Saturn, the sixth planet in order of distance from the sun, the third of the superior planets, and in ancient systems of astronomy the outermost member of the planetary system, but now known to travel within the orbits of two planets at least, Uranus and Neptune. Saturn moves at a mean distance from the sun amounting to 872,137,000 m. The eccentricity of his orbit being considerable, his greatest and least distances are respectively 920,973,000 m. and 823,301,000 m., the difference, 97,672,000 m., exceeding by fully 6,000,000 m. the earth's mean distance from the sun. The eccentricity of the orbit is 0.055996. Since the earth's mean distance from the sun is 91,430,000 m., it follows that when in opposition to the sun Saturn is at a distance from the earth of about 732,000,000 m. when nearest to the sun, and of about 829,500,000 m. when furthest from the sun. It appears therefore that notwithstanding his relatively enormous distance, which necessarily operates to diminish the changes of his apparent dimensions on account of the earth's motions, Saturn is seen under very different conditions in different oppositions.

For, remembering the difference of solar illumination when Saturn is in aphelion and perihelion, as well as the variation in the apparent size of his disk on account of his varying distance from the earth, we perceive that he must be more favorably placed for observation in a perihelion than in an aphelion opposition, in the ratio of (829,500,000)2 x (920,973,000)2 to (732,000,000)2 x (823,301,-000)2; that is, approximately as 8 to 5. This ratio would also represent the range of varying brightness of Saturn in different oppositions were it not for the rings, which greatly modify the apparent brightness of the planet. As the rings attain their greatest opening very nearly at the aphelion and perihelion of Saturn's orbit, the ratio just obtained fairly represents the relative brightness when he is in those parts of his orbit; but he appears much less bright relatively when at his mean distance than would be the case if he had no rings, for the rings then turn their edge almost exactly toward the earth. Saturn completes the circuit of his orbit in 10,759.2198 days, or 29 years 167.2 days, in an orbit inclined about 2° 29' 58" to the plane of the ecliptic. - Saturn comes next to Jupiter in volume and mass. In fact, he surpasses all the remaining planets taken together nearly three times.

His mean diameter is about 70,150 m., his polar diameter about 8,500 m. less, his equatorial diameter about 3,500 m. greater. His compression is about 1/10. He exceeds the earth 697 times in volume; but his density is only 0.13 of that of the earth, so that his mass only exceeds hers about 89.7 times. He rotates on his axis in about 10 1/2 hours, and his equator is inclined about 27° to the plane of his orbit. - Saturn when first observed with the telescope by Galileo presented a triple appearance, as if two smaller orbs were symmetrically placed on either side of a larger one. Afterward Galileo supposed the planet had two appendages resembling handles (ansoe) in shape and position. Sometimes the ansa appeared so large as to form a continuous ring; at other times they disappeared altogether. After they had been for some time invisible, they reappeared, and gradually increased in magnitude. Galileo and Hevelius were able to detect a dark space apparently enclosed within the rounded curve of the ansoe. Huygens explained these appearances as caused by an opaque, flat, thin, and circular ring surrounding the equator of Saturn, but nowhere touching the planet's globe.

This ring being inclined to the ecliptic and moving always parallel to itself, it follows that for half the Saturnian year one side of the ring is illuminated, and for the other half the other side. When the ring is turned edgewise either to the sun or to the earth, or when the earth is on the darkened side of the ring, it is invisible except in telescopes of great power. As the distance of the earth from the sun is very small compared with that of Saturn, it follows that the invisibility caused either by the ring's plane passing through the earth, or by the earth and sun being on different sides of the plane, must always occur when the planet is near one or other of the two points on its orbit where the plane passes through the sun. Later observations by Cassini, Bell, Sir W. Herschel, and others, but more particularly the recent observations of the Bonds at Harvard college and Dawes and Lassell in England, have added greatly to our knowledge of Saturn's ring system. The ring is found to consist of two chief bright rings separated by a circular gap; but each of the bright rings is probably divided into several subordinate rings. But perhaps the most remarkable part of this remarkable system is a dark ring within the bright system of rings.

This dark ring is so obvious with very moderate telescopic power that the idea is naturally suggested that it is a recent formation; and therefore the history of its discovery and of observations prior thereto deserves to be carefully noted. It appears from a paper by Galle of Berlin in the Nachrichten, No. 756, that a dark ring was seen with the large Berlin refractor in 1838. An account of Galle's observations, accompanied by drawings exhibiting the part of the ring seen across the body of the planet, was read by Encke before the Berlin academy in the same year. But little notice seems to have been attracted by this remarkable announcement; so that the actual discovery of the ring (made in such a way as to secure general recognition) must be attributed to Prof. G. P. Bond of Harvard and the Rev. W. R. Dawes in England, in November, 1850; Bond, who discovered the ring on Nov. 15, having the priority by a few days. Dawes at this time, and Bond somewhat later, called attention to the darkening of the inner bright ring toward the inside, where it adjoins on the dark ring.

Dawes's account, when he announced the discovery of the dark ring, was as follows: "The exterior portion of the inner bright ring to about one fourth of its whole breadth was very bright; but interior to this the shading off did not appear, as under ordinary circumstances, to become deeper toward the inner edge without any distinct or sudden gradations of shade; on the contrary, it was clearly seen to be arranged in a series of narrow concentric bands, each of which was darker than the next exterior one. Four such were distinctly made out; they looked like steps, leading down to the black chasm between the ring and the ball. The impression I received was that they were separate rings, but too close together for the divisions to be seen in black lines." Later Capt. Jacobs at Madras recognized the fact that the dusky ring is semi-transparent, the outline of the globe of the planet being distinctly visible through the ring. On this account, and because of the peculiar aspect of the dark ring when it crosses the body of the planet, the name "crape ring" has been assigned to it.

