For millennia, people have looked up at the full Moon and sworn that they could see something up there. Depending on the culture, it's anything from a face to a dragon. But why is anything there in the first place?
The Man in the Moon - which we'll use as the collective name for everything cultures have seen on the face of the Moon, which also includes a human figure, a rabbit, and a buffalo, among others - is pretty weak, as optical illusions go. Certainly, it's far less eye-popping than the Face of Mars (assuming you haven't seen the closeup) or the Old Man of the Mountain (assuming you visited before 2003).
Its big advantage is its location - it stretches all across the biggest, brightest object in the night sky, and it's there to be observed every month without fail. That's plenty of time to see things that aren't really there - and for a guide to all the permutations of pareidolia people have seen on the Moon's face, check out the image on the left. What's amazing isn't really that people see things in the Moon - it's that the Moon is oriented just right so that the illusion is there to be seen in the first place.
That's the finding of researchers at Caltech and the Hebrew University of Jerusalem, who traced the remarkable line of physics coincidences that combined to make the Moon appear as it does. For instance, the reason see the Man in the Moon so clearly is that the Moon is in geosynchronous orbit, meaning we always see the same face. If humans were around a couple billion years ago, we would have seen the Moon when it rotated much faster, meaning its face relative to Earth was always changing.
However, the Moon lost its rotational energy over time and became locked in place relative to us. When the Moon was first forming, the Earth's gravity stretched the satellite out of its original spherical shape so that it had two elongated ends - these ends correspond with the near and far sides of the Moon, and when the Moon was in the process of getting locked into synchronous orbit, it was pretty much guaranteed that one of these two elongated ends would end up facing Earth, because of the unequal distribution of mass.
The weird thing is that there's no immediately obvious reason why the near side of the Moon ended up being the near side, and why the far side is the far side. If anything, the situations seem like they should be reversed, considering the far side has bigger mountains and higher topography, which means more mass. The near side just has a bunch of smooth, dark plains formed by ancient lava - the same splotches that create the illusion of the Man in the Moon. Basically, the very natural features that create the illusion should also have worked against us ever actually seeing it.
But as the researchers discovered, the real determinant of which face we ended up seeing wasn't mass - it was the rate at which the Moon slowed its rotation down. The researchers found that depending on how quickly the Moon lost its rotational energy, the odds of one side ending up as the near side would change dramatically. Using our current best estimate, the odds were about two-to-one that the Man in the Moon would have ended up facing us. The researchers' computer simulations showed that if the Moon had lost energy 100 times faster, then the odds would truly have been 50/50, and tweaking the speed gives every other possible set of probabilities.
This isn't the end of the story - for one thing, there's no guarantee that the Moon had the same properties when it first became locked in synchronous orbit, and that would throw off these odds. But it's pretty much certain that one of humanity's biggest optical illusions is way more than just a trick of the mind - it's a showcase of all the physics phenomena that teamed up to make it visible in the first place.