fisher@scotty.DEC (04/10/85)
Charlie Sorsby asks, ~"if the earth is in the middle, won't the moon
and sun gravitational fields add rather than subtract?"
Consider the situation without the sun. There are TWO tides per day. One
is when the moon is roughly overhead, and the other is when the moon is
on the opposite side of the earth. The intuitive explanation for this is
that gravity has a stronger effect on things that are closer. If the moon
is on the opposite side of the earth, it is pulling on the earth harder than
it is pulling on the water at your feet. Thus the earth is pulled away from
the water, and the water rises.
Now add the sun opposite the moon with the earth in the middle. When the
sun is overhead, it is closer to the water at your feet than to the center
of mass of the earth. Thus it pulls harder on the water, tending to raise
it. Similarly the moon, on the other side, is pulling harder on the earth
than the water, again tending to raise the water. Voila! Spring tides.
(Note that in the above discourse, when I say "the water", I mean the water
on your side of the earth. There is nothing magic about water except that
it happens to be on the surface of the earth, and thus closer to whatever
is overhead than is the earth itself).
Burns
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