Have you ever been at a beach where at low tide you have to walk way out in the water just to get in up to your knees? Yet there are some places where you can hardly tell the difference between high and low tide.

The reason for this has nothing to do with the moon. Tides are caused by gravitation. Just as the earth pulls on the moon, so the moon attracts or pulls on the earth, but with much less force. The pull of the moon upon the earth draws the ocean waters nearest to it toward the moon as a broad swell, or wave. This produces high tide.

The water on the opposite side of the earth gets the least pull from the moon since it is farthest away, so it forms a bulge, too. So we have high tide on the side toward the moon and also on the side opposite the moon.

As the moon goes around the earth, these two high “heaps” of water and lower levels of water keep in about the same position on the earth’s surface in relation to the moon. In fact, if the earth’s surface were entirely covered with water, the rotation of high tides and low tides would be very regular.

But there are many things that interfere with this. One is the great bunk of the continents. They cause tidal currents which follow the shore-lines and pile up in certain places, such as bays.

On coasts that are gently sloping and straight, the incoming tide has room to spread out and may not rise very high. But where the incoming tide enters a narrow bay or channel, it cannot spread out, and the water may pile up to great heights. In the Bay of Fundy, for example, the difference between high and low tide may be more than 21 meters. Yet, in most of the Mediterranean Sea, the water rises no more than 0.5 meters at high tide.