Tides are periodic changes in local sea level occuring on time scales of hours. They are caused by the gravitational forces of the sun and moon, with negligible contribution from other astronomical bodies. Tides have a considerable effect on ship operations and on amphibious assault.

According to Newton's Law of Gravitation, the gravitational force from a distant body falls off as the square of the distance. As a result, the moon and sun exert a slightly larger gravitational force on the oceans on the near side of the earth than on the oceans on the far side of the earth. The result is a tendency for the oceans to flow towards the near and far points of the earth, the near point because the stronger gravitation here act against the centrifugal "force" of the earth's motion, and the far point because the weaker gravitation here acts with the centrifugal "force." It is convenient to speak of this slight change in gravitational force as if it was a separate tidal force superimposed on a uniform background gravitational force. This tidal force falls off as the cube of distance, faster than the background gravitational force, with the result that the sun's tidal forces are weaker than those of the moon in spite of the fact that the sun's background gravitation is much stronger than that of the moon.

In a world covered by a single ocean of uniform depth, one would see two high tides a day, the first roughly at the time the moon is directly overhead, and the other twelve hours later. Low tides would occur six hours after each high tide. The weaker tidal forces of the sun would act in concert with those of the moon to make tides more extreme at new and full moon, and would act against those of the moon to make tides less extreme at first and last quarter moon.

In the real world, the limited size and variable depth of ocean basins makes the picture more complex. Tidal forces set up oscillations in the ocean basins that sometimes only vaguely resemble the ideal picture. In some geographical locations, tides are measured in inches, while in areas where large ocean basins are funneled into narrow bays, such as Cook Inlet in southern Alaska, the tides can reach extremes of 30 feet (9 meters). Such large tide extremes can make port operations problematic. Where tides are relatively weak, large rivers can form deltas, while strong tides can scour out the mouth of a river to prevent delta formation and produce an estuary.

In regions where a sufficient record of tides exists, tides can be accurately predicted and taken into account when planning military operations. However, strategic locations in the far reaches of the Pacific often had inadequate tide records. At Tarawa the Americans encountered an unexpected "dodging tide" that made the water level over the fringing reef much lower than anticipated, leading to disastrous casualties among troops whose landing craft were unable to clear the reef, forcing them to wade hundreds of yards ashore.

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