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Tides |
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A tide is defined as the rising and falling of the sea, usually twice in each lunar day at a particular place, due to the attractive (gravitational) forces of the moon and the sun. Most people think that tides are caused mainly by the gravitational effects of the moon on the earth. However, the gravitational pull of the sun on the earth is about 200 times stronger than the gravitational pull of the moon on the earth. (Click here to see a mathematical calculation of the relative strength of the gravitational pull of the sun and moon on the earth.) So, tides are really the result of the difference in the gravitational pull of the moon and the sun on the earth on opposite sides of the earth. This difference in pull of the moon and the sun causes a bulge in the ocean water that is almost in line with the position of the moon: one bulge on the side of the earth toward the moon and one on the side of the earth opposite or away from the moon. The tides we observe are a result of the earth rotating under this bulge.
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The most familiar evidence of tides is the recurrence of high and low water levels that we see on the coastline. These tides usually reach high and low levels about every six hours plus 50 minutes. (The 50 minutes comes from the rotation of the earth and moon so that the moon is in approximately the same position relative to the earth about 50 minutes later each day.) While we think of the gravitational effects of the moon and sun on the oceans of the earth as the primary factors in determining the characteristics of tides, the shape of the coastline, the depth of the water, and the topography of the ocean floor along with the effects of prevailing winds all have a significant impact on the timing and height of the tides. Do the Great Lakes and large bodies of water other than the oceans experience tides? The short answer is ’yes’. However, in the case of the Great Lakes, the largest tides caused by the combined effects of the sun and moon are relatively small (estimated at less the five centimeters in height). These small variations are masked by the greater fluctuations in lake levels because of wind and barometric pressure changes so that the Great Lakes are considered to be non-tidal.
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