"In gazing at our fellow-planets on a clear night, as we see them stand out with pre-eminent brightness among the twinkling stars, who has not longed to penetrate the mystery of their being, and to know whether they, like our own Earth, are worlds full of life and movement? The vast distance that intervenes between us forbids us to expect a direct solution of the question, for no instrument yet made, or that we can hope to make, will bring their possible inhabitants within the range of our vision. We are reduced, therefore, to survey them with the sifting force of intellect, and to rest contented with such circumstantial proof as is derived from a knowledge of their general structure, and the analogies subsisting between them and our Earth.
"Among our nearest neighbors, Venus is nearly the size of our Earth; and Mercury and Mars, though considerably smaller, would still form worlds which, to our ideas, would not in their magnitude be so very different from our own. As before remarked, all the planets revolve in elliptical orbits round the Sun, and the time consumed in their journey constitutes their year. Their polar axis is not 'straight up and down,' but leans over or is inclined to the plane of their orbit, so that each pole is turned toward the Sun at one period of the year, and away from it at another. This arrangement insures the regular alternation of seasons and a variety of climates on their surface. The orbital inclination of Mars, for example, is much the same as that of the Earth, and, therefore, the relative proportion of his seasons must have a close resemblance to our own. It might be expected under these circumstances that ice would accumulate toward the poles in winter time, as on our Earth, and accordingly glacial accumulations have not only been observed by Astronomers, but it has been remarked that they occasionally diminish by melting during the heats of summer, while they increase in winter. Now as the planets, like the Earth, turn round on their axis with perfect regularity—and those just mentioned do so in very similar periods of time, hence, all have their days and nights.
"We have already stated that the Earth and its fellow-planets are kept steadily in their orbits by the exact adjustment of centrifugal and centripetal forces. Hence each moves in its regular order.
"Now by way of comparison, Astronomers have denominated the Sun as a globe two feet in diameter, or six feet in circumference. Starting from this globe let us wing our way across the space filled by the solar system. A short flight of thirty-seven millions of miles brings us to a world which, compared to the two-feet globe, is no larger than a grain of mustard seed, while it is so bathed in the Sun's dazzling rays that it is not easily distinguished when viewed from the Earth. This fussy little planet whirls round the Sun at the tremendous pace of 100,000 miles an hour, by which he proves his title to be called Mercury, the 'swift-footed,' of mythology. At a distance of sixty-eight millions of miles from the Sun we behold Venus, the brightest and most dazzling of the heavenly hosts. In comparative size she may be represented as a pea. She is our nearest neighbor among the planets, and the conditions under which she exists recall many of those under which we ourselves live. About ninety-five millions of miles from the Sun we come upon another 'pea' a trifle larger than the one representing Venus, and in it we hail our own familiar Mother Earth. Here we shall not now linger, but passing onward some fifty millions of miles we are attracted by the well-known ruddy glow of Mars—whose comparative size is that of a pin's head. His mean orbital speed is 54,000 miles an hour—nearly our own pace—but as he takes twice as much time to run round the Sun as we do, his year is consequently twice as long.
"Casting a glance behind, we are reminded of the growing distance that now separates us from the sun by the perceptible waning of his light.
"We next spread our wings for a very long flight. In passing through the "asteroid" zone of solar space, about 260 millions of miles from the sun, we may chance to fall in with some worlds of smaller dimensions than those we have been contemplating. We know very little about them, except that their ways are eccentric and mysterious. At length the shores of huge Jupiter are reached at a distance of nearly 500 millions of miles from the sun. To carry on the comparison, he is a "small orange" to the "pea" of our earth, or to the two feet globe that represents the sun. His orbit is a path 3,000 millions of miles long, which he accomplishes in an "annual" period of about 12 of our years. The sun's light has now shrunk considerably; but four brilliant moons or satellites, one or more of which are always "full," help to afford some compensation. But let us "onward" in our "outward-bound" course. We again pass through a space of nearly equal distance as that of Jupiter from the sun. We are now more than 900 millions of miles distant from the central pivot. Here we fall in with Saturn, whose comparative size may be represented by an orange considerably smaller than the last (bear in mind the comparative sizes, our earth as a "pea" to these each an orange). His year swallows up almost thirty of our own. And in this far distant region the Sun, though giving only about one ninetieth part of the light which we receive, is still equal to 300 full moons, and is at least sufficient for vision, and all the necessary purposes of life, while no fewer than eight satellites supplement the waning sun-light, besides a mysterious luminous "ring" of vast proportions.
"Twice as far away from the Sun as Saturn, Uranus, represented by a cherry, plods his weary course. Although his real diameter is 35,000 miles, his circumference over 100,000, being more than four times the size of our own earth, yet he is rarely seen by the naked eye. His annual journey round the Sun is 10,000 millions of miles, and he consumes what we should consider a lifetime, 84 years, in getting over it. Our little earth has now faded out of sight.
"Only a few years ago, Uranus was the last planetary station of our system, but the discovery of Neptune in 1846, gave us another resting-place on the long journey into space. Here, at a distance of nearly 3,000 millions of miles from the Sun, we may pause awhile before entering upon the more remote exploration of the 'starry universe.'
"We are approaching the frontier regions of our system, and the Sun's light and the power of his attraction are gradually passing away. Between the shores of our Sun-system and the shores of the nearest star-system—they also being suns—lies a vast, mysterious chasm, in the recesses of which may still lurk some undiscovered planets, but into which, so far as we yet know, the wandering comets alone plunge deeply.
"We now stand on the frontier of the Sun's domain, and are, in imagination, looking across one of those broad gulfs which, like impassable ramparts fence off the different systems of the universe from each other. It seemed needful that the great Architect should interpose some such barrier between the contending attractions of the giant masses of matter scattered through space; that there should be a sea of limitation in which forces, whose action might disturb each other, should die out and be extinguished. In it the flood-light of our glorious Sun gets weaker and weaker, and its bright disk wastes away by distance, until it shines only as a twinkling star. And the strong chain of its attraction which held with firm grasp the planets in their orbits, after dwindling by fixed degrees into a force that would not break a gossamer, is finally dissipated and lost.