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The moon pulls on the surface of the Earth. Although the moon does have an effect on the
solid surface of the Earth, it is so slight that we do not notice. On the other hand it is
very apparent that the moon has an effect on the liquid parts of the Earth. When the moon
orbits the Earth, it pulls the oceans on the side of the Earth towards the moon. This creates
a bulge on the Earth on the side near the moon. Also a bulge is created on the opposite side
of the Earth because the gravitational effect on the center of the Earth is greater than that on
the side opposite from the moon. These two bulges creates a cycle of tides where there is a
high tide every twelve hours and a low tide between these periods.
The sun also has a gravitational pull on the Earth. But since it is so far away, its pull is
about half that of the moon. When the sun, earth, and moon are lined up, the tides are greater since
both objects are pulling along the same line, these are called spring tides. On the other hand if the
sun and moon are perpendicular from one another, the pulls somewhat cancel each other out. Since the moon's
pull is stronger than the sun's, there are still tides but they are weaker than usual. These are called neap
tides.
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