The moon may be Earth’s closest neighbor and the only other celestial body that humans have set foot upon—but it’s still shrouded in mystery. For example, why do the near side and the far side of the moon look so different? There are two competing hypotheses, but in this piece for Slate, astronomer Phil Plait explains why he finds the more recently put forth reason (the one that doesn’t involve two proto-moons) more compelling.
The Two-Faced Moon