The phrase “Mercury is in retrograde” used to be one of those phrases that I was half-aware of, like “Age of Aquarius”, that you could employ as a signifier of hippy woo-ness. But it has an interesting history.

Observations of the stars, back in pre-telescope days, showed that most rotated uniformly, with the sky. But a few of them moved independently. Those mobile ones were called “planetes”, wanderers. And while the fixed stars kept moving in one direction, the planets sometimes appeared to go into reverse.

We now know that this is because the planets orbit the Sun at different intervals, so periodically the Earth’s orbit “catches up” with Mars or Mercury or whatever. But under the geocentric model, where everything orbited the Earth, that explanation wasn’t available. So the Greek astronomer Ptolemy introduced the idea of “epicycles”: orbits that were not simple circles, but a series of loops. This explained the backwards motion.

This was a brilliant explanation! It wasn’t perfect but did an excellent job of predicting where planets would be at a given time. The Copernican, heliocentric model — the Sun is fixed in the middle, everything orbits it – did even better. It was wrong too — the Sun is itself orbiting the galactic core — but less wrong. We now know, though, that the apparent retrograde motion of Mercury and other planets is an artefact of our own perspective; Mercury itself follows a simple elliptical path, not a flower-like series of epicycles.

As it happens, Mercury is in retrograde right now. And BBC Radio 4’s Women’s Hour dedicated a segment to it earlier this week. But it didn’t mention Ptolemy or the geocentric model. Instead, it had an astrologer saying that because Mercury is a symbol of communication, Mercury in retrograde implies various forms of mayhem. “Triple-check the links before you send out an important mass email,” she says, sagely.

I don’t want to suggest that this is some huge deal, but I find it really weird. Wouldn’t it be worth mentioning that astrology isn’t, you know, real? It feels like the BBC breathlessly reporting on the outcome of a WWE match without mentioning that it’s all fixed.

Maybe I’m a terrible killjoy. But it seems to me that the real story of “Mercury in retrograde”, its place in humanity’s search for understanding of the universe, is a lot more interesting than some entirely made-up babble about it meaning you might bump into your ex. By all means have the babble if you must, but couldn’t you sneak a little of the history of science in while you’re about it?

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Tom Chivers is a science writer. His second book, How to Read Numbers, is out now.

TomChivers

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