Gravity is the force that bodies exert on one another. Specifically, bodies attract each other with a force determined by their size and their distance apart. In the Earth's gravitational field, the moon is the biggest player. The two orbs attract each other with such force that the "deformable" part of the Earth -- namely, the water on the surface -- bulges out on the side closest to the moon. (Also on the side exactly opposite it, for complicated reasons.)
That bulge is high tide. It is also, in the language of physics, a "standing wave" of water on the Earth's surface. Because the planet is rotating, however, a new surface containing new water is constantly "becoming" the wave. That rotation creates the ebbing and flooding of the tides.
The fact that the tide advances across the ocean surface as a standing wave and standing trough makes for curious (and hard to believe) facts in this part of the world.
Because of its length, Chesapeake Bay is one of the few places where a full tidal wavelength can be seen. This means that a complete tide cycle can exist in the bay at one time. The tide can be high at Cape Charles at the entrance to the bay, and at Havre de Grace, Md., near the northern end of the estuary, while simultaneously being low at the mouth of the Potomac River, roughly halfway between.
What happens when the tide turns -- which is to say, when a section of the bay's surface moves across the peak or through the trough of the wave -- depends on local variables as much as large, astronomical forces. For example, in the Chesapeake near the Bay Bridge, the tide turns on the western side before it turns on the Eastern Shore. It also turns in the deep water of the bay before it turns on the surface water.
Friction between water and land is the main explanation for both observations.
The bay bottom drops off more gradually on the western side than on the east, giving it more frictional contact with the water. This tends to keep the water from reaching great velocity when the tide rises and falls, creating lateral movement of water known as "tidal currents." However, if the water near the shore tends not to move fast, it does tend to move early in a tide cycle because it feels the drag of the rotating Earth more than offshore water does.
"As a generality, a given stage of tide starts in the shallows and propagates into the center," says William Boicourt, a professor of oceanography at the Horn Point Laboratory at the University of Maryland Center for Environmental Science in Cambridge, Md.
The exception is the water in a bay's deepest channels or trenches. It has a lot of frictional contact with the bottom. It also has the weight of the water column bearing down on it. Like the shallow, shoreward water, the deep-trench water stays relatively still but also changes direction relatively easily.
What this means is that, for brief periods, an area of the bay can experience incoming and outgoing tides simultaneously. The tide can be incoming at the bay's depths, and outgoing at the surface; rising on the western shore, but ebbing on the Eastern Shore.
This paradoxical situation doesn't last long. But while it lasts, the bay swimmers try to exploit it.
When a tide turns (it doesn't matter whether it's been high or low) the water is temporarily still. It's neither coming nor going, neither flooding nor ebbing. Like stages of the tide, this period of slackness moves from west to east, at least in the part of the bay near the bridge.
The race directors time the start of the race so that most swimmers will be in the zone of relatively current-free water as it moves across the bay. The slack water, of course, doesn't carry them across. It simply lets them devote their muscle power toward moving in a west-to-east direction, without having to fight north-south currents at the same time.
Or at least that's the idea.
Weather conditions can greatly alter surface currents, and shorten or obliterate the time of slack water between tides. Last year, a weather service small-craft advisory nearly scrubbed the race, which depends on the services of 70 power boats and 50 sea kayaks to patrol the course. (The advisory was lifted an hour before starting time.) And even in the best of years, the slowest swimmers end up getting caught by the tide, making a difficult task even harder.
* Copyright 1998 The Washington Post Company