We have all been taught that the tides are chiefly the result of the gravitational attraction of the moon on the waters of the earth’s oceans.
But there are actually two rotational systems involved, and two bulges of water.
First, the moon goes around the earth, or so it appears.
Actually, both the earth and moon are rotating around the common center of mass, the center of gravity of the earth-moon system, which is a point inside the earth. About once a month, a circle around this common center is completed.
There is a slight excess of gravity force on the side of the earth toward the moon, producing a giant bulge in the water on that side.
On the other side, the side away from the moon, the centrifugal force of the earth going around the center of gravity is slightly larger than the gravitational force, so it pushes the water out, away from the moon, making another bulge.
Meanwhile, every day the earth rotates on its axis, so the earth is moving under those two bulges twice a day.
This accounts for the pair of high tides. The pair of low tides occurs because the water has to come from somewhere.
The centrifugal force involved can be visualized by thinking of a pan of water on a phonograph turntable. There would be a low tide in the middle and a high tide on the outside, because the force tends to push water out from the center of rotation.
The tidal forces are much more complicated than this simplified explanation, because the earth is not entirely covered by water.
There are continents in the way and the bulges in the ocean vary immensely because of these different land masses.
