Ocean Tides Vs. Earth Tides

Difference Between Ocean Tides and Earth Tides
Nearly three fourths of the earth’s surface is covered by oceans and seas. The level of these waters is called sea level.
Sea level is always changing. Each day a rhythmic rise and fall of sea level takes place in all the ocean waters of the world. This rhythmic rise and fall of the ocean waters is known as the ocean tides.
What are Ocean Tides?
To understand what the tides are, let us take an imaginary trip to an ocean beach somewhere on the Atlantic Ocean coast of North America.
Suppose you arrive at 10:00 A.M.According to a sign, this is just the time of high tide, or high water. You choose an uncrowded spot not far from the water’s edge, and there you leave your belongings while you go for a swim. When you come out, you notice that your belongings are farther from the water’s edge than they were when you entered the water. You sit down on the beach and watch the ocean. The sea level is steadily going down. More and more of the beach is being exposed, and the water’s edge is receding (drawing back) farther and farther. The tide is going out. It continues to do so for about 6 hours 13 minutes. Then it comes to a level at which further movement ceases. The ocean here is now at low tide, or low water.
Now the tide goes no lower. Instead, it begins to rise. It continues to do so until once again it reaches a high-water, or high-tide, mark. The time taken to go from low water to high water is nearly 6 hours 13 minutes—just about the same as the time taken to go from high water to low water.
By now you have undoubtedly left the beach, for nearly 12 1/2 hours have passed since you arrived and night has long since fallen. But if you were still present, you would be able to observe the high-water level. While it is not exactly the same height as the first high water you observed, it is close to that level.
Again the tide goes out and comes in. Each time it takes about 6 hours 13 minutes for its rise or fall. Thus, two consecutive (one following the other) high waters are about 12 hours 25 minutes apart. And it is twice that time—roughly 24 hours 50 minutes—from one day’s high water to the next day’s.
Suppose that you return to the beach at 10:00 A.M.the next day. The tide will be rising at this time, but it will not reach high water until about 10:50 A.M.In other words, high tide (and low tide, too) comes about 50 minutes later from one day to the next.
Earth Tides
The solid part of the earth is not a rigid, motionless mass. It too is influenced by the gravitational attraction of the moon and the sun. The entire earth from its center to its outer crust deforms (changes shape) periodically under the influence of the moon and sun. This phenomenon is called earth tides.
Earth tides are not noticeable to us, even though the ground beneath our feet is moving very slowly up and down twice a day. The movement is greatest near the equator and much less in the polar regions. In the United States, for example, earth tides cause an up and down movement of about 16 inches (40 centimeters). It happens so slowly that we cannot feel it, although sensitive scientific instruments can detect it.
Earth tides and ocean tides as well influence all precise measurements that scientists make as they stand on the surface of the earth. Research efforts are now focused on understanding the effects of tides on these measurements and how to accurately correct for them.

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