Translations:AY Honors/Oceans/Answer Key/29/en

High tide and Low tide

The high tide occurs twice per day, as does the low tide. These tides are caused by the way the moon's gravity acts differently on the rigid parts of the earth as compared to how it acts on the oceans. Since the earth is rigid (except for the large bodies of water that experience tides), the pull of the moon's gravity acts on the entire planet evenly - as if all of the gravitational force were applied to the earth's center. Thus, the equations relating gravity's force to distance work as if the distance were from the center of the earth, not as if it were on any point on the surface. But the oceans are different. Since they are fluid, and not rigid, the moon's pull acts as though each molecule of water were independent. So gravity acts on the ocean relative to the distance between the moon and the earth's surface (where the oceans are) and not the distance between the moon and the center of the earth.