Comment by dspillett

> But then isn’t everything on earth rotating about its own axis with respect to that external space?

From the point of view of the moon, for the purposes of action due to gravity, anything on Earth is essentially part of the Earth, not an entity that is massive enough to be considered separately. The aggregate centre of mass is what counts. Similar for the Sun looking at the Earth/Moon system: from that PoV Earth+Moon is it a single mass with a centre somewhere between the two major masses that form it.

If the Moon where sufficiently consistent in its shape and density, it could rotate freely in any direction while orbiting the Earth, that fact that it is more dense on one side means that it is more energy efficient for it to spin in step with its orbit such that the dense side keeps facing us. If something massive hit the moon (let's assume this somehow happens without significantly affecting its orbit or causing significant problems for Earth too!) it might push the rotation off for a bit, but it would slowly be pulled back into sync. If something sufficiently massive simply landed on the moon, that would affect the mass distribution and the exact face that points at us would slowly change to reach a new equilibrium.