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.
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 feature
s, 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.
All this, of course, will require time on a scale the mind finds it difficult to conceive, and before it happens it is quite probable that the human race will have vanished from the earth. This may seem, then, like a Wellsian fantasy of a world so remote that we may dismiss it from our thoughts. But already, even in our allotted fraction of earthly time, we can see some of the effects of these cosmic processes. Our day is believed to be several seconds longer than that of Babylonian times. Britain’s Astronomer Royal recently called the attention of the American Philosophical Society to the fact that the world will soon have to choose between two kinds of time. The tide-induced lengthening of the day has already complicated the problems of human systems of keeping time. Conventional clocks, geared to the earth’s rotation, do not show the effect of the lengthening days. New atomic clocks now being constructed will show actual time and will differ from other clocks.
Although the tides have become tamer, and their range is now measured in tens instead of hundreds of feet, mariners are nevertheless greatly concerned not only with the stages of the tide and the set of the tidal currents, but with the many violent movements and disturbances of the sea that are indirectly related to the tides. Nothing the human mind has invented can tame a tide rip or control the rhythm of the water’s ebb and flow, and the most modern instruments cannot carry a vessel over a shoal until the tide has brought a sufficient depth of water over it. Even the Queen Mary waits for slack water to come to her pier in New York; otherwise the set of the tidal current might swing her against the pier with enough force to crush it. On the Bay of Fundy, because of the great range of tide, harbor activities in some of the ports follow a pattern as rhythmic as the tides themselves, for vessels can come to the docks to take on or discharge cargo during only a few hours on each tide, leaving promptly to avoid being stranded in mud at low water.
In the confinement of narrow passages or when opposed by contrary winds and swells, the tidal currents often move with uncontrollable violence, creating some of the most dangerous waterways of the world. It is only necessary to read the Coast Pilots and Sailing Directions for various parts of the world to understand the menace of such tidal currents to navigation.
‘Vessels around the Aleutians are in more danger from tidal currents than from any other cause, save the lack of surveys,’ says the postwar edition of the Alaska Pilot. Through Unalga and Akutan passes, which are among the most-used routes for vessels entering Bering Sea from the Pacific, strong tidal currents pour, making their force felt well offshore and setting vessels unexpectedly against the rocks. Through Akun Strait the flood tide has the velocity of a mountain torrent, with dangerous swirls and overfalls. In each of these passes the tide will raise heavy, choppy seas if opposed by wind or swells. ‘Vessels must be prepared to take seas aboard,’ warns the Pilot, for a 15-foot wave of a tide rip may suddenly rise and sweep across a vessel, and more than one man has been carried off to his death in this way.
On the opposite side of the world, the tide setting eastward from the open Atlantic presses between the islands of the Shetlands and Orkneys into the North Sea, and on the ebb returns through the same narrow passages. At certain stages of the tide these waters are dotted with dangerous eddies, with strange upward domings, or with sinister pits or depressions. Even in calm weather boats are warned to avoid the eddies of Pentland Firth, which are known as the Swilkie; and with an ebb tide and a northwest wind the heavy breaking seas of the Swilkie are a menace to vessels ‘which few, having once experienced, would be rash enough to encounter a second time.’
Edgar Allan Poe, in his ‘Descent into the Maelstrom,’ converted one of the more evil manifestations of the tide into literature. Few who have read the story will forget its drama—how the old man led his companion to a mountain cliff high above the sea and let him watch the water far below in the narrow passageway between the islands, with its sinister f
oam and scum, its uneasy bubbling and boiling, until suddenly the whirlpool was formed before his eyes and rushed with an appalling sound through the narrow waterway. Then the old man told the story of his own descent into the whirlpool and of his miraculous escape. Most of us have wondered how much of the story was fact, how much the creation of Poe’s fertile imagination. There actually is a Maelstrom and it exists where Poe placed it, between two of the islands of the Lofoten group off the west coast of Norway. It is, as he described it, a gigantic whirlpool or series of whirlpools, and men with their boats have actually been drawn down into these spinning funnels of water. Although Poe’s account exaggerates certain details, the essential facts on which he based his narrative are verified in the Sailing Directions for the Northwest and North Coasts of Norway, a practical and circumstantial document:
Though rumor has greatly exaggerated the importance of the Malström, or more properly Moskenstraumen, which runs between Mosken and Lofotodden, it is still the most dangerous tideway in Lofoten, its violence being due, in great measure, to the irregularity of the ground … As the strength of the tide increases the sea becomes heavier and the current more irregular, forming extensive eddies or whirlpools (Malström). During such periods no vessel should enter the Moskenstraumen.
These whirlpools are cavities in the form of an inverted bell, wide and rounded at the mouth and narrower toward the bottom; they are largest when first formed and are carried along with the current, diminishing gradually until they disappear; before the extinction of one, two or three more will appear, following each other like so many pits in the sea … Fishermen affirm that if they are aware of their approach to a whirlpool and have time to throw an oar or any other bulky body into it they will get over it safely; the reason is that when the continuity is broken and the whirling motion of the sea interrupted by something thrown into it the water must rush suddenly in on all sides and fill up the cavity. For the same reason, in strong breezes, when the waves break, though there may be a whirling round, there can be no cavity. In the Saltström boats and men have been drawn down by these vortices, and much loss of life has resulted.
The Sea Around Us Page 18
