olin_piclist@embedinc.com (Olin Lathrop)
> Here's a thought experiment:  Pretend the planet was rigid with no oceans.

Unusual imagery, but equivalent to the usual explanation, which goes like
this: If the Earth deformed losslessly, the tidal bulges would always be
in line with the moon (ignoring solar tides for the moment). However, the
frictional losses cause the bulges to be dragged forward (in the direction
of the Earth's rotation) with respect to that line. This causes the gravity
vector that the moon sees to point slightly forward (in the direction of
the moon's orbital motion) of the center of the Earth. This actually
transfers energy from the Earth to the moon, raising its orbit and
increasing its orbital period.

All of this energy -- both the energy transferred to the moon and the
energy that goes into the frictional losses -- comes from the rotational
energy of the Earth. The Earth is slowing down (which is why we need leap
seconds), and some day it will have one face locked facing the moon, just
like the moon has one face locked toward the Earth already.

When we extract energy from the tides, we're taking advantage of the
difference in the heights of the land tide and the water tide. Doing
so looks like additional frictional losses within the overall system.
In other words, we're transferring more of the tidal strain from the
relatively lossless water to the relatively lossy rock. The Earth
slows down slightly more than it otherwise would if we didn't do that.

-- Dave Tweed
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