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The Solar System - Harper's


THE earth is a huge oblate or orange-shaped sphere, spinning on its shorter axis like a humming-top, yet at such a rate of speed as to seem standing still; it goes once round in twenty-four hours, its rotation being both the cause and the measure of day and night. The highest mountains range from four to five miles in height; the greatest depth of the ocean is probably little more than five miles, although Ross let down 27,000 feet of sounding-line in vain on one occasion. So that the earth's surface is very irregular; but its mountainous ridges and oceanic valleys are no greater things in proportion to its whole bulk, than the roughness of the rind of the orange it resembles in shape. The geological crust—that is to say, the total depth to which geologists suppose themselves to have reached in the way of observation—is no thicker in proportion than a sheet of thin writing paper pasted on a globe two feet in diameter. The surface of the earth is some 148,500,000 of miles in extent; and only one-fourth of that large space is dry land, the rest being ocean and ice. The atmosphere rises all round to a height between forty-five and fifty miles above the sea-level. The solar radiance sends such heat as it brings no deeper any where than 100 feet into the surface or scurfskin of the dry land—from forty to a hundred feet, one-third of the sun's heat being absorbed by the air. Yet the deeper man digs beyond the hundred feet, the warmer he finds the earth, and that at a somewhat determinable rate of increase. Supposing that rate of increment to go on toward the centre, it is computable that the solid underwork of the world, say granite by way of conjecture, must be in a state of fusion at no vast depth from the ground on which we tread. Let the scientific imagination descend a little lower, and it will find the melted granite in the form of a fiery vapor or gas—the dry steam of a red-hot liquid, in which the rock-built foundations of "the everlasting hills" melt like icebergs. But this is conjectural and probable, not observable and proved.

Far away from this spinning and perilous globe of ours, at the distance of some 95,000,000 miles, stands the sun. A ray of light, starting from his surface at any given moment, takes eight minutes to reach us, although light runs at the speed of 195,000 miles in a second. The sun is 1,380,000 times as large as the earth, and 355,000 times as heavy; but the stuff of which he is made is just about a fourth part as dense as the average matter of this world. The sun is of as light a substance, taking his whole body, as coal; whereas the earth is twice as heavy as brimstone, striking the mean between the air, the ocean, the dry land, and the internal vapor. The sun has an atmosphere like the earth, or rather he has two. One of them, close upon his solid surface, seems to resemble our own; it bears cloudy bodies in its upper levels. The other is a sort of fiery gas, surrounding the former, kindled and sustained in the calorific and luminous state, no man knows or can conjecture how. Storms in the lower atmosphere are constantly blowing this phosphorescent airy envelope aside, so as to afford us glimpses down into the (comparatively) dark and black recesses beneath. These are the spots on the sun. Galileo inferred the rotation of the sun on his axis from the motions of those spots. The explanation of those spots, afforded by the discoveries of Wilson and Herschel, diminishes the value of the inference; but no Copernican can doubt that the sun is forever turning, and that with unimaginable swiftness and impetuosity.

At the distance, then, of more than ninety-five millions of miles, this dim spot which men call earth, this great globe and all its dwellers, this ever-spinning planet, revolves around the sun once every year, that revolution being both the cause and the measure of that space of time. Its orbit is not a circle; it is an ellipse, but not very far removed from the circular path. The terrestrial axis is not at right angles to that ellipse, else there were no seasons; it is somewhat inclined. The earth, once regarded as the fixed and solid centre of creation, is now to be conceived of as a globular sphere of some fire-blown stream, bounded by a film of rock like a soap-bubble, carrying an unresting sea in the hollows of its rind, swathed in a soft gauze of air, going round upon itself every day, running round the sun every year; and all that with so much silence, security, and stillness of speed that nobody ever suspects the dread predicament of physical circumstance in which he wakes and sleeps, lives and dies, does good or evil, and passes away to judgment. It is difficult to realize the truth, now that it is told; for the knowledge of the intellect is one thing, and the consent of the whole man is quite another.

