Saturn, as observed by Cassini in August 1676.
“In the year 1676, after Saturn had emerged from the Sun’s rays, Sig. Cassini saw him in the morning twilight with a darkish belt upon his globe, parallel to the long axis of his ring as usual. But what was most remarkable, the broad side of the ring was bisected right round by a dark elliptical line, dividing it, as it were, into two rings, of which the inner ring appeared brighter than the outer one, with nearly the like difference in brightness as between that of silver polished and unpolished—which, though never observed before, was seen many times after with tubes of 34 and 20 feet, and more evidently in the twilight or moonlight than in a darker sky.”
From the time when Galilei’s inadequate glass revealed the “threefold” aspect of Saturn, and led up to Huygens’s solution of the mystery in 1654, this planet has been successively interrogated with the improved telescopes which every generation has produced. Cassini, W. Herschel, Encke, Bond, Lassell, Dawes, and Hall are names familiar to us as having materially advanced our knowledge of this unique orb, both as to his surface-configuration and as to his numerous retinue of satellites.
Belts and Spots on the Planet.—Parallel belts are seen on the surface of Saturn, but they are much fainter than those on Jupiter, and they seldom display the spots and other irregularities interspersed with the belts of the latter planet. Well-bounded spots have rarely been distinguished on the disk of Saturn; the belts normally appear equal in tone, without breaks, condensations, abrupt curves, or branches, so that the rotation-period has only been accurately determined by Herschel and Hall. And in these cases the markings were certainly atmospheric, and probably affected by proper motions similar to those operating on Jupiter.
Cassini and Fatio remarked two bright streaks on the planet as early as 1683. Sir W. Hershel, in 1790, observed a very dark spot near the margin of the limb, and a few modern observers have been successful in distinguishing either bright or dark spots or patches, though no continuous and useful observations appear to have been secured. In the winter of 1793 Herschel noticed a very distinct quintuple belt, which consisted of three dusky and two intervening light zones. The dark belts presented a dusky yellow hue, while the spaces separating them were white. He recognized the evidences of rotation in the quintuple belt; for on the same nights, after a few hours’ interval, it exhibited considerable variation. Though seen with great precision at first, it became indistinct at a later hour, and the individual belts were placed at unequal distances.
Rotation-Period.—Prof. A. Hall, at Washington, discovered on Dec. 7, 1876, a well-defined white spot, 2″ or 3″ in diameter, and situated just below the ring of Saturn. He watched this object till Jan. 2 following, when it had become faint and indistinct, and the planet being low and the weather unfavourable no further observations were made. The spot had fortunately been seen at four other observatories in the United States, Prof. Hall having notified its existence to them; and on discussing the results, a rotation-period was found not differing largely from Herschel’s value derived from the quintuple belt in 1793. These are, in fact, the only two determinations on which we may place confidence. They are as below:—
| h m s | Probable error. | ||
|---|---|---|---|
| 1793. | Sir W. Herschel | 10 16 0·4 | 2 min. |
| 1877. | Prof. A. Hall | 10 14 23·8 | 2·3 sec. |
Schröter, from different spots, computed periods of 11h 40m 30s, 11h 51m, and more than 12h; but these are probably excessive. The difference of 1m 37s between the values of Herschel and Hall is relatively a trivial one, as the markings observed were doubtless atmospheric and subject to irregularities of motion. As to the rotation of the ring, Herschel, in 1789, detected some bright marks on it, and deduced the period as 10h 32m 15s·4[43]. Many astronomical works give the rotation-period of Saturn as 10h 29m 16s·8; and this is adopted in Chambers’s ‘Descriptive Astronomy,’ 4th edit. vol. i. p. 653. The mistake has its origin in Laplace’s Système du Monde, where it is stated that Saturn rotates in 0·428 of a day, and the ring in 0·437, which, reduced to hours, minutes, and seconds, give 10h 16m 17s·2 and 10h 29m 16s·8.
The equator of Saturn is usually the brightest part of the disk. On its S. side, in recent years, it has been bounded by a very dark narrow belt. Further S. the whole disk seems involved in a faint shading, of a decidedly yellowish hue. Sometimes a considerable number of belts are visible; but they are evidently liable to changes, so that the same number and arrangement are not preserved from year to year.
The Rings.—As to the luminous rings, the extreme diameter of the outer one is about 40″, or more than 170,000 miles; and the black division, separating it from the inner one, is 0″·4, or 1700 miles. The outer ring has a breadth of 2″·4, or 10,000 miles; while the inner one measures 3″·9, or 17,000 miles. The outer ring is less luminous than the inner; the latter, round its outer edges, is extremely brilliant, and has sometimes been described as the brightest part of the Saturnian system. The inner part of this ring is much shaded-off, and offers a strong contrast to the silvery whiteness of the other portion.