Otto Struve has shown that the inner or dusky ring is not a modern appendage to the planet, as might be suggested by the fact that it remained so long undiscovered; but that at the beginning of the 18th century the dark band thrown across the planet was distinguished from the shadow of the bright rings, and was called by observers the equatorial belt. Another curious result of Struve's researches was to show, by comparisons of measures made by Huygens, Cassini, Bradley, Sir W. Herschel, W. Struve, Encke, and himself, that "the inner edge of the inferior bright ring is gradually approaching the body of the planet, while at the same time the total breadth of the two bright rings is constantly increasing." The theory now generally accepted respecting the rings is that they are composed of minute satellites, like sand on the seashore for multitude. The following table shows the position of the ring, system and the proportions of the ring and globe:

Longitude of ring's rising node.................... 167° 44'

Inclination of ring to the ecliptic................... 28° 10'

Annual precession of rising node................... 3.145"

Exterior diameter of outer ring in miles............ 166,920

Interior " " " " ............ 147,670

Exterior diameter of inner ring.................... 144,310

Interior " " " .................... 109,100

Inferior diameter of the dark ring.................. 91,780

Breadth of outer bright ring....................... 9,625

" of inner bright ring....................... 17,605

" of division between rings................. 1,680

" of dark ring.............................. 8,660

" of system of bright rings.................. 28,910

" of entire system of rings.................. 37,570

Space between planet and dark ring................ 10,322

Saturn is attended by eight satellites, the largest of which, Titan, sixth in order of distance from the planet, is the largest satellite of the solar system, and probably is as large as the planet Mercury. Much confusion prevails, as Sir John Herschel complains, in the nomenclature of the Saturnian satellites, owing to the order of distances not coinciding with that of discovery. The elements of the satellites, and the names of their discoverers, are as follows, counting from within outward:

No.

Sidereal revolution.

Distance in radii ofSaturn 1400286

Diameter in miles.

Discoverer.

1....

0d. 22h. 37m.

3.3607

1,000?

Sir W. Herschel.

2....

1 8 53

4.8125

........

do.

3....

1 21 18

5.3396

500?

G. D. Cassini.

4....

2 17 41

6.8398

500?

do.

5....

4 12 25

9.5528

1,200

do.

6....

15 22 41

22.1450

3,000

C. Huygens.

7....

21 7 8

26.7834

?

W. Bond.

8....

79 7 54

64.3590

1,800

G. D. Cassini.

Saturn's surface is marked by belts, somewhat similar to Jupiter's. Owing to the inclination of Saturn's equator, his belts afforded an opportunity, wanting in Jupiter's case, for testing the question whether the sun is the chief agent in causing the belts. It is clear that if this were so the belts would follow the sun, the equatorial zone, a well marked band of whitish color, remaining no longer equatorial when the sun was far to the north or south of the Saturnian celestial equator. And as the sun viewed from Saturn takes more than 29 years in completing the circuit of the star sphere, being half that time north of the Saturnian equator and the remaining half south of that circle, it is clear that there would be ample time for the sun to draw the cloud zone north of Saturn's equator during the summer of Saturn's northern hemisphere, and south of the equator during the summer of Saturn's southern hemisphere. This would happen if the sun caused and therefore ruled the Saturnian cloud belts, as he causes and rules the great cloud belt of the zone of calms. But in Saturn's case nothing of this kind is observed. His great equatorial cloud zone remains equatorial all the year round.

No clearer evidence could be desired of the fact that this cloud zone is neither sun-raised nor sun-ruled, but is due to some cause in the Saturnian globe itself. This cause can be no other, it would seem, than an intense heat pervading the whole globe of the planet. For other reasons, drawn from a consideration of the condition of Jupiter and Saturn on the nebular hypothesis, Prof. Peirce has recently adopted the opinion that these two great planets are thus instinct with their primeval fires. - Measurements of Saturn have led to the singular result that the planet appears to vary in shape. The disk is usually elliptical, but sometimes shows a figure in which the two diameters from 45° Saturnian latitude N. to 45° S. appear the greatest, the equatorial diameter less, and the polar diameter the least. This appearance has been called Saturn's "square-shouldered" aspect. It was first noticed by Sir W. Herschel, and as he recognized it with different telescopes he was satisfied that it was not a mere optical delusion. It has since been noticed by other observers, as the Bonds, Airy, and Coolidge, who possessed far too great skill in observing to be readily deceived in a matter so simple. It has been ascribed to optical illusion, but probably without sufficient ground.

In fact it has never been shown why the illusion should be noticed at one time and not at others, or how it can be occasioned. It appears to the writer that a sufficient, a reasonable, and a probable interpretation is afforded by the theory that the atmosphere of Saturn is subject to changes, either by the formation and precipitation of cloud masses at an enormous elevation, or in some other way, which cause the apparent figure of the disk to alter while the real globe of Saturn, far within the visible boundary, remains unchanged in shape. This theory corresponds well with results to which the study of the planet Jupiter seems to lead us, as already shown. (See Jupiter).