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Dr. Donald E. Simanek wrote:
However the implication in the above is that the bulges are somehow *due
to* the rotation about the center of mass. This isn't true. The two tidal
bulges would be there even if there were no rotation, that is if you were
to 'nail down' the earth and moon to the fabric of space, the bulges would
still be there. The rotation only modifies the bulges somewhat. Many
textbooks I've seen give students the wrong impression that these tidal
bulges depend on rotation, and depend on some mysterious property of the
center of mass. And of course students will swallow even a bogus
'explanation' if it 'sounds good' without critically examining the details
of evidence, logic, and fundamental physics.
Or imagine a Cavendish style experiment with heavy balls hanging *at rest*
in a torsion balance arrangement. The balls should have tidal bulges, but
I doubt they'd be even measurable on this scale of experiment. It's hard
enough just to measure the net force they exert on each other.
It's about time someone here posted a bibliography list of papers and
textbooks which do this correctly, as resources for those who happen to
use textbooks which are careless on these matters. The question comes up
one one of my discussion groups about every six months. Better yet,
someone with more time than I have right now, could prepare a document on
this, which I'd be more than happy to put (or link) on my web page.
Perhaps we could produce a central web site with good explanations of
things textbooks frequently mess up. My system administrator recently
allocated me 50% more space for my web site, so there's room for things
like this. I'd be willing to prepare the graphics for such a project.