Why the Man in the Moon is Always 'Looking' at Earth

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.

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:

The bulge continued to point toward Earth as the moon rotated through it, causing the moon's interior to squish and flex as the bulge changed position. The internal friction from this flexing acted as a brake that slowed the moon's spinning until its rotation rate matched its revolution rate, when it settled into a synchronous orbit.

In this way, as a result of Earth's gravity, the moon became locked into an orientation with its long axis pointing toward our planet.

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.

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