when he rises. The distance from the sun is calculated to be 125,000,000 of miles. He revolves around that luminary in 686 days, 23 hours, of our measurement of time, moving at the average rate of 47,000 miles an hour; and moves round his own axis from west to east in somewhat more than twenty-four hours. By his diurnal motion the equator of Mars is carried round at the rate of 556 miles every hour. His quantity of light and heat is equal to only about one half of that which we receive; and, to his inhabitants, the sun must appear but half as large as he does to us. No moon has as yet been found attending him. He is sometimes of a gibbous form, but never horned; sometimes half, or three-quarters illuminated, but never full; which proves that his orbit includes that of the earth, and that he shines by reflected light. Our globe and the moon must appear to Mars, like two moons, a larger and a smaller, changing places with each other; appearing, sometimes horned, sometimes half or three-quarters illuminated; but never full, nor ever more remote from each other than one quarter of a degree. To the inhabitants of Mars, the earth must appear as large as Venus does to us, and never more distant from the sun than forty-eight degrees. Dark spots have been observed upon the surface of this planet; and bright spots about his poles. Of all the planets, Mars is most like the earth. Its diurnal motion is nearly the same; the obliquity of his ecliptic not very different; and, when compared to the other planets, his distance from the sun and the time of his revo lution round that glorious body, most resemble those of the earth. Mars appears to be surrounded by a considerable atmosphere; belts have also been observed on his disc. Jupiter was formerly ranked as next in distance from the sun; yet his distance from that luminary was observed to be disproportionate to that of Mars, of the Earth, and of Venus, so that some astronomers conjectured there must have been another planet between him and Mars, lost by some violent operation; or that some planet or planets actually existed in that space. This latter circumstance is now found to be the fact. Modern astronomy has discovered four very small globes moving round the sun, in orbits between those of Mars and Jupiter. But as Herschel, and other astronomers, seem to regard them as differing from the rest of the planets, the consideration of them may be deferred, till the description which astronomy gives of the primary planets be finished. Jupiter is the brightest of the planets, excepting Venus, and he exhibits the same appearance of belts on his surface, as does Mars; but they are much larger and more permanent. The number of these belts is very variable; as sometimes only one can be discerned, and at other times eight are discernible. They are generally, but not always, parallel to one another; their continuance is very uncertain, as they sometimes remain unchanged for three months together, and, at other times, new belts have been formed in a few hours. Jupiter is the largest of the planets. His diameter is 81,000 miles, which is a thou VOL. I. sand times more than that of the earth. At the distance of 426,000,000 of miles he revolves round the sun in eleven of our years, three hundred and fourteen days, and twelve hours, at the rate of 25,000 miles an hour. He moves round his own axis, from west to east in nine hours and fifty-six minutes. By this amazingly rapid rotation, his equator is carried round 25,920 miles every hour. The figure of this planet is an oblate spheroid; and the plane of his equator is very nearly coincident with that of his orbit; so that its inhabitants can know very little difference of seasons. To them, the sun must appear but one twenty-eighth part as large as he does to us, and the light and heat they receive from that luminary, are comparatively small in proportion to what we enjoy. But to counterbalance this apparent disadvantage, Jupiter is attended by four moons, some larger, and some smaller than the earth. The first of these his satellites at 229,000 miles distance from his centre, revolves round him in one day, eighteen hours and thirty-six minutes of our time. The second is distant from the planet 364,000 miles, and goes round him in three days, thirteen hours, and fifteen minutes. The third performs its revolution in seven days, three hours and fifty-nine minutes, at the distance of 580,000 miles, and the fourth, at the distance of 1,000,000 of miles from his centre, in sixteen days, eighteen hours, and thirty minutes. These are sometimes seen to pass over the disc of Jupiter like dark spots. The three moons of Jupiter nearest to his body, pass through his shadow in every revolution they make round him; and by means of their eclipses, astronomers have discovered that the light from the sun reaches the earth in eight minutes, and have also determined the longitudes of various places on our globe, with greater facility and certainty, than by any other method yet known. Saturn, the next planet in the system, is distant from the sun, 780,000,000 miles, and moving at the rate of 18,000 miles an hour, performs his revolution round that glorious star, in twenty nine years, one hundred and sixty-seven days, and five hours of our time. Its diameter is 67,000 miles, and consequently this planet is six hundred times as large as the earth. Saturn has round him a thin broad ring, which appears double when viewed through a powerful telescope, and when it exhibits this appearance, the inner ring is brightest. The breadth of this ring is 21,000 miles, and its distance from the body of the planet is the same. Herschel has proved that the ring revolves in its own plane, in ten hours, thirty minutes, and fifteen seconds. The light of the ring is generally more vivid than that of the planet. As the plane of this ring keeps always parallel to itself, it disappears twice in every revolution of the planet, that is, about once in fifteen of our years, and he sometimes appears quite round for nine successive months. At other times, the distance between the body of the planet and the ring is very perceptible. When viewed from Saturn, it must appear like a vast luminous arch in the heavens; visible and invisible in turn, for fifteen years. The axis of Saturn is probably inclined to the ring. This planet receives only one-ninetieth part of the light and heat of the sun, which we do. Herschel gives the following dimensions of the two rings, with the space between them, Inner diameter of the smaller ring, Outside diameter of the larger ring, 204,883 Breadth of the inner ring, 20,000 Breadth of the outer ring, 7,200 Breadth of the vacant space between the two rings, } - 2,839 Herschel supposes that the ring consists of as solid materials as Saturn itself. It casts a very strong shadow upon that planet. Seven moons revolve round Saturn, which supply him with light during the absence of the sun. They are so small, and at such a distance from the earth, that they cannot be seen but through very powerful telescopes. The sixth and seventh, discovered by Herschel, are the smallest. The fourth is the brightest, Saturn is still more flattened at the poles, than even Jupiter. This circumstance appears to be occasioned by its ring; which being in the plane of its equator, and equally as dense as the planet itself, acts more powerfully upon the equatorial regions of Saturn than upon any other part of his disc; and by diminishing the gravity of those parts, it aids the centrifugal force in flattening the poles of the planet. |