In common language we would state that the shape of the path of the earth around the sun was sometimes much more elongated and elliptical than at others. The line drawn through the longest part of an ellipse is called the major axis. Now the sun does not occupy the center of this line, but is placed to one side of it; or, in other words, occupies one focus of the ellipse. It will thus be seen that the earth, at one time during its yearly journey, is considerably nearer to the sun than at others. The point where it approaches nearest the sun is called Perihelion, and the point where it reaches the greatest distance from the sun is called its Aphelion. It will be readily seen that the more elliptical its orbit becomes the greater will be the difference between the perihelion and aphelion distance of the sun. At present the earth is about three millions of miles nearer the sun in perihelion than in aphelion. But we must remember the orbit of the earth is now nearly circular. There have been times in the past when the difference was about thirteen millions of miles. We must not forget to add, that the change in the shape of the earth’s orbit is not a regular increase and decrease between well-known extremes. It is caused by the attraction of the other planets. It has been calculated at intervals of ten thousand years for the last million years. In this way it has been found that “the intervals between connective turning points are very unequal in length, and the actual maximum and minimum values of the eccentricity are themselves variable. In this way it comes about that some periods of high eccentricity have lasted much longer than others, and that the orbit has been more elliptical at some epochs of high eccentricity than at others.”14 We have just seen that the earth is nearer the sun at one time of the year than at another. At present the earth passes its perihelion point in the Winter of the Northern Hemisphere, and its aphelion point in the Summer. We will for the present suppose that it always reaches the points at the same season of the year. Let us see if the diminished distance from the sun in Winter has any thing to do with the climate.
If so, this effect will be greatly magnified during a period of high eccentricity, such as the earth has certainly passed through in the past. We will state first, that the more elliptical the orbit becomes, the longer Summer we have, and the shorter Winter. Astronomically, Spring begins the 20th of March, and Fall the 22d of September. By counting the days between the epochs it will be found that the Spring and Summer part of the year is seven days longer than the Fall and Winter part. But if the earth’s orbit becomes as highly eccentrical as in the past, this difference would be thirty-six days.15
This would give us a long Spring and Summer, but a short Fall and Winter. This in itself would make a great difference. We must beer in mind, however, that at such a time as we are here considering, the earth would be ten millions of miles nearer the sun in Winter than at present. It would certainly then receive more heat in a given time during Winter than at present.16 Mr. Croll estimates that whereas the difference in heat received during a given time is now one-fifteenth,17 at the time we are considering it would be one-fifth. Hence we see that at such a time the Winter would not only be much shorter than now, but at the same time would be much milder.
These are not all the results that would follow an increase of eccentricity. The climate of Europe and North America is largely modified by those great ocean currents—the Gulf Stream and the Japan current. Owing to causes we will not here consider, these currents would be greatly increased at such a time. As a result of these combined causes, Mr. Croll estimates that during a period of high eccentricity the difference between Winter and Summer in the Northern Hemisphere would be practically obliterate. The Winter would not only be short, but very mild, and but little snow would form, while the sun of the long Summers, though not shining as intense as at present, would not have to melt off a great layer of snow and ice, but the ground became quickly heated, and so warmed the air. Hence, if Mr. Croll be correct, a period of high eccentricity would certainly produce a climate in the Northern Hemisphere such as characterized many of the mild interglacial epochs as long as the earth passed its perihelion point in Winter.
We have so far only considered the Northern Hemisphere. As every one knows, while we have Winter, the Southern Hemisphere has Summer. So at the very time we would enjoy the mild short Winters, the Southern Hemisphere would be doomed to experience Winters of greatly increased length and severity. As a consequence, immense fields of snow would be formed, which, by pressure, would be changed to ice, and creep away as a desolating glacier. It is quite true that the short Summer sun would shine with increased warmth, but owing to many causes it would not avail to free the land from snow and ice.
As Mr. Geikie points out, “An increased amount of evaporation would certainly take place, but the moisture-laden air would be chilled by coming into contact with the vast sheets of snow, and hence the vapor would condense into thick fogs and cloud the sky. In this way the sun’s rays would be, to a large extent, cut off, and unable to reach the earth, and consequently the Winter’s snow would not be all melted away.” Hence it follows that at the very time the Northern Hemisphere would enjoy a mild interglacial climate, universal Spring, so to speak, the Southern Hemisphere would be encased in the ice and snow of an eternal Winter.
But the earth has not always reached its perihelion point during the Winter season of the Northern Hemisphere. Owing to causes that we need not here consider, the earth reaches its perihelion point about twenty minutes earlier each year, so if it now passes its perihelion in Winter of the Northern Hemisphere, in about ten thousand years from now it will reach it in Summer, and in twenty-one thousand, years it will again be at perihelion in Winter. But see what important consequences follow from this. If during a period of high eccentricity we are in the enjoyment of short mild Winters and long pleasant Summers, in ten thousand years this would certainly be changed. Our Summer season would become short and heated; our Winters long and intensely cold. Year by year it would be later in the season before the sun could free the land from snow, and at length in deep ravines and on hill-tops the snow would linger through the brief Summer, and the mild interglacial age will have passed away, and again the Northern Hemisphere will be visited by snow and ice of a truly. Glacial Age. If, therefore, a period of high eccentricity lasts through the many thousand years, we must expect more than one return of glacial cold interspersed by mild interglacial climates.
We have tried in these last few pages to give a clear statement of what is known as Croll’s theory of the Glacial Age. There is no question but what the earth does thus vary in its position with regard to the sun, and beyond a doubt this must produce some effect on the climate, and we can truthfully state that the more the complicated question of the climate of the earth is studied, the more grounds do scholars find for affirming that indirectly this effect must have been very great. And yet we can not say that this theory is accepted as a satisfactory one even by the majority of scholars. Many of those who do not reject it think it not proven. Therefore, before interrogating the astronomer as to the data of the Glacial Age, according to the terms of this theory, let us see what other causes are, adduced; then we can more readily accept or reject the conclusions as to the antiquity of man which this theory would necessitate us to adopt.
The only other cause to which we can assign the glacial cold, that is considered with any favor by geologists, is geographical; that is to say, depending on the distribution of land and water. Glaciers depend on the amount of snow-fall. In any country where the amount of snow-fall is so great that it is not all evaporated or melted by the Summer’s sun, and consequently increases from year to year, glaciers must soon appear, and these icy rivers would ere-long, flow away to lower levels. If we suppose, with Sir Charles Lyell, that the lands of the globe were all to be gathered around the equator, and the waters were gathered around the poles, it is manifest that there would be no such a thing as extremes of temperature, and it is, perhaps, doubtful whether ice would form, even in polar areas.18 At any rate, no glaciers could be formed, as there would be no land on which snow could gather in great quantities.
If, however, we reverse this picture, and conceive of the land gathered in a compact mass around the poles, shutting out the water, but consider the equatorial region of the earth to be occupied by the waters of the ocean, we would manifestly have a very different scene. From the ocean moisture-laden winds would flow over the polar lands. The snowfall would necessarily be great. In short, we can not doubt but what all the land of the earth would be covered with glaciers.19