Why is the far side of the moon on, well, the far side of the moon?
The moon is tidally locked. That means that it rotates in sync with its orbit around Earth, so that the same side of the moon always faces us. It used to rotate faster, but Earth’s pull slowed it down and locked it in place.
The lunar far side is much more elevated than the near side, which has a group of facelike splotches that some call the man in the moon. All that mass on the far side is pretty attractive, gravitationally speaking. So it would seem to make more sense if the far side of the moon had been the one pulled in our direction.
A new study in the journal Icarus offers an explanation for the moon’s seemingly unbalanced configuration. [Oded Aharonson, Peter Goldreich and Re'em Sari, "Why Do We See the Man in the Moon?"] Which way the moon locked into place depended on how asymmetric it was, as well as on the torque the moon felt from Earth’s gravity.
Given those factors, the researchers concluded that the chances were about two to one that the moon would lock into place in its current orientation. Which means that in a universe of infinite wonders, the man in the moon is still an odds-on favorite.
[The above text is a transcript of this podcast.]