There are several strong bands in the spectrum of Saturn, due to absorption in the planet's atmosphere. They are identical with those shown by the spectra of Jupiter, Uranus and Neptune, though of greater intensity than in the spectrum of the former and of less intensity than in those of the two last-mentioned planets. Their origin is still unknown.
The Rings.—But Saturn's most remarkable feature, and that which renders it unique, so far as our knowledge goes, is the magnificent system of rings by which it is surrounded. That Saturn differs in appearance from other bodies was seen at once by Galileo when he turned his little telescope towards it in 161o, but his instrument was not sufficiently powerful to show clearly what it was that he saw. He noticed that the planet had a small attendant on each side, and accordingly represented it as a triple body. But during the next few years the appendages dwindled and finally disappeared, greatly to his perplexity and chagrin, as he feared he must have been misled by some kind of illusion. When they subsequently reappeared they continued to present a difficult problem to the telescopic observers of the day—some times seeming like arms stretching out on each side of the central body, and sometimes like curved handles—and a number of curious drawings have come down to us which show how puzzled the observers were, but how near some of them came to the solution of the mystery. The true explanation was ultimately arrived at by Huygens in 1655, but, wishing for further time to make sure of his solution and yet secure himself against the possible loss of priority in the discovery, he published the follow ing series of letters :—aaaaaaa ccccc d eeeee g h innn 1111 mm nnnnnnnnn 0000 pp q rr s ttttt uuuuu; which, when properly ar ranged, form the sentence :—"Annuls cingitur, tenui, piano, nusquam cohaerente, ad eclipticam inclinato" (It is girdled by a thin flat ring, nowhere touching, inclined to the ecliptic). These last few words explain the various appearances which so sorely puzzled the earlier observers with their imperfect instruments. The plane of the ring is inclined about 27° to the planet's orbit, and about 28° to the ecliptic, and keeps parallel to itself through out the planet's revolution. There are accordingly two opposite portions of the orbit, viz., near longitudes 172° and 352°, where Saturn is in Leo and Aquarius respectively, at which the ring can be presented edgewise to the earth, and when this event happens (as it does either once or three times during each passage of the ring plane across the earth's orbit) the ring—owing to its thinness—disappears from view even in powerful instruments. At intermediate positions, viz., when the planet is in Taurus and Gemini and in Sagittarius, it appears opened out at an angle of 27°, and is then seen to project slightly beyond the polar diameter of the planet's globe.
The next important telescopic discovery as regards Saturn was the detection by G. D. Cassini, in 1675, of a black line or gap dividing the ring into two concentric rings. This is generally known as "Cassini's division." The ring exterior to this division is narrower and less bright than the inner ring, while the outer portion of the latter is the most brilliant part of the whole sys tem. Within the second ring is yet another feature of great interest, viz., a third ring, commonly known as the Crape ring, of which the brightness is so feeble that it long escaped detection. It was first recorded by Galle, at Berlin, in 1838, but its existence was strangely forgotten till it was independently rediscovered, in 185o, by G. P. Bond at Harvard and W. R. Dawes in England. It can be readily traced with a comparatively small telescope as a dusky band where it crosses the planet's globe, but is not so easily seen in the portions projected against the dark sky. The three rings are often denoted by the letters A, B and C. From time to time other divisions besides that of Cassini have been reported, but they seem to have been merely partial and tem porary, except that known as "Encke's division," in ring A, which is, perhaps, permanent, though probably not really a complete division. It usually appears as a pencil-like shading rather than a sharp black line, and sometimes merely as the boundary of the darker outer portion of the ring. The figures given by different authorities for the dimensions of the ring system differ some what, but the following are approximately correct : The breadth of the Cassini division is probably rather over 2,000 miles. The thickness of the rings is apparently between 20 and 40 m. only. It is noteworthy that all three are at least partly transparent ; Saturn itself can be distinctly seen through the Crape ring, and on some recent photographs it also shows through the outer ring. Moreover, on Feb. 9, 1917, M. A. Ainslie, at Black heath, and J. Knight, at Rye, observed that a seventh magnitude star (B.D. 1714) , remained visible during its occultation by the outer ring, though Ainslie considered that it lost something like three-fourths of its light ; it appeared to travel some distance along the Cassini division but did not pass behind ring B ; and on March 14, 1920, during the occultation of Lalande 20,654 (Mag. 7.3), the star remained conspicuously visible in a 6-in. refractor at W. Reid's observatory, Rondebosch, South Africa, even when behind the brightest part of ring B, and despite the fact that, in consequence of the obliqueness of the line of sight, its light had to traverse a distance through the ring equal to eight times its real thickness.