You might make a case that the centrifugal explanation is easier for some people
to understand, but claiming that gravity doesn't cause the tides is just plain
bogus! What force are you really proposing? The Tide Fairy? The only force
at work here is gravity. It must be possible to explain the tides completely
(not counting local effects) simply by looking at gravity.

The Moon's gravity affect each portion of the Earth differently. You can divide
the total force into two components, one that affects the Earth equally, and the
other represents the differences. The sum of all of the differences nets out to
zero, so the first force can be thought of as the force that pulls the Earth
around the Moon-Earth center. When you subtract that out, what you're left with
are the differential forces that push out the bulges, and pull down the poles.

One conceptual problem with the differential view is that it appears that the
far side bulge is being pushed away from the Moon. That its not really the
case: it is being pulled toward the Moon, but with less force than the rest of
Earth. Its only when you subtract off the large primary force, which maintains
the orbit, that it appears that the far side bulge is being pushed away from the
Moon.



"Nav" wrote in message
...
Well Peter, I have to disagree there. The gravitational force acts only
toward the center of mass of the system. This cannot by itself produce
two bulges. To clarify this, try imagining the forces of gravity in 2D
on a piece of paper. In all cases, water would be pulled toward the
center of the Earth-Moon pair. This would lead to less water on the far
side and more water as you move toward the moon... -two bulges would not
be present.

Cheers

Peter S/Y Anicula wrote:
You make it sound as if the gravitational forces explains the bulge
under the moon and the centrifugal forces explains the bulge on the
side of the earth that turns away from the moon.
That is not right.

The gravitational difference alone can explain that there are bulges
on both side of the earth. That's why it is sometimes the only factor
mentioned when trying to keep the explanation simple.
The centrifugal element can only explain that there is a bulge on the
part of the earth that turns away from the moon.
That is why it is one of the elements (and there are others), that is
sometimes left out of the explanation.

While I think that in some cases it is a good idea to include the
centrifugal element in the explanation, I don't know exactly how many
elements one should include to make it a good explanation - but I
haven't yet seen a complete explanation in a popular publication.

Peter S/Y Anicula
Sailor
The seven seas