Why the tide is not a bathtub

A tide is not the moon pulling the sea toward itself like a blanket. It is the whole ocean responding to a difference in gravity from one side of the earth to the other, while the earth turns underneath.

Gravity falls off with distance. The side of the earth nearer the moon is pulled slightly harder than the center of the earth, and the center is pulled slightly harder than the far side. Relative to the earth as a whole, water on the near side is drawn toward the moon, and water on the far side is left behind. Two bulges, not one. That is the part a bathtub cannot show you, because a bathtub is not a planet with an ocean wrapped around it, and the difference in the moon’s pull across a bathtub is nothing.

The earth rotates once a day under those bulges. A coast passes through a bulge, and the water is high. It passes out, and the water is low. It meets the other bulge, and the water is high again. That is why so many places see two high tides and two low tides in about a day. The day is a little longer than twenty-four hours, because the moon has moved in its orbit while the earth turned, so the next high tide is about fifty minutes later than yesterday’s. The clock on the kitchen wall and the clock in the harbor are not the same clock.

Not every coast cooperates. Some places, because of the shape of the basin, show one high and one low a day. Others mix the two patterns. The bulges are the open-ocean idea. A real shoreline is a bay, a shelf, a continent in the way. The wave of the tide has to fit the basin it is entering. If the basin’s size and depth happen to reinforce the wave, the range is huge. The Bay of Fundy is the famous case, not a typical beach. A theory that predicts “the” height of the tide without a map is not finished.

The sun does the same kind of work and a smaller share of it. It is vastly more massive than the moon, but much farther away, and tides depend on the difference in pull across the earth, which distance weakens quickly. When the sun and the moon are lined up with the earth — new moon and full moon — their bulges add, and the range is larger. Those are spring tides, from the sense of springing up, not from the season. When the moon is at a right angle to the sun — the quarters — the bulges partly cancel, and the range is smaller. Those are neap tides. A full moon does not make a storm. It makes, all else equal, a larger ordinary range.