It’s human nature to see shapes and patterns all around us, and ascribe a meaning to what is actually just a random coincidence. The phenomenon is called pareidolia, and includes things like seeing the Virgin Mary in a piece of burnt toast, for example.
But some examples are more persistent than others — like the Man in the Moon. It’s not a real face, of course, just a quirk of how the dark areas (the lunar maria, or “seas”) and lighter highlands of the lunar surface are arranged. Yet the illusion is powerful enough to have a Western mythology dating back thousands of years, inspiring all manner of nursery rhymes and literary references.
And there’s some interesting physics at work here as well, at least according to a new paper in the journal Icarus. See, the Man in Moon is always staring at us here on Earth — or, if you want to be all science-y about it, those particular features of the lunar surface always face Earth.
It happens because the moon is locked in what’s known as a “synchronous orbit”: for every orbit it completes around the Earth, the moon also rotates exactly one time. So we always see that face.
But it didn’t necessarily have to be that way; why is this side of the moon, and not the other, the one that faces Earth? There had to be a 50/50 chance of it being one way or the other. Or so astronomers have thought — until now.
Two Caltech astronomers, along with an Israeli colleague, think that there’s a perfectly good explanation why the Man in the Moon always faces us — and it’s not due to the proverbial coin toss. Rather, Oded Aharanson, Peter Goldreich, and Re’em Sari propose that it’s due to the fact that the Moon spun around its axis much faster in the past than it does today. And the rate at which it gradually slowed its pace could explain why it eventually became locked in the current orientation.
When the moon formed some four billion years ago, it was a blob of hot molten stuff. The Earth’s gravitational pull stretched it a bit, elongating it like a football, and that shape stuck when the Moon cooled off. The Man in the Moon is at one of those oblong ends.
Back then — about a couple billion years ago — any inhabitants of Earth would have seen varying sides of the moon, not just the fixed face. But that relentless gravitational pull from Earth eventually slowed down the Moon’s rate of spin on its axis, and tidal forces created yet another bulge, one that moved around in such a way that it always pointed toward Earth.
And this is where the physics starts to get interesting. Per the official press release:
So far, so good, but it still seems like a bit of a coin toss when it comes to which side of the moon faces Earth. The Caltech team ran a series of computer simulations, plugging in many different rates of slowing, and found they could “load” the coin however they wanted, so that either side of the moon would always face the Earth when it hit that locking point — depending on that rotational energy dissipation rate.
For instance, there really would be a 50/50 chance of the current orientation if the rate of dissipation had been, say, 100 times faster. Instead, that rate was much slower, so there would be two-to-one odds that the Man in the Moon would find himself always facing Earth. This assumes, of course, that the properties of the present-day moon were similar to those in its distant past.
Not every culture sees a Man in the Moon. There are myths and legends based on perceiving a woman, a rabbit, a frog, a moose, a buffalo, or even a dragon in the full moon. But since those illusion arise from the same patterns of light and dark shadows, one assumes the same analysis would apply.
Images: (top) The “man in the moon.” Source: NASA Goddard/Arizona State University. (bottom) Still from Georges Melies’ silent film, La Voyage dans la Lune (1902). Public domain.