An extraordinary resemblance exists between Mars and the Earth. Physical, geographical, and climatological conditions, days and nights, seasons, celestial perspectives, all are alike in these two planets, with the sole difference that the globe of Mars is half as small again as that of the Earth; so that, if a man were transported to Mars, he might believe himself to be, not in a strange planet, but in a little known corner of the Earth, such as Australia or Polynesia.
As we pursue our journey through the heavens, ever increasing our distance from the Sun, we shall find, after Mars, the group of the Asteroïds. We shall not linger before this cluster of small stars, which is no doubt nothing but a collection of the dismembered fragments of a planet, which formerly existed in this particular point of space, and was dashed to pieces by some formidable accident in the universe. These little stars, like the important planets, have each their names, such as Vesta, Pallas, Circe, &c., &c. Maximiliana, and Feronia are placed at the two extremities, with respect to distance from the Sun. These remains of a broken star continue to circulate around the Sun, like the planet which they formerly composed.
After the Asteroïds comes great Jupiter.
Jupiter is the largest planetary sphere in our solar system, being 1400 times greater than the Earth. Its distance from the Sun is 200,000,000 miles. In consequence of this distance, its year is as long as twelve of our years. Notwithstanding its colossal dimensions, Jupiter turns with such rapidity upon its axis, that it accomplishes an entire revolution in twelve hours, so that its day and night are respectively only ten hours long. The shortness of Jupiter's nights are compensated by the existence of four moons, or satellites, which revolve around this planet, and give it permanent light. This illumination by reflection, added to very long twilights, must make Jupiter's nights nearly equal to the day in brightness.
Though Jupiter suffers under the disadvantage of very short days, it has on the other hand the inappreciable advantage of perfect equality in the length of its days and nights, and of that of the four seasons over all its parallels. The axis of Jupiter is hardly at all oblique, and therefore Jupiter, like the planet Saturn, enjoys a sort of perpetual spring, that is to say, an equable distribution of solar heat and light along the same degrees of latitude. Jupiter, unlike Mars and Venus, has no vicissitudes of seasons, no sudden and painful transitions from cold to heat in the same place. The climates are invariable in each latitude, and the seasons are hardly discernible.
The globe of Saturn is 734 times larger than that of the Earth, and is 364,000,000 leagues from the Sun. It takes thirty years to perform its revolution around the central star, and its year is therefore thirty times as long as ours.
Saturn, like Jupiter, has very short days. It revolves on its axis in ten hours, so that its day and night respectively are but five hours. But it has eight moons, or satellites, which accompany it, and give it light, thus, as in the case of Jupiter, supplementing the shortness of its days. There is hardly any obliquity of the axis of Saturn, so that its days and nights are always equal. There is a perpetual equinox, and the climates are invariable, while variation of seasons hardly exists. In Saturn, as in Jupiter, perpetual spring reigns. Saturn has one peculiarity which does not belong to any other body in our solar system. It is placed in the centre of a ring, of the same nature as its own, and which surrounds it on every side. This ring (see plate 5), is surrounded by a second, and the second by a third, and the whole are called the rings of Saturn. This circular envelope is exceedingly thin—only ten leagues in thickness—but very wide; its width is 12,000 leagues. It is not motionless, but it revolves with the globe which it surrounds.
The strange disposition of the rings of Saturn affords a proof of the inexhaustible riches of nature, and the variety of forms which the Creator has called into being in the vast universe. It ought to guard us against our constant tendency to model all the worlds which we do not know, upon the type of the earth.
Hardly anything is known about the peculiarities of Uranus, a planet which is only eighty-two times larger than the earth, but which is 732,000,000 of miles from the sun, and takes eighty-four years to accomplish its revolution around the central star.
Plate 5 shows the relative proportions of Uranus and the earth. The prodigious distance of Uranus from our globe, added to its small size, renders it almost inaccessible to observation.