How do the Moon and the Sun Cause Tides?

            The ocean rises and falls regularly. There is one complete rise and fall about every 12 hours. These regular up-and-down movements of the ocean are called tides. Scientists have discovered that two things help cause tides: (1) the gravitational attraction exerted by the sun and the moon on the earth, and (2) the movements of the earth and moon.
            The gravitational force of the moon pulls on all the land and water of the earth. This pull is strongest on the parts of the earth that are nearest to the moon at any one time. Since water moves more easily than the land, water is always pulled up a little on the side of the earth closest to the moon. That is, a high tide is produced on the side of the earth nearest the moon. At the same time, there is a high tide on the opposite side of the earth. This is because the water on that side of the earth is farther from the moon than the solid center part of the earth is. So the moon pulls harder on the solid part and pulls it away from the water on the other side. In between these two points the water level is lower.
            As the earth rotates, the land is carried toward the pulled-up water closest to the moon. If you are on a seashore as this happens, you see the water level rise. We say that the tide is rising or coming in, and we call this the flood tide. When the water reaches the highest level, it is high tide. As the earth keeps on rotating, this part is no longer nearest the moon and the water level falls. Then we say that the tide is falling or going out, and we call this the ebb tide. When the water reaches the lowest level, it is low tide. Then, when the earth is halfway around – that is, when this point is farthest from the moon – there is another high tide. This high tide is followed by a low tide. After this low tide, the next high tide comes when this part of the earth is again nearest the moon.
            For centuries, sailors have known that the hours for high and low tides are not the same every day. The time needed to complete two high tides is not 24 hours, but about 24 hours and 50 minutes. The reason for this extra 50 minutes is the movement of the moon around the earth. The moon revolves around the earth in the same direction that the earth is rotating. It goes around once in about 29½ days. So the earth must rotate a little more than a full turn to bring any point on it back to the place where the water is pulled up toward the moon.
            The sun's gravitational pull on the earth also has an effect on the tides. The sun pulls harder on the earth than does the moon. But because the sun is so far away, the pull on the side of the earth nearest the sun is not much greater than the pull on the side away from the sun. So the tides created by the sun are not as high as those created by the moon. In most places, the effect of the sun is less than half that of the moon. The tides we observe are combinations of these two types. Twice each month the tides caused by the moon and the sun come at the same time, causing the highest and lowest tides. And twice each month they come six hours apart, causing the least noticeable tides.