Jeff Morris wrote:

"Nav" wrote in message
...
...

If you still haven't got the idea then I really
can't make it any clearer and you'll just have to ponder why University
Departments of Oceanographics (and NOAA etc.) all say that THE TIDES ON
EARTH are due to the difference between centrifugal and gravity forces.


I have never claimed that you can't derive the tides by computing the difference
between gravitational pull and the centrifugal force. My point has been that
centrifugal force is constant across the Earth, and thus does not explain why
the bulges are in opposite directions. The centrifugal force is exactly
canceled by the average gravitational pull, and what is left over is the
differential force. Your claim has been that the centrifugal force varies
across the Earth, and that's what I've taken exception to.


F= mr omega^2. The distance from the barycenter to all points on earth
is NOT the same. Anyway, the site clearly shows in Fig. 2 that it is the
_DIFFERENCE_ between gravity and centrifugal force that makes the tides,
not gravity alone. There really is no point going on any more is there?
For myself, I'm sick of repeating the same point over and over.

EOT

Cheers


You keep citing the NOAA page,
http://www.co-ops.nos.noaa.gov/restles3.html
So I went back and read that in detail. In it is the "disclaimer":

"While space does not permit here, it may be graphically demonstrated that, for
such a case of revolution without rotation as above enumerated, any point on the
earth will describe a circle which will have the same radius as the radius of
revolution of the center-of-mass of the earth around the barycenter. Thus, in
Fig. 1, the magnitude of the centrifugal force produced by the revolution of the
earth and moon around their common center of mass (G) is the same at point A or
B or any other point on or beneath the earth's surface. Any of these values is
also equal to the centrifugal force produced at the center-of-mass (C) by its
revolution around the barycenter."

it goes on to develop differential gravity:

"While the effect of this centrifugal force is constant for all positions on the
earth, the effect of the external gravitational force produced by another
astronomical body may be different at different positions on the earth because
the magnitude of the gravitational force exerted varies with the distance of the
attracting body."

In other words, this site actually agrees with what I have been saying.
Frankly I owe an apology to the site's author, since I maligned it without
reading carefully. In fact, although it leads with a provocative line about a
"little known aspect of the moon's orbital motion," and has a rather confusing
diagram, its basic approach is correct and in full agreement with my claim.
Apparently the site was actually changed at some point about two years ago
because of complaints on another board.