[Choobus]

a different tim wrote

Bit of confusion here, I think. Yes, Earth's rotation is slowing down, yes, it's the tidal forces, but it's the moon that the earth would end up synchronous with, not the sun (the converse - the moon being synchronous with the earth from the moon's point of view, is already the case). The phenomenon is known as spin/orbit coupling or spin/orbit resonance, depending on who you talk to. It will take about 15 billion years for the earth to have no rotation with respect to the moon. This is longer than the universe has been around and certainly a lot longer than the sun is expected to last.
http://www.scienceblog.com/community...199600486.html

There is also an earth/sun tidal force which would take even longer to synchronise the earth with the sun. It wouldn't necessarily end up as a 1:1 resonance - there are other ratios of spin/orbit that are stable, for example Mercury (formerly thought to be in 1:1 ratio, in other words synchronous) turns out to be in a stable 3:2 spin/orbit resonance.

Don't have the maths for the full explanation so if you want to go deeper you'll need to ask someone else.

For the record, this is usually referred to as orbital resonance as most physicists understand "spin-orbit coupling" to refer to the quantum effect whereby an electron spin is weakly coupled to it's angular momentum state. (Maybe it's different in Astronomy departments