Alice Ferguson, age 12, of Beaumont, Tex., for her question:
WHAT CAUSES THE OCEAN TIDES?
A tide is the rise and fall of ocean waters on a definite time schedule. Tides of the ocean are caused mainly by the pull of the moon on the earth. The moon’s gravity pulls up the water directly below the moon, forming a high tide there.
As a high tide occurs on one side of the world, another is occurring on the other side. As the earth turns, high tides occur at each place on the ocean twice each day.
Men have known for thousands of years that the moon had some relationship to the tides. Before the year A.D. 100, the Roman naturalist Pliny wrote of the moon’s influence on the tides. But the physical laws of tides were not worked out until after the English scientist Sir Isaac Newton discovered the law of gravitation in the 1600s.
The moon’s gravity actually 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.
As the earth turns on its axis, one tidal bulge always stays under the moon and the other stays on the opposite side of the earth.
The sun also exerts a pull on the earth. But the sun is about 390 times as far from the earth as the moon, so the tide producing force of the sun is only 46 percent as great as that produced by the moon. As a result, the tides caused by the sun are 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.
The shape, size and depth of seas or oceans make differences !n the way the tide acts.
The range of the tide differs from day to day according to the position of the sun and the moon. When the moon and the sun are pulling along the same line, as they do at full moon and new moon, the tide rises higher than usual.
When the sun combines with the pull of the moon to produce tides that are higher than normal, the result is something called spring tides. Spring tides occur about twice a month near the times of the new and full moons. The moon then lies either between the earth and the sun, or on the opposite side of the earth from the sun.
Neap tides result when the pull of the sun is at right angles to the pull of the moon. Neap tides do not rise as high as normal tides. They occur about twice each month, when the moon is near its first and third quarters. At these times, the moon is on either side of the earth.
All bodies of water, large or small, are subject to the tide producing forces of sun and moon. But it is only where oceans and continents meet that tides are great enough to be noticed.
In the inland bodies of water the regular rise and fall of the tide is often so small that it is completely masked by changes !n level due to wind and weather. As an example, Lake Superior has a tide that rises and falls only about two inches.