SATURN is a very conspicuous planet, though not so brilliant as Jupiter. The period of his sidereal revolution round the earth is 10,759 days. lie moves from west to east nearly in the plane of the ecliptic, and exhibits irregularities similar to those of Jupiter and Mars. He becomes retrograde both before and after his opposition, when at the dis tance of about 109° from the sun. His retrograde motion continues about 139 days, and during its continuance he de. scribes an arc of about 6°. His diame ter is a maximum at his opposition, and his mean apparent diameter is 18". Saturn, when viewed through a good telescope, makes a more remarkable appearance than any of the other planets. Galileo first discovered his uncommon shape, which he thought to be like two small globes, one on each side of a large one, and he published his discovery in a Latin sentence, the meaning of which was, that he had seen him appear with three bodies, though, in order to keep the discovery a secret, the letters were transposed. Having viewed him for two years, he was surpnscd to see him be come quite round, without these ap pendages, and then, after some time, to assume them as before. These adjoin ing globes were what are now called the anew of his ring, the true shape of which was first discovered by Huygens, about fatty years after Galileo, first with a telescope of twelve feet, and then with one of twenty-three feet, which magni fied objects one hundred times. From the discoveries made by him and other as tronomers, it appears that this planet is surrounded by a broad thin ring, the edge of which reflects little or none of the Sun's light to us, but the planes of the ring reflect the light in the same manner that the planet itself and if we stippo,e the diameter of Saturn to be divided into three equal parts, the diameter of the ring is about seven of these parts. The ring is detached from the body of Saturn in such a man lier, that the distance between the in nermrist part of the ring and the body is equal to its breadth. Both the out ward and rim of the ring is pro. jetted into an ellipsis, more or less ob long, according to the different degrees of obliquity with which it is viewed. Sometimes our eye is in the plane of the ring, and then it becomes invisible, either because the outward edge is not fitted to reflect the Sun's light, or more probably because it is too thin to be seen at such a distance. As the plane of this ring keeps always parallel to it self, that is, its situation in one part of the orbit is always parallel to that in any other part, it disappears twice in every revolution of the planet, that is, about once in fifteen years, and he sometimes appears quite round for nine months to gether. At other times, the distance between the body of the planet and the ring is very perceptible, Insomuch that Mr. Whiston tells us of Dr. Clarke's father having seen a star through the opening, and supposed him to have been the only person who ever saw a sight so rare, as the opening, though cer tainly very large, appears very small to us.
When Saturn appears round, if our eye be in the plane of the ring, it will appear as a dark line across the middle of the planet's disk, and if our eye be elevated above the plane of the ring, a shadowy belt will be visible, caused by the shadow of the ring, as well as by the interposition of part of it be tween the eye and the planet. The shadow of the ring is broadest when the Sun is most elevated, but its ob scure parts appear broadest, when our eye is most elevated above the plane of it. When it appears double, the ring next the body of the planet appears brightest. When the ring ap pears of an elliptical form, the parts about the ends of the largest axis are called the ansx, as has been already men tioned. These, a little before and after the disappearing of the ring, are of un equal magnitude ; the largest antis is longer visible before the planet's round phase, and appears again sooner than the other. On the first of October, 1714,
the largest ansm was on the east side, and on the twelfth on the west side, of the disk of the planet, which makes it pro bable that the ring has a rotation round an axis. Herschel has demonstrated, that it revolves in its own plane in 10h 32' 15.4". The observations of this phi losopher have added greatly to our knowledge of Saturn's ring. According to him there is one single, dark, con siderably broad line, belt, or zone, which he has constantly found on the north side of the ring- As this dark belt is subject to no change whatever, it is probably owing to some permanent construction of the surface of the ring; this construe tion cannot be owing to the shadow of a chain of mountains, since it is visible all round on the ring ; for there could be no shade at the ends of the ring ; a similar argument will apply against the opinions of very extended caverns. It is pretty evident that this dark zone is contained between two concentric cir cles, for all the phenomena correspond with the projection of such a zone. The nature of the ring Dr. Herschel thinks no less solid than that of Saturn itself, and it is observed to cast a strong shadow up on the planet. The light of the ring is also generally brighter than that of the planet, for the ring appears sufficiently bright when the telescope affords scarcely light enough for Saturn. The Doctor con cludes that the edge of the ring is not flat, but spherical, or spheroidical. The di menstons of the ring, or of the two rings with the space between them, Dr. Her schel gives below : Inner diameter ef smaller ring 146,345 Outside diameter of ditto - 184.393 Inner diameter of larger ring 190.248 Outside diameter of ditto - 204,883 Breadth of the inner ring - 20,000 Breadth of the outer ring - 7,200 Breadth of the vacant space, or dark zone - - - - 2,339 There have been various conjectures relative to the nature of this ring. Some persons have imagined that the diameter of the planet Saturn was once equal to the present diameter of the outer ring, and that it was hollow, the present body being contained within the former sur face, in like manner as a kernel is con tained within its shell ; they suppose that, in consequence of some concussion, or other cause, the outer shell all fell down to the inner body, and left only the ring at the greater distance from the centre, as we now perceive it. This con jecture is in some measure corroborated by the consideration, that both the planet and its ring perform their rotations about the same common axis, and in very nearly the same time. But from the observations of Dr. Herschel, he thus concludes : "It does not appear to me that there is suffi cient ground for admitting the ring of Saturn to be of every changeable nature, and 1 guess that its phenomena will here after be so fully explained, as to recon cile all observations. In the meanwhile we must withhold a final judgment of its construction, till we can have more ob hervations. Its division, however, into two very unequal parts can admit of no doubt." The diameters of Saturn are not equal : that which is perpendicular to the plane of his ring appears less by one-eleventh than the diameter situated in that plane. If we compare this form with that of Jupiter, we have reason to conclude that Saturn turns rapidly round his shorter axis, and that the ring moves in the plane of his equator. Herschel has confirmed this opinion by actual observa tion. He has ascertained the duration of a revolution of Saturn round his axis to amount to 0.428 day. Huygens observed five belts upon this planet nearly parallel to the equator.