The human brain is wired to see all kinds of patterns in various shapes. The most common one is that of the human face, most often encountered in our day to day lives, be it in the coffee, a fire hidrant or a cut off potato (I saw Jesus!). The moon makes no exception either. For millenia, man has worshiped the moon and thought a god, the “man from the moon”, was watching down on the rest of us mortals during the night. Scientists now explain why this particularly fascinating side of the moon faces the Earth, and not the other.

The Earth facing side of the moon (left), featuring a human face-like pattern, and the opposite facing side (right). (c) NASA

The moon is covered by lunar maria that form dark spots and lighter highlands on its surface, all of which come together in such a manner that it bears a striking resemblance to a typical human face (eyes, nose, mouth), however its also often been interpreted as a rabbit, dragon, frog, buffalo and all kinds of critters.  Ever noticed, though, how night after night the moon is facing Earth from the exact position every time?

Yes, the moon does spin. The reason why it’s always facing from the same direction is due to the fact that the moon is locked in a synchronous orbit with Earth, causing it to rotate exactly one time with every complete orbit around Earth. From down here, it looks as if the moon doesn’t ever spin. The study we’re discussing in this present article doesn’t discuss this, though.

Instead astronomers at Caltech sought to address the life-long question of why does the moon face Earth with that particular side, and not the other. For years, scientists seemed to have come to the conclusion that in the end, it was all just a flip of a coin matter – 50/50. The researchers, however, argue that it’s far from being that simple.

Around four billion years ago, the moon was a nothing more than a molten rock, constantly changing and morphing. Back then, the moon was rotating at a much faster velocity than it is now, and was much closer to Earth as well, providing for a shifting scenery of the moon during the night. Over the years, Earth’s gravitational pull slowly slowed down the moon until it stabilized into a synchronous position, however it also elongated the natural satellite – the moon isn’t a sphere. These tips of lunar ellipse are actually the two major candidates for the Earth facing sides. Still, it’s still 50/50, right?

Here’s well the physics get really interesting. Besides these elongation, the moon is also subjected to tidal forces, which tugged on the moon, creating a slight bulge in the process. This bulge caused the moon to always position itself towards Earth pointing alongside the bulge’s axis. As time passed, this constant shifting of position caused internal friction and acted as a sort of break, slowing down the moon’s orbit until it reached today’s level. Still doesn’t explain the whole man on the moon side, thing.

The researchers ran a computer simulation and discovered that the main factor which determines which side of the moon is facing Earth is rate of rotational energy dissipation. If the moon had lost its rotational energy at a rate 100 times faster than it really did, than indeed the chances would’ve been 50/50. However, at its actual rate of energy dissipation the man in the moon side had about two-to-one odds of facing us.

“The real coincidence is not that the man faces Earth,” said Oded Aharonson, professor of planetary science at the California Institute of Technology (Caltech). Instead, the real coincidence is that the moon’s dissipation rate was just the right amount to create such fascinating physics and load the coin

The findings were reported in the journal Icarus.

Caltech press release via Discovery News