Читать книгу Outlines of the Earth's History: A Popular Study in Physiography - Nathaniel Southgate Shaler - Страница 9

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We have seen that the matter in the visible universe everywhere tends to gather into vast associations which appear to us as stars, and that these orbs are engaged in ceaseless motion in journeys through space. In only one of these aggregations—that which makes our own solar system—are the bodies sufficiently near to our eyes for us, even with the resources of our telescopes and other instruments, to divine something of the details which they exhibit. In studying what we may concerning the family of the sun, the planets, and their satellites, we may reasonably be assured that we are tracing a history which with many differences is in general repeated in the development of each star in the firmament. Therefore the inquiry is one of vast range and import.

Following, as we may reasonably do, the nebular hypothesis—a view which, though not wholly proved, is eminently probable—we may regard our solar system as having begun when the matter of which it is composed, then in a finely divided, cloudy state, was separated from the similar material which went to make the neighbouring fixed stars. The period when our solar system began its individual life was remote beyond the possibility of conception. Naturalists are pretty well agreed that living beings began to exist upon the earth at least a hundred million years ago; but the beginnings of our solar system must be placed at a date very many times as remote from the present day.[1]

According to the nebular theory, the original vapour of the solar system began to fall in toward its centre and to whirl about that point at a time long before the mass had shrunk to the present limits of the solar system as defined by the path of the outermost planets. At successive stages of the concentration, rings after the manner of those of Saturn separated from the disklike mass, each breaking up and consolidating into a body of nebulous matter which followed in the same path, generally forming rings which became by the same process the moons or satellites of the sphere. In this way the sun produced eight planets which are known, and possibly others of small size on the outer verge of the system which have eluded discovery. According to this view, the planetary masses were born in succession, the farthest away being the oldest. It is, however, held by an able authority that the mass of the solar system would first form a rather flat disk, the several rings forming and breaking into planets at about the same time. The conditions in Saturn, where the inner ring remains parted, favours the view just stated.

Before making a brief statement of the several planets, the asteroids, and the satellites, it will be well to consider in a general way the motions of these bodies about their centres and about the sun. The most characteristic and invariable of these movements is that by which each of the planetary spheres, as well as the satellites, describes an orbit around the gravitative centre which has the most influence upon it—the sun. To conceive the nature of this movement, it will be well to imagine a single planet revolving around the sun, each of these bodies being perfect spheres, and the two the only members of the solar system. In this condition the attraction of the two bodies would cause them to circle around a common centre of gravity, which, if the planet were not larger or the sun smaller than is the case in our solar system, would lie within the mass of the sun. In proportion as the two bodies might approach each other in size, the centre of gravity would come the nearer to the middle point in a line connecting the two spheres. In this condition of a sun with a single planet, whatever were the relative size of sun and planet, the orbits which they traverse would be circular. In this state of affairs it should be noted that each of the two bodies would have its plane of rotation permanently in the same position. Even if the spheres were more or less flattened about the poles of their axes, as is the case with all the planets which we have been able carefully to measure, as well as with the sun, provided the axes of rotation were precisely parallel to each other, the mutual attraction of the masses would cause no disturbance of the spheres. The same would be the case if the polar axis of one sphere stood precisely at right angles to that of the other. If, however, the spheres were somewhat flattened at the poles, and the axes inclined to each other, then the pull of one mass on the other would cause the polar axes to keep up a constant movement which is called nutation, or nodding.

The reason why this nodding movement of the polar axes would occur when these lines were inclined to each other is not difficult to see if we remember that the attraction of masses upon each other is inversely as the square of the distance; each sphere, pulling on the equatorial bulging of the other, pulls most effectively on the part of it which is nearest, and tends to draw it down toward its centre. The result is that the axes of the attracted spheres are given a wobbling movement, such as we may note in the spinning top, though in the toy the cause of the motion is not that which we are considering.

If, now, in that excellent field for the experiment we are essaying, the mind's eye, we add a second planet outside of the single sphere which we have so far supposed to journey about the sun, or rather about the common centre of gravity, we perceive at once that we have introduced an element which leads to a complication of much importance. The new sphere would, of course, pull upon the others in the measure of its gravitative value—i.e., its weight. The centre of gravity of the system would now be determined not by two distinct bodies, but by three. If we conceive the second planet to journey around the sun at such a rate that a straight line always connected the centres of the three orbs, then the only effect on their gravitative centre would be to draw the first-mentioned planet a little farther away from the centre of the sun; but in our own solar system, and probably in all others, this supposition is inadmissible, because the planets have longer journeys to go and also move slower, the farther they are from the sun. Thus Mercury completes the circle of its year in eighty-eight of our days, while the outermost planet requires sixty thousand days (more than one hundred and sixty-four years) for the same task. The result is not only that the centre of gravity of the system is somewhat displaced—itself a matter of no great account—but also that the orbit of the original planet ceases to be circled and becomes elliptical, and this for the evident reason that the sphere will be drawn somewhat away from the sun when the second planet happens to lie in the part of its orbit immediately outside of its position, in which case the pull is away from the solar centre; while, on the other hand, when the new planet was on the other side of the sun, its pull would serve to intensify the attraction which drew the first sphere toward the centre of gravity. As the pulling action of the three bodies upon each other, as well as upon their equatorial protuberances, would vary with every change in their relative position, however slight, the variations in the form of their orbits, even if the spheres were but three in number, would be very important. The consequences of these perturbations will appear in the sequel.

In our solar system, though there are but eight great planets, the group of asteroids, and perhaps a score of satellites, the variety of orbital and axial movement which is developed taxes the computing genius of the ablest astronomer. The path which our earth follows around the sun, though it may in general and for convenience be described as a variable ellipse, is, in fact, a line of such complication that if we should essay a diagram of it on the scale of this page it would not be possible to represent any considerable part of its deviations. These, in fact, would elude depiction, even if the draughtsman had a sheet for his drawing as large as the orbit itself, for every particle of matter in space, even if it be lodged beyond the limits of the farthest stars revealed to us by the telescope, exercises a certain attraction, which, however small, is effective on the mass of the earth. Science has to render its conclusions in general terms, and we can safely take them as such; but in this, as in other instances, it is well to qualify our acceptance of the statements by the memory that all things are infinitely more complicated than we can possibly conceive or represent them to be.

We have next to consider the rotations of the planetary spheres upon their axes, together with the similar movement, or lack of it, in the case of their satellites. This rotation, according to the nebular hypothesis, may be explained by the movements which would set up in the share of matter which was at first a ring of the solar nebula, and which afterward gathered into the planetary aggregation. The way of it may be briefly set forth as follows: Such a ring doubtless had a diameter of some million miles; we readily perceive that the particles of matter in the outer part of the belt would have a swifter movement around the sun than those on the inside. When by some disturbance, as possibly by the passage of a great meteoric body of a considerable gravitative power, this ring was broken in two, the particles composing it on either side would, because of their mutual attraction, tend to draw away from the breach, widening that gap until the matter of the broken ring was aggregated into a sphere of the star dust or vapour. When the nebulous matter originally in the ring became aggregated into a spherical form, it would, on account of the different rates at which the particles were moving when they came together, be the surer to fall in toward the centre, not in straight lines, but in curves—in other words, the mass would necessarily take on a movement of rotation essentially like that which we have described in setting forth the nebular hypothesis.

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.