Mike Tullett wrote in message ...

I'm not baffled, as I taught *some* aspects of dynamic oceanography for over
20 years and thus claim some knowledge of the topic.


So you have been teaching it as long as I have been trying to
understand it? And this is not bull sh then:

To understand the second bulge in the lunar component, we have
to treat the earth and moon as one system with a common centre of
mass. This is known as the Barycentre and lies within the earth,
nearly 3000 miles from the earth's centre.

Not only is the moon rotating about this point once very 28 days, but
so is the earth's centre, i.e. it wobbles as it orbits the sun.

One can argue to keep the earth rotating around that point, requires
an inward acting force (on the opposite side to the moon - a
centripetal force) and this weakens the "gravity" there thus allowing
that second bulge to form. I am simplifying things a lot and diagrams
would be useful.

This site looks quite sound - "Why are there two high tides each day"

http://www.math.nus.edu.sg/aslaksen/teaching/tides.html

Mind you, this is very much the Newtonian approach and assumes very
deep water (much deeper than is really the case), and no land to
affect the
tidal bulges as the encircle the earth.

1.

First of all can we agree that due to the surrounding mass of the
earth there is virtually no gravity or rather almost zero
gravitational attraction at the centre of the earth? This being
similar to what would happen to the smaller mass in a Cavendish
gravity experiment if two equal weights were counterbalanced each side
of it. (The centre mass is "not attracted to either." As it were)

(And by extension showing that the tidal raising ability of the full
moon is much less than at any other phase or portion of a phase.)

2.

By further extension the barycentre is the centre of the sun, moon and
earth. Therefore they will have no "pull" on an object at point zero.

3.
The simplification of an inward acting force (if that were a
possibility) requires that it is uni- or bi- directional in order to
affect only one or two sides of the earth; yet we have shown that the
barycentre is by definition a centre of gravity.

As regards Newton, he did not resolve the problem of tidal action
except that he acknowledged the seas only obeyed the perception in the
Sea of Aethiope and nowhere else.

Today of course we know that there are numerous varieties of tides
from the one a day that follows the sun at Fiji, to the complex double
highs of the Solent and the extraordinary ranges such as at Bristol.
And that some tides, such as those at Seattle, have a mixed tropical
and whatever the name for our type is.

As it happens it appears that the tide around the British Islands seem
to work on a three phase pulse where the tides at Orkney, Liverpool
and Dover are nearly at high or low tide at the same time but those on
the west of Ireland, Hull and Bristol are on the opposite part of the
range.

As for the rest of the stuff on that site I just get past boredom
trying to put it all straight. You must choose to believe what you
wish. I know I can't convince you otherwise.