First off, tides are caused by the moon and, to a
lesser extent, the sun exerting a gravitational
force on the earth. Since the gravitational force
decreases as 1/(distance squared), the surface of
the earth nearest the sun or moon is pulled away
from the center of the earth a little bit. Also,
the far side of the earth lags a little bit. The
effect of this is that the earth has bulges that
point towards and away from the moon and the sun.
As the earth rotates, different parts of the
planet experience these bulges and thus tides.
Tides are very complicated, but, in general, they
change as the moon rotates around the earth and
thus changes its position with respect to the sun.
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