Hi Phil

Keep up the good work! But there is an error on your pages where you
describe how the moon forms two tides. You cannot explain this correctly
without noting that the earth-moon pair rotate about a common point. It
is this rotation that causes the two tides. If you like, you can explain
it as a centrifugal force acting to throw water out while gravity pulls
the water to water the moon. Since the gravity term is weaker on the
outside of the earth the centrifugal term dominates and the result is
two tides.

Many regards

Mark Cannell
Professor,
University of Auckland.

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


"Nav" skrev i en meddelelse
...
Hi Phil

Keep up the good work! But there is an error on your pages where you
describe how the moon forms two tides. You cannot explain this
correctly
without noting that the earth-moon pair rotate about a common point.
It
is this rotation that causes the two tides. If you like, you can
explain
it as a centrifugal force acting to throw water out while gravity
pulls
the water to water the moon. Since the gravity term is weaker on the
outside of the earth the centrifugal term dominates and the result
is
two tides.

Many regards

Mark Cannell
Professor,
University of Auckland.

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

We can certainly look at the gravitational force from the moon and the
gravitational force of the earth seperatly, and then ad the two, to
have a look at the combined forces.

Peter S/Y Anicula


"Nav" skrev i en meddelelse
...
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

Yes, you can. Where is the center of mass of the earth moon system?

Cheers

Peter S/Y Anicula wrote:

We can certainly look at the gravitational force from the moon and the
gravitational force of the earth seperatly, and then ad the two, to
have a look at the combined forces.

Peter S/Y Anicula


"Nav" skrev i en meddelelse
...

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

If the center of mass was the only factor involved, wouldn't the bulge be on
one side of the earth only?
Scout

"Nav" wrote in message
...
Yes, you can. Where is the center of mass of the earth moon system?

Cheers

Peter S/Y Anicula wrote:

We can certainly look at the gravitational force from the moon and the
gravitational force of the earth seperatly, and then ad the two, to
have a look at the combined forces.

Peter S/Y Anicula


"Nav" skrev i en meddelelse
...

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

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

Jeff Morris wrote:

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 are you talking about? I never said that gravity was not a part of
the equation.


Let me repost:
"The gravitational force acts only toward the center of mass of the
system. This cannot by itself produce two bulges."

Note the "by itself".

I'll repeat myself, the key is to understanding the _two tides_ problem
is that the system is rotating and "centrifugal" forces are balanced
only at the centers of the masses by gravity.

Cheers

"Nav" wrote in message
...


Jeff Morris wrote:

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 are you talking about? I never said that gravity was not a part of
the equation.


Let me repost:
"The gravitational force acts only toward the center of mass of the
system. This cannot by itself produce two bulges."

Note the "by itself".

That's exactly the point - gravity is the only force at work here. Gravity does
cause the bulges. The centrifugal forces are a "fiction" caused by the
accelerating reference frame. Why is it accelerating? Because of gravity!



I'll repeat myself, the key is to understanding the _two tides_ problem
is that the system is rotating and "centrifugal" forces are balanced
only at the centers of the masses by gravity.


That is a simplified way to look at it. If it helps your understanding, fine.
Your problem, however, is that you're insisting that this is the *only* way to
understand the problem. The are numerous correct ways to look at this. You
don't have to use centrifugal force to explain the far bulge.

Frankly, for me, it doesn't help at all, because the centrifugal force is
constant throughout the Earth. If it produces the bulge on the far side, how
can it also produce a bulge in the opposite direction on the near side? The
answer, of course, is that you have to add the centrifugal force to
gravitational force. which is different throughout the Earth. The resulting
force is exactly the same as the differential gravity from the other model. Why
is this? Because the centrifugal force is a "fiction" - it is simply the
opposite of the net gravitational force that causes the Earth to rotate around
the Earth-Moon system. In the differential model you subtract this out, in the
centrifugal model you add it.

So I have trouble thinking of centrifugal force as pushing out the far bulge;
for me the bulge is caused because the far side receives less pull from the Moon
than the rest of the Earth. However, arguing that one model is more correct
than the other is like arguing whether A+B=C or A=C-B.

The gravitational force acts only toward the center of mass of the
system. This cannot by itself produce two bulges.

When you say that, you are mixing two explanations. That doesn't work.

We can certainly look at the gravitational force from the moon and the
gravitational force of the earth separately, and then ad the two, to
have a look at the combined forces. If you do not include part of the
rotation element, it works just fine.

If you only look at the gravitational forces, you can explain the two
bulges!

It is an abstraction. Not the "truth". Even if you include the
rotation it is still an incomplete abstraction. We are discussing
different incomplete models. We haven't yet reached anything near the
"truth".

When discussing different models it is important not to mix elements
casually.
I'm surprised that a mere sailor have to teach this to a professor.

Peter S/Y Anicula

"Nav" skrev i en meddelelse
...
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