Outlines of the Earth's History: A Popular Study in Physiography

by Nathaniel Southgate Shaler

In the stages of concentration the planetary nebulæ might well repeat those through which the greater solar mass proceeded. If the volume of the material were great, subordinate rings would be formed, which when they broke and concentrated would constitute secondary planets or satellites, such as our moon. For some reason as yet unknown the outer planets—in fact, all those in the solar system except the two inner, Venus and Mercury and the asteroids—formed such attendants. All these satellite-forming rings have broken and concentrated except the inner of Saturn, which remains as an intellectual treasure of the solar system to show the history of its development.

To the student who is not seeking the fulness of knowledge which astronomy has to offer, but desires only to acquaint himself with the more critical and important of the heavenly phenomena which help to explain the earth, these features of planetary movement should prove especially interesting for the reason that they shape the history of the spheres. As we shall hereafter see, the machinery of the earth's surface, all the life which it bears, its winds and rains—everything, indeed, save the actions which go on in the depths of the sphere—is determined by the heat and light which come from the sun. The conditions under which this vivifying tide is received have their origin in the planetary motion. If our earth's path around the centre of the system was a perfect circle, and if its polar axis lay at right angles to the plane of its journey, the share of light and heat which would fall upon any one point on the sphere would be perfectly uniform. There would be no variations in the length of day or night; no changes in the seasons; the winds everywhere would blow with exceeding steadiness—in fact, the present atmospheric confusion would be reduced to something like order. From age to age, except so far as the sun itself might vary in the amount of energy which it radiated, or lands rose up into the air or sunk down toward the sea level, the climate of each region would be perfectly stable. In the existing conditions the influences bring about unending variety. First of all, the inclined position of the polar axis causes the sun apparently to move across the heavens, so that it comes in an overhead position once or twice in the year in quite half the area of the lands and seas. This apparent swaying to and fro of the sun, due to the inclination of the axis of rotation, also affects the width of the climatal belts on either side of the equator, so that all parts of the earth receive a considerable share of the sun's influence. If the axis of the earth's rotation were at right angles to the plane of its orbit, there would be a narrow belt of high temperature about the equator, north and south of which the heat would grade off until at about the parallels of fifty degrees we should find a cold so considerable and uniform that life would probably fade away; and from those parallels to the poles the conditions would be those of permanent frost, and of days which would darken into the enduring night or twilight in the realm of the far north and south. Thus the wide habitability of the earth is an effect arising from the inclination of its polar axis.

Fig. 3.—Inclination of Planetary Orbits (from Chambers).

As the most valuable impression which the student can receive from his study of Nature is that sense of the order which has made possible all life, including his own, it will be well for him to imagine, as he may readily do, what would be the effect arising from changes in relations of earth and sun. Bringing the earth's axis in imagination into a position at right angles to the plane of the orbit, he will see that the effect would be to intensify the equatorial heat, and to rob the high latitudes of the share which they now have. On moving the axis gradually to positions where it approaches the plane of the orbit, he will note that each stage of the change widens the tropic belt. Bringing the polar axis down to the plane of the orbit, one hemisphere would receive unbroken sunshine, the other remaining in perpetual darkness and cold. In this condition, in place of an equatorial line we should have an equatorial point at the pole nearest the sun; thence the temperatures would grade away to the present equator, beyond which half the earth would be in more refrigerating condition than are the poles at the present day. In considering the movements of our planet, we shall see that no great changes in the position of the polar axis can have taken place. On this account the suggested alterations of the axis should not be taken as other than imaginary changes.

It is easy to see that with a circular orbit and with an inclined axis winter and summer would normally come always at the same point in the orbit, and that these seasons would be of perfectly even length. But, as we have before noted, the earth's path around the sun is in its form greatly affected by the attractions which are exercised by the neighbouring planets, principally by those great spheres which lie in the realm without its orbit, Jupiter and Saturn. When these attracting bodies, as is the case from time to time, though at long intervals, are brought together somewhere near to that part of the solar system in which the earth is moving around the sun, they draw our planet toward them, and so make its path very elliptical. When, however, they are so distributed that their pulling actions neutralize each other, the orbit returns more nearly to a circular form. The range in its eccentricity which can be brought about by these alterations is very great. When the path is most nearly circular, the difference in the major and minor axis may amount to as little as about five hundred thousand miles, or about one one hundred and eighty-sixth of its average diameter. When the variation is greatest the difference in these measurements may be as much as near thirteen million miles, or about one seventh of the mean width of the orbit.