Precisely as the earth goes round the sun from year to year, the moon goes round the earth from month to month, and that at a distance of some 240,000 miles; the same lunar side or hemisphere being always turned toward us, although that satellite turns upon her own axis as well as the earth and the sun. The earth is in repose so far as the moon is concerned; it is her sun. The two combined, being as true a unity as any chemical molecule which is composed of two atoms, go round the sun as if they were one; the earth carries her moon with her. So that it is possible, if not probable in the first instance, that the sun, though in repose as to the earth and her moon (and, indeed, to all the planets yet to be mentioned) may be in motion on some vast orbit of his own; an orbit along which he carries all his planetary adherents with him, just as the earth takes her moon round the sun. It is curious to perceive how, not only in the case of our own moon, but in the cases of the moons of Jupiter, Saturn, and Uranus, and actually in those of all the planets considered as the moons of the sun, the Platonic epicycle really holds good. The earth turns on her heel, with the moon held out at arm's length, while she goes round the amphitheatre before the solar eye; so do the other moon-bearers. So does the sun himself upon a vaster arena and before a greater spectator, like another Briareus; holding out his seventeen planets, and nobody knows how many comets, in his hundred hands. The moons, of those solar planets which have them, represent the epi-epicyclical orbits of the Ptolemaic theory. It is curious, and also touching, to notice how often the errors of man are thus the shadows of truth. Were it not for the preceding shadows, indeed, the substance would never arrive; and therefore the Ptolemaics of the world are second, in value and in merit, only to epochal discoverers like Copernicus.

Suppose the sun to be represented by a radiant little orb two feet in diameter, in order to bring it within the measure of our eye; then this great globe of ours, with all its stupendous histories, is no bigger than a full-sized pea in proportion, revolving at the distance of 215 feet. Neptune, the outermost and last discovered of the planets, would stand at the distance of a mile and a quarter from a sun of that imaginary size, and it would be no larger than a cherry. Another cherry at the distance of three-quarters of a mile would stand for Uranus. Saturn would be a small orange at two-fifths of a mile from our two-feet solar body. A middle-sized orange, at the distance of a quarter of a mile, would be his Jupiter. At some 500 feet the nine little planets, commonly called asteroids, probably enough the fragments of an exploded orb, and now moving in a sort of group, would be represented by as many grains of sand. A pin-head, at 327 feet, would do for Mars. Then comes the earth. Still nearer the sun, namely at 142 feet from our present model, revolves Venus, of the dimensions of a pea. And finally little Mercury wheels along his orbit, with a radius of 82 feet, and the dimensions of a mustard seed.

Add the terrestrial moon, the four moons of Jupiter, the ring within ring that whirls round Saturn like an endless moon, the eight ordinary moons of that extraordinary planet, the moons of Uranus and Neptune (yet uncertain in their number), and it is impossible to say how many comets, not to forget the enormous groups or hosts of comparatively small stones or meteors, which are believed to be revolving round the solar centre like pigmy asteroids; and the Copernican conception of the mere constitution of the solar system, as developed by time and toil, is completed. The sun is 882,000 miles in diameter; the earth is 7926; Juno is 79; Saturn, 79,160, and so forth. The earth is more than five times as heavy as water; Saturn is as light as cork. The earth rotates in twenty-four hours; Jupiter in ten. The earth revolves in a year; Mars in a year and ten months; Mercury in about three months; Venus in seven and a half months; Jupiter in eleven years; Saturn in twenty-nine; Uranus in eighty-four; Neptune in a hundred and sixty-four. A summer in Mercury lasts some three weeks; in Neptune forty-one years. Light leaps from the sun to the earth in eight minutes; to Neptune in four hours. In short, the reader has to consider thousands of discovered facts, to carry with him a whole world of indubitable inference, and to study a truly wonderful bringing of the whole machinery, or rather organization, to geometrical law, before he can apprehend how glorious a whole the Copernican astronomy has become.