Why Tides Rise and Fall

The churning of the great bodies of water on the earth is a vital part of the health of the planet. As the ocean’s water rises and falls with the ebb and flow of the tides, nutrients are stirred about to keep them distributed for use by sea life forms. The way that the tides operate is fairly well understood as a function of the interaction of the moon’s gravity with the water on the earth.

The moon may be smaller than the earth, but it is still a giant ball of material.

A large object like the moon has a large gravity field. The combination of the gravitational pull of the earth and the moon keeps the moon orbiting the earth at a relatively static distance between roughly 220,000 and 250,000 miles. At this distance, the pull of the moon’s gravity is strong enough to cause the water in the oceans and other extremely large bodies of water like the great lakes to shift from side to side.

As the moon orbits the earth, the influence of its gravity field moves from one side of the earth to the other.

If you could imagine the earth as sitting still and not rotating, this idea is easier to grasp. Place the moon under the earth, and its gravity does not affect the ocean on the other side of the earth very much. As the moon rises in the east, the water in the ocean is pulled toward the moon. This creates a rising tide. Depending on the distance that the moon is from the earth, the tides may rise more or less.

When the moon continues its orbit to the middle of the sky, the water is pulled from both sides back toward the center of the oceans.

This causes the low tide to begin on the eastern side of the ocean. As the moon moves more toward the western side of the ocean, the tide begins to rise on that side. It will continue to rise until the moon drops below the western horizon. At that point, the east side of the ocean is at low tide and the west side is at high tide.

Once the moon is safely tucked away below the edge of the earth, the western tide will begin to recede.

This is the beginning of the low tide for that side of the ocean. The same action happens on the opposite side of the earth as moon travels over that part of the planet.  The tide sequence is repeated the next time the moon reaches its starting point at the eastern side of the ocean.