Dance of the Tides p.3
Real Tides Are Complicated

The simple models presented in these activities can help you understand the mechanics of tides, but the real tides in the oceans are more complicated. For example, the high tide arrives at Santa Cruz two hours later than at the Golden Gate Bridge, even though the moon is above both places at the same time. Move down the coast, and the time delays get longer. The delay is caused because water is a fluid, and under the pull of the moon, it moves from place to place.

As the water flows, it is spun by the Coriolis force—an effect caused by the rotating earth—which is delayed by friction as it drags against the continental shelf and influenced by a variety of other factors. The result is that the tidal bulges in the ocean do not line up perfectly with the moon. Actually, there are more than forty factors affecting the rise and fall of the tides at any particular spot on the earth. Tides are difficult to understand because such a large number of factors influences their behavior.

In order to study the ebb and flow of waters in San Francisco Bay, engineers built a scale model of the bay. By raising and lowering the height of the water at the mouth of the Golden Gate in the model, they can directly observe the response of the waters of the bay. When an oil spill occurred, the engineers used the model to predict where the tidal currents would carry the oil so that clean-up crews could be ready and waiting. (If you want to visit this model, call the Bay Model Visitor Center at 415/332-3870 for the operating schedule.)

Today, tides are primarily modeled with powerful computers. Because tides are influenced by winds and changes in atmospheric pressure, however, even with high-tech tools, we cannot always predict tides accurately. As we learn more about the earth, the moon, and the oceans, we’ll gain an increased understanding of the tides.