Fig. 685.—Drift Ice.
Doctor Croll has pointed out that the great eccentricity of the earth’s orbit existed about 210,000 years ago, when there was a difference between the nearest and farthest position of the earth and the sun of 12,000,000 of miles at least.[28] This is a very considerable distance even in the enormous spaces which intervene between us and the other planets of the solar system, and about that time the Glacial period arrived. Perhaps we may make this clearer by going back to the precession of the equinoxes.
The earth moves in an orbit called an ellipse, and the sun is not in the centre of this nearly circular path. We can now understand that the earth comes nearer to the sun sometimes and recedes at others. These points of nearest approach and greatest distance are termed perihelion and aphelion. In the latter case we are about 9,000,000 of miles farther from the sun than when in perihelion—that is, when the greatest “eccentricity” is reached. In addition to this the axis of the earth is continually changing in direction by reason of solar attraction at the equator. This shifting, as explained in the astronomical section, is very slight every year, and in the course of 24,000 years the conditions of the seasons will have completely changed round and back again,—for the northern and southern conditions will be reversed in our hemisphere. Day and night come twenty minutes earlier every year. We are now nearer the sun in winter as shown in diagram (page 497); when we change we shall be nearest the sun in summer and farthest in winter.
Fig. 686.—The Mer de Glace.
Doctor Croll, who has done much in his most interesting paper on changes of climate[29], tells us how this eccentricity of the earth’s orbit produced indirectly the Glacial epoch. He shows how, if in a period of the greatest “eccentricity” our winter came in aphelion, we should receive one-fifth less heat than now, but a correspondingly greater heat in summer. But if our winter under such circumstances fall (as now) in perihelion, the difference between winter and summer would be practically nil, because the sun during a period of the earth’s great eccentricity “could not warm the hemisphere whose summer happened to arrive in perihelion.” No doubt the sun’s rays would be very powerful, but the earth being covered with ice and snow could not be warmed; fogs would accrue and hide the sun, as at present in Antarctic summers, when the cold is very great. The warm ocean currents would be stopped, and the northern portion of our hemisphere would be, as it undoubtedly was, frozen over and covered with snow.
Fig. 687.—Mammoth and Irish Elk.
When we consider the millions of years since the earth is supposed to have been launched into space, we can imagine that the Glacial periods would occur frequently, and considering the very slow “precession” movement there, and the alternating tropical climate with graduations of temperature for thousands of years they would last long. The great Glacial period is computed to have begun 240,000 years ago and lasted 160,000 years with alternations of comparative summer; and so the years went on, season succeeding season, altering the appearance of the earth, and causing successive changes in the distribution of animal and vegetable life. Then the great mammalia, the mammoth and hippopotamus, with the hyæna, lion, and other felidæ came, and went when Arctic animals usurped their places. At the later Glacial epoch man must have arrived in Britain, and “this being so,” says Professor Geikie, “it is startling to recall in imagination those grand geological revolutions of which he must have been a witness.... He entered Britain at a time when our country was joined to Europe across the bed of the German Ocean; at a time when the winters were still severe enough to freeze over the rivers in the south of England; at a time when glaciers nestled in our upland and mountain valleys, and the Arctic mammalia occupied the land. He lived here long enough to witness a complete change of climate, to see the Arctic mammalia vanish from England, and the hippopotamus and its congeners take their places. At a later date, and while a mild and genial climate still continued, he beheld the sea slowly gain upon the land, until, little by little, step by step, a large portion of our country was submerged—a submergence which, as we know, reached in Wales to the extent of 1,300 feet or thereabouts.”