In a posting earlier today, Christine Buckley wrote:
<< They [the Sun and the Moon} exert maximum gravitational pull on the water
nearest to them. When the moon is on the side of the earth directly opposite
the sun, their forces
lessen each other. At least, I think that's it. I gave away my copy of
'Understanding Science' last year!>>
I do not think that this is correct. It is true that the Sun and the Moon
exert a greater gravitational force on the side of the Earth nearest to them.
BUT, this gravitational force is counterbalanced by the centifugal force of
the Moon ratating round the Earth and by the Earth rotating round the Sun.
Actually the Sun and the Earth both rotate round the common centre of gravity
of the Sun and the Earth, but this is inside the sphere of the Sun. The Earth
also rotatres round the common centre of gravity of the Earth and the Moon,
but this is very close to the Earth. So while the gravity of the Sun and Moon
directly cause bulges in the Earth's oceans on the sides respectively facing
the Sun and the Moon, each bulge is matched by a similar bulge on the sides
of the Earth OPPOSITE the Sun and the Moon. So, there are four bulges in the
waters of the Earth's oceans, one facing the Sun, one opposite the Sun, one
facing the Moon and one opposite the Moon
At full moon and new moon, the bulges caused by the Moon (and the opposing
centrifugal force) and by the Sun (and its opposing centifugal force)
coincide and that is when the highst tides occur, because the Sun and the
Moon and the opposing centifugal forces are all pulling together. The
gravitational forces from the Sun and the Moon do not cancel each other out,
but on the contrary, reinforce each other.
When an eclipse whether of the Sun or the Moon) takes place, the Moon is
actually passing through the plane of the Earth's oribit round the Sun, so
that the Sun and the Moon are more exactly in line. Accordingly the bulges at
the time of an eclipse coincide to the maximum extent and the greatest tides
take place. (There are, in fact, other relevant factors affecting the hieght
of the tides), such as the distance of the Earth from the Sun and the
distance of the Moon from the Earth, caused by the eccentricity of the
respective orbits orbits. These factors also affect the height of the tides,
but the effect of these is much smaller than the effects of the Sun and
Moon'sgravitational and the opposing centrifugal forces.)
The tides are the major effect which the Moon has on the Earth. But there are
other, more mysterious influences which are not fully understood. They are,
perhaps, caused by the amount of light reflected to the Earth at night by the
Moon when it is full. One of the most impressive effects is the way some
marine animals mate throughout the world in unison at the time of the full
Moon. I understand there are many other phenomena of the same kind.
I understand that there small tides in the Mediterranean, and that they are
in part are caused by the Sun and the Moon. These are tides local to the
Mediterranean and are not caused by the ocean tides, because the Straits of
Gibraltar are too narrow and shallow to permit sufficient movement of water
through them.
I very much doubt, however, whether the gravity of the Sun and Moon has any
effect on the ebbing and flowing of springs. The terms "ebb" and"flow" are
merely general terms which are independently applied to the separate motions
of the tides and of the springs. springs.
David Lister.
Grimsby, England.
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