The moon, in additional to being a beautiful addition to the night sky, is also responsible for the changing ocean tides that we experience daily here on Earth. Basically, the moon has gravitational power, just like Earth does, and the moon's gravitational force exerts a powerful pull on the oceans on both sides of our planet. This causes the oceans to bulge away from the Earth. The ocean facing the moon bulges toward the moon, while the ocean on the other side of the Earth bulges in the opposite direction -- this is caused by the Earth actually pulling away from the ocean and toward the moon. The rotation of the Earth, along with the moon's gravitational pull, causes ocean tides to change.
High tide occurs when a body of water is facing the moon and the moon is pulling the water toward it with its gravity. High tide also occurs when an ocean is facing directly away from the moon, or when the moon is pulling the mass of the Earth away from the water. When the moon is neither overhead nor on the opposite side of the Earth, a body of water settles back into low tide. This sequence of high tide and low tide generally happens twice a day everywhere on the planet, approximately every 12 hours and 25 minutes, as the Earth rotates and the moon orbits. Each tide lasts about six hours.
So why aren't there tides in smaller bodies of water, like ponds or small lakes? It's because the entire body of water is the same distance from the moon throughout, so all the water experiences an equal gravitational pull [source: ONR].
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