CHAPTER XVIII.

On the Ring of Saturn.

ART. 514. WHEN speaking of the appearances of some of the planets when viewed through a telescope (Art. 402.), we mentioned the discovery of a curious ring which encompassed the body of saturn. This ring, according to the observations of Liuygens, is about 22000 miles broad, and its greatest diameter in proportion to that of the planet, as 9 to 4Later observations have furnished us with a much better acquaintance with this curious phenomenon. De la Lande and De la Place inform us that Cassini saw the breadth of this ring divided into separate parts, nearly equal in breadth. And Mr. Hadley, with an excellent 5 feet reflector, saw the ring divided into two parts. Mr. Short, and some few others, thought they saw several divisions upon the ring but the long-continued and accurate observations of Dr. Herschel seem to confirm the division of the ring into only two concentric parts, almost beyond the possibility of doubt. This gentleman says, there is one single, dark, considerably 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 construction cannot be owing to the sha

dow 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 opinion of very extended caverns. It is pretty evident that this dark zone is contained between two concentric circles; 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 upon 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 concludes that the edge of the ring is not flat, but spherical or spheroidical. The dimensions of the

ring, or of the two rings with the space between them, Dr. Herschel gives as below:

Inner diameter of smaller ring

Outside diam. of ditto

Miles.

.

1-46345

184393

Breadth of the vacant space, or dark zone

20000

7200

2839

515. The ring revolves in its own plane in 1032 154, being nearly the same time (Art. 402.) as that in which the planet performs a rotation upon its axis. The ring, being nearly a circle, appears elliptical from its oblique position; and it appears most open when saturn is 90° from the nodes of the ring upon the orbit of the planet, or when saturn's longitude is about 2 179 and 8 170. In such a situation the minor axis is very nearly equal to half the major, when the observations are reduced to the sun; consequently the plane of the ring makes an angle of about 30° with the orbit of saturn. Or, according

to some observations, the inclination of the ring to the ecliptic is about 31° 22′.

516. 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 surface, in like manner as a kernel is contained 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 conjecture is in some measure corroborated by the onsideration, 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 sufficient ground for admitting the ring of ' saturn to be of a very changeable na ure, and I guess that its phenomena will hereafter be so fully "explained, as to reconcile all observations. In the "meanwhile we must withhold a final judgment of "its construction, till we can have more observa❝tions. Its division, however, into two very une66 qual parts, can admit of no doubt."

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517. As saturn moves round the sun in an orbit nearly circular, his obliquely posited ring, like our earth's axis, keeps parallel to itself, and is therefore turned edgewise to the sun twice in a saturnian year, which is nearly as long as thirty of our years. But the ring, though probably considerably broad, is invisible (except when very good telescopes are used) whenever its plane passes through the sun, or the earth, or between them: the chief phenomena of the ring may be illustrated by fig. 7, Pl. VII. where S represents the sun, ABCDEFGH the orbit of saturn, IKL MN O that of the earth; both planets

moving according to the order of the letters. When saturn is at A, his ring is directed towards the sun at S, and he is then seen from the earth with common telescopes, as though he had lost his ring, let the earth be in whatever part of its orbit it may, except between N and O; for whilst it describes that space, saturn is apparently so near the sun as to be hid in his beams. As saturn passes from A to C, his ring appears more and more open to an observer on the earth at C the ring appears most open; and becomes narrower and narrower, as saturn passes from C to E. When he comes to E, the ring is again directed edgewise both to the sun and earth, and as neither of its sides is illuminated, it is again invisible to us. But as he goes from E to G, his rings open more and more to our view on the under side; and seems just as open at G as it did at C; and may be seen in the nighttime from the earth in any part of its orbit, except about M, when the sun obscures the planet from our view. As saturn passes from G to A, his ring turns more and more edgewise to us, and therefore it seems to become more and more narrow; and at A it disappears, as before. Also, while saturn goes from A to E, the sun shines on the upper part of his ring, and the underside is dark; and whilst he passes from E to A, the sun shines on the under side of his ring, and the upper side is dark. According to the computation of Dr. Maskelyne, the plane of the ring passed through the earth on January 29, 1790; the earth passing from the northern, or dark, to the southern or enlightened side of the ring; the ring, therefore, then became visible, and will continue so till 1803.

518. The mean distances of the earth and saturn from the sun, are respectively as I : 9°54972 (Art. 360.); therefore the length of the semidiameter of the earth's orbit will be the sine of 6° 1' 2" in saturn's orbit. Saturn, then, describes an arch of 6° 1' 2" from the instant the plane of his ring produced touches the earth's orbit, to the instant it passes

through the sun and as saturn's periodic time in his orbit is about 10761615 days, he will be 179.83 days in running 6° 1' 2". The plane of his ring then takes nearly 360 days to run over the diameter of the earth's orbit; but as this is nearly the part of it, it is therefore 355 X 360 days in running

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through a space equal to the whole orbit: but the earth takes 365.25 days nearly to run the same distance; hence, the velocity of the earth in its orbit, is to the simultaneous velocity of the ring of saturn,

as

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355 × 360: 365-25, or nearly as 3098 to 1. Hence, during the 179.83 days the ring is passing over a part of its orbit equal and nearly parallel to I M, the plane of the ring may pass through the earth three times, though sometimes not so many. This might be pursued much farther; but it is unnecessary here. Those who are desirous of seeing more on this head, are referred to a treatise entitled, Essai sur les Phénomènes relatifs aux Disparitions périodiques de l'Anneau de Saturne, par M. Dionis du Séjour, in which they will find a curious analytical solution of the problem respecting the times of the appearance and disappearance of this ring, wherein is also exhibited a new species of curves.

* I cannot recommend this work more highly than by extracting the encomium upon it (signed by D'Alembert, Borda, Bezout, Vandermonde, La Place, and Fouchi) from the Register of the Royal Academy of Sciences at Paris, for 1775:- Tels sont les objets que M. du Séjour a traités dans son outrage; et Pon voit qu'il n'a rien laissé à désirer sur la théorie des phases de l'anneau de saturne. L'élégance, la fincsse, et la simplicit. des méthodes dont ik a fait usage, rendent cet ouvrage très intéressant pour les géomètres; et la discussion des phénomènes depuis 1600 jusqu'en 1900, le rend nécessaire aux astronomes qui voudront dans la suite observer avec precision ces apparences: ainsi nous croyons qu'il mérite d'étrẹ imprimé avec l'approbation et le privilège de l Academie.