What turns the Earth on its own axis, at a constant speed, day in day out, each and every day ?

Originally posted by AmethystSD
And why should always the same side be facing Earth? My understanding is that the experts don't agree on the answers.

Sorry, but they do agree. It's called tidal lock, it's usually covered in astronomy 101.
en.wikipedia.org...
Basically, the moon's far side feels our gravity less than the moon's near side. This causes the moon's rotation to gradually slow until it syncs to its orbital period around the earth, which has already happened. For the same reason, our gravity has pulled the moon's core and mantel towards the earth-facing side slightly. The moon exerts the same tidal force on earth, giving us our tides (hence the name), and gradually slowing our rotation. Earth is much more massive though, so it has much more inertia to resist the slowing-effect of the moon's tidal force.

[edit on 12-1-2009 by ngchunter]