Charlie Randolph, age 9, of Nashua, N.H., for his question:

WHAT CAUSES OCEAN TIDES?

A tide is the rise and fall of ocean waters on a definite time schedule. The moon's gravity 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.

Because of the pulling, the moon's gravity produces two bulges on the ocean. These bulges are the positions of high tide.

As the earth turns on its axis, the land and water rotate together. But one tide bulge always stays on the opposite side of the earth. 7herefore, the earth's rotation 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 actually produces the tides.

The tides caused by the sun and by the moon combine to produce the tides seen along the world's seacoasts.

Differences in the coastline and in the channels of the sea bottom may make a difference in the times that the tide wave hits the different ports along the same coastline.

One tide acts very much like another. From its lowest point, the water rises gradually for about six hours, until it reaches high tide, or high water. Then it begins to fall, continuing for about six hours until it reaches low tide.

After 12 hours, the tide cycle begins again.

The difference between high water and low water is called the range of the tide. As the water rises and falls, it moves toward and away from the coast. This movement of the water is called the tidal current.

When the water moves toward the coast or inland, it is the flood current. When it flows seaward, it is called the ebb current.

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 and is called a spring tide.

When the sun and moon pull at right angles, as when the moon is in its first and third quarters, the tide does not rise as high as usual and is called a neap tide.