What is it about one place that will bring 40 or 50 feet of water rising about its shores, while at another place lying under the same moon and sun, the tide will rise only a few inches? What, for example, can be the explanation of the great tides on the Bay of Fundy, while only a few hundred miles away at Nantucket Island, on the shores of the same ocean, the tide range is little more than a foot?
The modern theory of tidal oscillation seems to offer the best explanation of such local differences—the rocking up and down of water in each natural basin about a central, virtually tideless node. Nantucket is located near the node of its basin, where there is little motion, hence a small tide range. Passing north-eastward along the shores of this basin, we find the tides becoming progressively higher, with a 6-foot range at Nauset Harbor on Cape Cod, 8.9 feet at Gloucester, 15.7 feet at West Quoddy Head, 20.9 feet at St. John, and 39.4 feet at Folly Point. The Nova Scotia shore of the Bay of Fundy has somewhat higher tides than the corresponding points on the New Brunswick shore, and the highest tides of all are in Minas Basin at the head of the Bay. The immense movements of water in the Bay of Fundy result from a combination of circumstances. The bay lies at the end of an oscillating basin. Furthermore, the natural period of oscillation of the basin is approximately 12 hours. This very nearly coincides with the period of the ocean tide. Therefore the water movement within the bay is sustained and enormously increased by the ocean tide. The narrowing and shallowing of the bay in its upper reaches, compelling the huge masses of water to crowd into a constantly diminishing area, also contribute to the great heights of the Fundy tides.
The tidal rhythms, as well as the range of tide, vary from ocean to ocean. Flood tide and ebb succeed each other around the world, as night follows day, but as to whether there shall be two high tides and two low in each lunar day, or only one, there is no unvarying rule. To those who know best the Atlantic Ocean— either its eastern or western shores—the rhythm of two high tides and two low tides in each day seems ‘normal.’ Here, on each flood tide, the water advances about as far as the preceding high; and succeeding ebb tides fall about equally low. But in that great inland sea of the Atlantic, the Gulf of Mexico, a different rhythm prevails around most of its borders. At best the tidal rise here is but a slight movement, of no more than a foot or two. At certain places on the shores of the Gulf it is a long, deliberate undulation—one rise and one fall in the lunar day of 24 hours plus 50 minutes—resembling the untroubled breathing of that earth monster to whom the ancients attributed all tides. This ‘diurnal rhythm’ is found in scattered places about the earth—such as at Saint Michael, Alaska, and at Do Son in French Indo-China—as well as in the Gulf of Mexico. By far the greater part of the world’s coasts—most of the Pacific basin and the shores of the Indian Ocean—display a mixture of the diurnal and semidiurnal types of tide. There are two high and two low tides in a day, but the succeeding floods may be so unequal that the second scarcely rises to mean sea level; or it may be the ebb tides that are of extreme inequality.
There seems to be no simple explanation of why some parts of the ocean should respond to the pull of sun and moon with one rhythm and other parts with another, although the matter is perfectly clear to tidal scientists on the basis of mathematical calculations. To gain some inkling of the reasons, we must recall the many separate components of the tide-producing force, which in turn result from the changing relative positions of sun, moon, and earth. Depending on local geographic features, every part of earth and sea, while affected in some degree by each component, is more responsive to some than to others. Presumably the shape and depths of the Atlantic basin cause it to respond most strongly to the forces that produce a semidiurnal rhythm. The Pacific and Indian oceans, on the other hand, are affected by both the diurnal and semidiurnal forces, and a mixed tide results.
The island of Tahiti is a classic example of the way even a small area may react to one of the tide-producing forces to the virtual exclusion of the others. On Tahiti, it is sometimes said, you can tell the time of day by looking out at the beach and noticing the stage of the tide. This is not strictly true, but the legend has a certain basis. With slight variations, high tide occurs at noon and at midnight; low water, at six o’clock morning and evening. The tides thus ignore the effect of the moon, which is to advance the time of the tides by 50 minutes each day. Why should the tides of Tahiti follow the sun instead of the moon? The most favored explanation is that the island lies at the axis or node of one of the basins set in oscillation by the moon. There is very little motion in response to the moon at this point, and the waters are therefore free to move in the rhythm induced by the sun.
If the history of the earth’s tides should one day be written by some observer of the universe, it would no doubt be said that they reached their greatest grandeur and power in the younger days of Earth, and that they slowly grew feebler and less imposing until one day they ceased to be. For the tides were not always as they are today, and as with all that is earthly, their days are numbered.
In the days when the earth was young, the coming in of the tide must have been a stupendous event. If the moon was, as we have supposed in an earlier chapter, formed by the tearing away of a part of the outer crust of the earth, it must have remained for a time very close to its parent. Its present position is the consequence of being pushed farther and farther away from the earth for some 2 billion years. When it was half its present distance from the earth, its power over the ocean tides was eight times as great as now, and the tidal range may even then have been several hundred feet on certain shores. But when the earth was only a few million years old, assuming that the deep ocean basins were then formed, the sweep of the tides must have been beyond all comprehension. Twice each day, the fury of the incoming waters would inundate all the margins of the continents. The range of the surf must have been enormously extended by the reach of the tides, so that the waves would batter the crests of high cliffs and sweep inland to erode the continents. The fury of such tides would contribute not a little to the general bleakness and grimness and uninhabitability of the young earth.
Under such conditions, no living thing could exist on the shores or pass beyond them, and, had conditions not changed, it is reasonable to suppose that life would have evolved no further than the fishes. But over the millions of years the moon has receded, driven away by the friction of the tides it creates. The very movement of the water over the bed of the ocean, over the shallow edges of the continents, and over the inland seas carries within itself the power that is slowly destroying the tides, for tidal friction is gradually slowing down the rotation of the earth. In those early days we have spoken of, it took the earth a much shorter time—perhaps only about 4 hours—to make a complete rotation on its axis. Since then, the spinning of the globe has been so greatly slowed that a rotation now requires, as everyone knows, about 24 hours. This retarding will continue, according to mathematicians, until the day is about 50 times as long as it is now.
And all the while the tidal friction will be exerting a second effect, pushing the moon father way, just as it has already pushed it out more than 200,000 miles. (According to the laws of mechanics, as the rotation of the earth is retarded, that of the moon must be accelerated, and centrifugal force will carry it farther away.) As the moon recedes, it will, of course, have less power over the tides and they will grow weaker. It will also take the moon longer to complete its orbit around the earth. When finally the length of the day and of the month coincide, the moon will no longer rotate relatively to the earth, and there will be no lunar tides.