"Nav" wrote in message
...

I don't think its fair to do this - you can mathematically eliminate effects
by
shifting the reference frame, but "locking" objects together is changing the
problem at a more fundamental level.

I don't see it that way, the explanation for the two tides based on
differential gravity alone does not care whether the earth is "moon
locked" at (say) an L point -and that why it is not the correct
explanation in my opinion. Of course it all comes down to gravity and
the energy of the system but the simplest close answer should consider
the rotation as well.

The rotation is certainly considered in the differential explanation. The "net
force" which is subtracted out in the differential model is the force which
casues the Earth to rotate around the moon. It is exactly oppostite the
"fictional" centrifugal force.




In this case, how to you "lock" the Earth?
In fact, the crux of this problem is that different parts of the Earth are
actually acting somewhat independently.

However, this brings up an interesting point. At some point in the distant
future the tides will be eliminated.

(Well not really, unless you ignore the Sun). But I think this point
reinforces what I've been trying to get across, without considering the
rotation(s) about the center of mass you don't get a two tide situation.
Any description that does not explicitly consider the relative motion
will not generate two tides -do you agree?

How will this happen? Because the tides
lag the Moon the high tide is not directly under the Moon, but offset. This
creates soon torque that is transferring energy from the Earth to the Moon.
The
result is that the Earth is slowing down, and the Moon's orbit is
increasing.
This will continue (some say) until the Earth's rotation slows down to match
the
Moon, and the bulge stays under the Moon. The Earth and Moon will at that
point
be locked together. Because the Moon is smaller, it has already assumed
this
orientation WRT the Earth.

If we work this backwards we find the in the distant past the Moon's orbit
was
much closer to the Earth, and the Earth's day much shorter. Exactly how
much
depends on what other theory you're trying to support or disprove. However,
we
do know the effect is real - the measurement using equipment left behind by
the
astronauts shows the distance increasing about 4 cm a year, and the Earth's
day
lengthening by 1.5 milliseconds a century.


I never looked it up but would have guessed the rate of slow down would
be larger than that.

I also thought that at first glance. But consider: 1.5 millisecs/century is
1500 seconds/100 mil years, or a bit under a half hour per 100 mil years. So
when was the distance zero? About 5 billion years ago! Of course, the math is
a lot more complex than that, but it shows how long a time a billion years is.


From that number you can calculate the energy cost
of the tidal forces... Here's a thought, at current rate of energy
consumption growth how long before even this energy source would be
insufficient for our needs?

Well, there are a few places tapping tidal energy. But I think the big score
would be to tap the thermal energy in the Earth's core. Why should Iceland be
the only place that gets a free ride?