Jellby <me### [at] privacynet> wrote:
> That's it. The solid earth can be considered rigid, while the water in
> oceans obviously isn't. The portion of water close to the Moon is pulled
> towards it more strongly than the rock, because it's closer to it, and it
> can deform. The portion of water in the far side is pulled more weakly, so
> in relation to the solid earth it looks like it's being pushed away.
I'm not sure this is right. The best (and definitely correct) explanation so
far was Darren's, from earlier in this thread:

"Tides are caused when any large body orbits a point. Consider two rocks
on the moon, one on the ground very close to the Earth, one on the
ground on the side we never see. The one on the ground close to the
Earth is going slower than it would if it were all by itself in the same
orbit without the moon. A lower orbit is a faster orbit, so the rock
there is going too slow, so it should fall down towards the earth. A
higher orbit is a slower orbit, but the rock on the far side is actually
travelling faster than the rock on the near side instead of slower, so
it would normally be "flung away" from the center. The smaller the
radius of orbit compared to the size of the orbiting body, the more
evident the effect. The stronger the gravity, of course, the more
evident the effect."

This is how it was taught to me during my Physics degree.

It's interesting to note that the only reason we notice tides is because our
moon is unusually large - relative to its parent body, it's the largest
moon in the solar system by some orders of magnitude. In light of the
sci-fi element to this thread, it's also worth recounting two of Larry
Niven's short stories, "There Is A Tide", and "Neutron Star", both of which
hinge upon understanding tidal effects. "Neutron Star" in particular
features a good explanation of tides.

Bill

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