Mercury retrograde, explained without astrology

Astrologers regularly blame Mercury retrograde for a variety of everyday communication problems. But underneath those interpretations lies a much more interesting story about the limits of our Earth-bound perspective and the discovery of the solar system.

Every four months or so, the planet Mercury goes into apparent retrograde motion. This isn’t significant for astronomers, who have understood this phenomenon as a basic fact of the solar system for some 400 years. But it’s a religious holiday of sorts in astrology, and an increasingly popular topic on the internet.

Maybe Taylor Swift has something to do with that:

But don’t be spooked if you’re a science-minded person. There’s an interesting story for you here too. Apparent retrograde motion of planets is an illusion generated by the combined movements of Earth and the observed planet (in this case, Mercury, but it could be any planet).

For reasons that are much easier to explain in the video above than in text descriptions, planets appear to temporarily reverse the direction of their orbit, from the point of view of Earth, whenever they pass by Earth or when Earth passes by them.

This is clearest with Mars, which we lap every two years. The image in the thumbnail of the video above, captured by astrophotographer Tunç Tezel, is actually Mars retrograde, not Mercury. I couldn’t find any images of Mercury retrograde, and I suspect they’re nearly impossible to make since the planet is so often obscured by sunlight. If a planet makes an illusory loop and no one sees it, is it an illusion at all? If so, Mercury retrograde happens three or four times each year.

So why do astrologers think the fake backward motion of a rock 48 million miles away could make you get into a fight with your mom or break your iPhone? I’ve searched on astrology websites for some claim to a physical mechanism, but, refreshingly, they don’t bother. The scheme appears to require belief in Roman mythology, which assigned communication to the god Mercury. He has since undergone a bit of mission creep. From Astrologyzone.com:

Mercury rules all types of communication, including listening, speaking, learning, reading, editing, researching, negotiating, selling, and buying. Mercury also rules all formal contracts and agreements, as well as important documents such as book manuscripts or term papers, agreements, deeds, contracts, leases, wills, and so forth. Included under this planet’s domain are all types of code, including computer codes, as well as transportation, shipping, and travel.

I’ll let you decide what that means for your life, but for an astrology-free explanation of retrograde motion and its role in the history of astronomy, watch the full video on the Vox YouTube channel.