The Moving Tides
In every country the Moon keeps ever the rule of alliance with the Sea which it once for all has agreed upon.
THERE IS NO DROP of water in the ocean, not even in the deepest parts of the abyss, that does not know and respond to the mysterious forces that create the tide. No other force that affects the sea is so strong. Compared with the tide the wind-created waves are surface movements felt, at most, no more than a hundred fathoms below the surface. So, despite their impressive sweep, are the planetary currents, which seldom involve more than the upper several hundred fathoms. The masses of water affected by the tidal movement are enormous, as will be clear from one example. Into one small bay on the east coast of North America—Passamaquoddy—2 billion tons of water are carried by the tidal currents twice each day; into the whole Bay of Fundy, 100 billion tons.
Here and there we find dramatic illustration of the fact that the tides affect the whole ocean, from its surface to its floor. The meeting of opposing tidal currents in the Strait of Messina creates whirlpools (one of them is Charybdis of classical fame) which so deeply stir the waters of the strait that fish bearing all the marks of abyssal existence, their eyes atrophied or abnormally large, their bodies studded with phosphorescent organs, frequently are cast up on the lighthouse beach, and the whole area yields a rich collection of deep-sea fauna for the Institute of Marine Biology at Messina.
The tides are a response of the mobile waters of the ocean to the pull of the moon and the more distant sun. In theory, there is a gravitational attraction between every drop of sea water and even the outermost star of the universe. In practice, however, the pull of the remote stars is so slight as to be obliterated in the vaster movements by which the ocean yields to the moon and the sun. Anyone who has lived near tidewater knows that the moon, far more than the sun, controls the tides. He has noticed that, just as the moon rises later each day by fifty minutes, on the average, than the day before, so, in most places, the time of high tide is correspondingly later each day. And as the moon waxes and wanes in its monthly cycle, so the height of the tide varies. Twice each month, when the moon is a mere thread of silver in the sky, and again when it is full, we have the strongest tidal movements—the highest flood tides and the lower ebb tides of the lunar month. These are called the spring tides. At these times sun, moon, and earth are directly in line and the pull of the two heavenly bodies is added together to bring the water high on the beaches, and send its surf leaping upward against the sea cliffs, and draw a brimming tide into the harbors so that the boats float high beside their wharfs. And twice each month, at the quarters of the moon, when sun, moon, and earth lie at the apexes of a triangle, and the pull of sun and moon are opposed, we have the moderate tidal movements called the neap tides. Then the difference between high and lower water is less than at any other time during the month.
That the sun, with a mass 27 million times that of the moon, should have less influence over the tides than a small satellite of the earth is at first surprising. But in the mechanics of the universe, nearness counts for more than distant mass, and when all the mathematical calculations have been made we find that the moon’s power over the tides is more than twice that of the sun.
The tides are enormously more complicated than all this would suggest. The influence of sun and moon is constantly changing, varying with the phases of the moon, with the distance of moon and sun from the earth, and with the position of each to north or south of the equator. They are complicated further by the fact that every body of water, whether natural or artificial, has its own period of oscillation. Disturb its waters and they will move with a seesaw or rocking motion, with the most pronounced movement at the ends of the container, the least motion at the center. Tidal scientists now believe that the ocean contains a number of ‘basins,’ each with its own period of oscillation determined by its length and depth. The disturbance that sets the water in motion is the attracting force of the moon and sun. But the kind of motion, that is, the period of the swing of the water, depends upon the physical dimensions of the basin. What this means in terms of actual tides we shall presently see.
The tides present a striking paradox, and the essence of it is this: the force that sets them in motion is cosmic, lying wholly outside the earth and presumably acting impartially on all parts of the globe, but the nature of the tide at any particular place is a local matter, with astonishing differences occurring within a very short geographic distance. When we spend a long summer holiday at the seashore we may become aware that the tide in our cove behaves very differently from that at a friend’s place twenty miles up the coast, and is strikingly different from what we may have known in some other locality. If we are summering on Nantucket Island our boating and swimming will be little disturbed by the tides, for the range between high water and low is only about a foot or two. But if we choose to vacation near the upper part of the Bay of Fundy, we must accommodate ourselves to a rise and fall of 40 to 50 feet, although both places are included within the same body of water—the Gulf of Maine. Or if we spend our holiday on Chesapeake Bay we may find that the time of high water each day varies by as much as 12 hours in different places on the shores of the same bay.
The truth of the matter is that local topography is all-important in determining the features that to our minds make ‘the tide.’ The attractive force of the heavenly bodies sets the water in motion, but how, and how far, and how strongly it will rise depend on such things as the slope of the bottom, the depth of a channel, or the width of a bay’s entrance.
The United States Coast and Geodetic Survey has a remarkable, robotlike machine with which it can predict the time and height of the tide on any past or future date, for any part of the world, on one essential condition. This is that at some time local observations must have been made to show how the topographic features of the place modify and direct the tidal movements.
Perhaps the most striking differences are in the range of tide, which varies tremendously in different parts of the world, so that what the inhabitants of one place might consider disastrously high water might be regarded as no tide at all by coastal communities only a hundred miles distant. The highest tides in the world occur in the Bay of Fundy, with a rise of about 50 feet in Minas Basin near the head of the Bay at the spring tides. At least half a dozen other places scattered around the world have a tidal range of more than 30 feet—Puerto Gallegos in Argentina and Cook Inlet in Alaska, Frobisher Bay in Davis Strait, the Koksoak River emptying into Hudson Strait, and the Bay of St. Malo in France come to mind. At many other places ‘high tide’ may mean a rise of only a foot or so, perhaps only a few inches. The tides of Tahiti rise and fall in a gentle movement, with a difference of no more than a foot between high water and low. On most oceanic islands the range of the tide is slight. But it is never safe to generalize about the kinds of places that have high or low tides, because two areas that are not far apart may respond in very different ways to the tide-producing forces. At the Atlantic end of the Panama Canal the tidal range is not more than 1 or 2 feet, but at the Pacific end, only 40 miles away, the range is 12 to 16 feet. The Sea of Okhotsk is another example of the way the height of the tide varies. Throughout much of the Sea the tides are moderate—only about 2 feet—but in some parts of the Sea there is a 10-foot rise, and at the head of one of its arms—the Gulf of Penjinsk—the rise is 37 feet.