The first and most evident effect arising from these changes of the orbit comes from the difference in the amount of heat which the earth may receive according as it is nearer or farther from the sun. As in the case of other fires, the nearer a body is to it the larger the share of light and heat which it will receive. In an orbit made elliptical by the planetary attraction the sun necessarily occupies one of the foci of the ellipse. The result is, of course, that the side of the earth which is toward the sun, while it is thus brought the nearer to the luminary, receives more energy in the form of light and heat than come to any part which is exposed when the spheres are farther away from each other in the other part of the orbit. Computations clearly show that the total amount of heat and the attendant light which the earth receives in a year is not affected by these changes in the form of its path. While it is true that it receives heat more rapidly in the half of the ellipse which is nearest the source of the inundation, it obtains less while it is farther away, and these two variations just balance each other.

Although the alterations in the eccentricity of its orbit do not vary the annual supply of heat which the earth receives, they are capable of changing the character of the seasons, and this in the way which we will now endeavour to set forth, though we must do it at the cost of considerable attention on the part of the reader, for the facts are somewhat complicated. In the first place, we must note that the ellipticity of the earth's orbit is not developed on fixed lines, but is endlessly varied, as we can readily imagine it would be for the reason that its form depends upon the wandering of the outer planetary spheres which pull the earth about. The longer axis of the ellipse is itself in constant motion in the direction in which the earth travels. This movement is slow, and at an irregular rate. It is easy to see that the effect of this action, which is called the revolution of the apsides, or, as the word means, the movement of the poles of the ellipse, is to bring the earth, when a given hemisphere is turned toward the sun, sometimes in the part of the orbit which is nearest the source of light and heat, and sometimes farther away. It may thus well come about that at one time the summer season of a hemisphere arrives when it is nearest the sun, so that the season, though hot, will be very short, while at another time the same season will arrive when the earth is farthest from the sun, and receives much less heat, which would tend to make a long and relatively cool summer. The reason for the difference in length of the seasons is to be found in the relative swiftness of the earth's revolution when it is nearest the sun, and the slowness when it is farther away.

There is a further complication arising from that curious phenomenon called the precession of the equinoxes, which has to be taken into account before we can sufficiently comprehend the effect of the varying eccentricity of the orbit on the earth's seasons. To understand this feature of precession we should first note that it means that each year the change from the winter to the summer—or, as we phrase it, the passage of the equinoctial line—occurs a little sooner than the year before. The cause of this is to be found in the attraction which the heavenly bodies, practically altogether the moon, exercises on the equatorial protuberance of the earth. We know that the diameter of our sphere at the equator is, on the average, something more than twenty-six miles greater than it is through the poles. We know, furthermore, that the position of the moon in relation to the earth is such that it causes the attraction on one half of this protuberance to be greater than it is upon the other. We readily perceive that this action will cause the polar axis to make a certain revolution, or, what comes to the same thing, that the plane of the equator will constantly be altering its position. Now, as the equinoctial points in the orbit depend for their position upon the attitude of the equatorial plane, we can conceive that the effect is a change in position of the place in that orbit where summer and winter begin. The actual result is to bring the seasonal points backward, step by step, through the orbit in a regular measure until in twenty-two thousand five hundred years they return to the place where they were before. This cycle of change was of old called the Annus Magnus, or great year.

If the earth's orbit were an ellipse, the major axis of which remained in the same position, we could readily reckon all the effects which arise from the variations of the great year. But this ellipse is ever changing in form, and in the measure of its departure from a circle the effects on the seasons distributed over a great period of time are exceedingly irregular. Now and then, at intervals of hundreds of thousands or millions of years, the orbit becomes very elliptical; then again for long periods it may in form approach a circle. When in the state of extreme ellipticity, the precession of the equinoxes will cause the hemispheres in turn each to have their winter and summer alternately near and far from the sun. It is easily seen that when the summer season comes to a hemisphere in the part of the orbit which is then nearest the sun the period will be very hot. When the summer came farthest from the sun that part of the year would have the temperature mitigated by its removal to a greater distance from the source of heat. A corresponding effect would be produced in the winter season. As long as the orbit remained eccentric the tendency would be to give alternately intense seasons to each hemisphere through periods of about twelve thousand years, the other hemisphere having at the same time a relatively slight variation in the summer and winter.