Debbie Queen, age 12, of Gastonia, N.C., for her question:
WHAT MAKES THE TIDE GO IN AND OUT?
When water reaches its lowest point, it will then rise gradually for about six hours until it reaches high tide, or high water. Then it immediately starts to fall, and for six hours this is continued until low tide, or low water, is reached. The cycle then begins again. The difference between the high water and the low water is called the range of the tide.
Tide is simply the rise and fall of ocean waters on a definite time schedule.
You’ll find tides on every body of water, large and small, although you’ll probably not notice the effect except at the edge of the ocean. Lake Superior, for example, only rises and falls about 2 inches.
Gravitational forces of the sun and the moon cause the tides.
Gravity from the moon pulls the water nearest the moon slightly away from the solid part of the earth. At the same time, the moon pulls the solid earth slightly away from the water on the opposite side of the earth. In this way, the moon’s gravity produces two bulges on the ocean. These bulges are the positions of high tide.
When the earth turns on its axis, the water and land rotate together. But one tidal bulge always stays under the moon, and the other tidal bulge always stays on the opposite side of the earth. In this way, the rotation of the earth brings a high tide to most places on the ocean about twice a day. The two high tides at a given place do not usually rise equally high, because the centers of the tidal bulges usually lie on opposite sides of the equator. The centers are located there instead of on the equator because the moon is usually located either north or south of the plane of the equator.
The sun and the moon pull harder on the side of the earth nearest them than they do on the center of the earth because the center is farther away. It is this difference in pull that produces the tides.
Because the sun is 390 times as far from the earth as the moon, the tide producing force of the sun is only 46 percent as high as those caused by the moon. The tides caused by the sun and by the moon combine to produce the tides seen along the seacoast.
Tides in the air are much like those in the ocean. At the earth’s surface, the speed of these tides, which are called lunar winds, is about 1/20th of a mile per hour. Although they are too low to be felt, scientists detect them by studying variations in weather statistics.
High and low tides in the air come twice daily. There are also high stages that are equivalent to the ocean’s spring tide. Lunar winds blow eastward in the morning and westward in the evening.