts from the track’s right side, you’ll see the vehicle move from right to left as it speeds around the far side. When it reaches the track’s far left side, the car’s motion appears to reverse itself. It moves from left to right as it zooms past you on the track’s near side. When it gets to the right side of the track, the process repeats.
For astronomers, our racetrack is a lot bigger. After Mercury appears as far to “the right” of the sun as it gets in our sky—what we call greatest western elongation—its apparent motion against the background stars is from west to east (right to left for Northern Hemisphere observers). We call this “apparent prograde motion,” meaning forward motion.
A few weeks later Mercury’s orbit takes the planet behind the sun from our viewpoint as it continues to move eastward. It eventually reaches its greatest eastern elongation from the sun, reverses course and starts to move westward in the sky. Now the innermost planet zips toward the sun again on the side of its orbit nearest to Earth. This time, it passes between Earth and our star, and it eventually reaches its greatest westward point from the sun. Then Mercury’s orbital dance begins once again.
That westward motion, from left to right, as we see it, is called “apparent retrograde motion.” The full extent of Mercury’s prograde-to-retrograde pattern takes somewhat longer than the planet’s 88-day orbital period because Earth is moving around the sun at the same time. After completing one three-month revolution around the sun, Mercury must move a little bit farther in its orbit to catch up and lap us. This prograde-to-retrograde cycle is known as a synodic period, and Mercury takes roughly 115 days to complete it.
All the sun’s planets exhibit this sort of backward-and-forward motion. Venus takes about 584 days to complete one synodic period, while Mars takes about 780 days. In the latter case, Earth’s motion is faster than Mars, so we’re the ones catching up to it. We see Mars show retrograde motion for about two months out of its 26-month orbit.
Mercury displays retrograde motion several times every Earth year. The next occurrence will stretch from August 23 to September 14.
Great! So that’s the cool astronomy stuff. But what does Mercury in retrograde have to do with electronics malfunctioning or social interactions taking a negative turn?
Well, nothing. Most fundamentally, the change from prograde to retrograde motion is an illusion born of our orbital perspective. Mercury is always moving in the same direction around the sun, just like a race car is always moving in one direction around the track. As our angle to either object changes, we see the apparent motion change direction, even as the real motion is constant.
Furthermore, despite astrologers’ claims, there’s no physical reason why Mercury’s motion, apparent or actual, would matter to your smartphone’s electronics or your social life here on Earth. For example, being the smallest planet in the solar system, Mercury also has the weakest gravitational field—meaning that the planet’s pull simply can’t affect us in any significant sense. Heck, Venus is more massive than Mercury and gets much closer to Earth, so if there were some physical way for remote planets to reach across space to wreck your life, then the retrograde motion of Venus would be the bane of our existence. But you don’t hear astrologers complain about that nearly as much.
In reality—where I prefer to live—stuff just sometimes breaks, and even friends can be fickle. If poorly understood cosmic forces constantly intervene in the details of our lives, it is puzzling indeed that no one has ever managed to empirically measure any such effect. Also, if you hear about the Mercury-in-retrograde effect, during these periods, you’re more likely to remember when things go wrong versus when they go right—a form of confirmation bias.
In fact, astrologers are right about one thing: knowledge of these motions can be helpful—just not in the way that they think. When, on the far side of its orbit and in the midst of its prograde cycle, Mercury passes behind the sun, the planet soon appears as an evening star—a lovely if somewhat dim point of light in the western twilight sky after sunset. I’m more of an evening than a morning person, so this arrangement makes viewing Mercury easier for me, and I’m always delighted when I can catch it in the gloaming. Some weeks later, when midretrograde Mercury passes in front of the sun, the planet becomes a morning star, and early risers—that is, people who are not me—can spot it toward the eastern horizon before sunrise.
This means Mercury’s retrograde motion is a blessing, not a curse! When the planet appears to reverse its orbital course, it also becomes much more visible in the evening sky. Note that “much more” is relative here; Mercury’s close proximity to the sun means that the planet only is visible close to the horizon around dawn or dusk and never appears in a truly dark sky. Right now, it sets shortly after the sun, about 25 degrees toward our home star’s east (Northern Hemisphere observers’ left). You’ll need binoculars if you want to spot it, and even then, it’ll be difficult to see.
So next time you hear someone muttering about Mercury in retrograde, remember: the fault lies not in our stars but in our head.