The high and low tide are caused by the gravitational
forces between the earth and the moon. However, the
source of the real effect takes some explaining to
grasp. The incorrect way of thinking is that the
moon attracts all the water to itself, therefore
causing a high tide on the side of earth close
to the moon, and a low tide on the side far from
the moon. However, if this was the case, there would
be a high tide once per day. But there are TWO high
tides a day. The reason is, the part of the earth
both AWAY from the moon, and CLOSE to the moon BOTH
get high tides...
Now we need to explain why this is so. The earth and
moon rotate around each other, each pulling the other towards
itself. The moon attracts every piece of matter on earth.
Since gravity is inversely proportional to the square
of the distance, this force is greater on the side
of the earth closer to the moon, and lesser on the side
of the earth further from the moon. Since the earth is
quite a rigid object, this difference in forces fails
to deform the earth (much). However, it succeeds quite
well in deforming the oceans -- which are not as rigid.
Since the waters on the moon side are attracted more
strongly than average, they tend to bulge TOWARDS the moon,
hence causing a high tide. The waters on the opposite
side of the moon, since they are attracted less strongly
than average, tend to 'lag behind' the rigid earth,
and bulge AWAY from the moon, which in this case, is also
AWAY from the earth, again, causing a high tide.
Low tide occurs at about right angles to the moon,
where the force on the waters match the average pull
of the moon on the earth closely.
The question that usually follows this is, why doesn't
the sun cause any tides (comparable to that of the moon)
although its pull on the earth is larger? The answer is,
although the gravitational pull of the sun on the earth
is larger than that of the moon, due to the much greater
distance, the force changes very little from one end
of the earth to the other. Since it is the difference
in the force than the average magnitude of the force
that matters for creating tides, the net effect is much
less than that for the moon.
Answered by: Yasar Safkan, Ph.D. M.I.T., Software Engineer, Istanbul, Turkey
'Strange is our situation here upon the earth. Each of us comes for a short visit, not knowing why, yet sometimes seeming to divine a purpose.'