CHAPTER XII.

THE RINGS OF SATURN.

Until about the middle of the present century the rings of Saturn were universally regarded as solid and continuous. The labors, however, of Professors Bond and Pierce, of Cambridge, Massachusetts, as well as the more recent investigations of Prof. Maxwell, of England, have shown this hypothesis to be wholly untenable. The most probable opinion, based on the researches of these astronomers, is, that they consist of streams or clouds of meteoric asteroids. The zodiacal light and the zone of small planets between Mars and Jupiter appear to constitute analogous _primary_ rings. In the latter, however, a large proportion of the primitive matter seems to have collected in distinct, segregated masses. These meteoric zones have probably presented--what are not elsewhere found in the solar system--cases of commensurability in the planetary periods. The interior satellites of Saturn are so near the ring as doubtless to exert great perturbative influence. Unfortunately, the elements of the Saturnian system as determined by different astronomers are somewhat discordant. This, however, is by no means surprising when we consider the great distance of the planet and the small magnitude of some of the satellites. For convenience of reference the mean apparent distances of the satellites, together with their periodic times, are given in the following table. The former are taken from Hind's _Solar System_; the latter from Herschel's _Outlines of Astronomy_.

TABLE I.--THE SATELLITES OF SATURN.

+-----------+------------------------+---------------+ | | | MEAN APPARENT | | NAME. | SIDEREAL REVOLUTION. | DISTANCE. | +-----------+------------------------+---------------+ | | _d._ _h._ _m._ _s._ | ´´ | | Mimas | 0 22 37 22·9 | 26·78 | | Enceladus | 1 8 53 6·7 | 34·38 | | Tethys | 1 21 18 25·7 | 42·57 | | Dione | 2 17 41 8·9 | 54·54 | | Rhea | 4 12 25 10·8 | 76·16 | | Titan | 15 22 41 25·2 | 176·55 | | Hyperion | 22 12? | 213·3? | | Japetus | 79 7 53 40·4 | 514·52 | +-----------+------------------------+---------------+

The late Professor Bessel devoted much attention to the theory of Titan, whose mean distance he found to be 20·706 equatorial radii of the primary. Struve's measurements of the ring are given in the second column of the following table. Sir John Herschel, however, regards the Russian astronomer's interval between the rings as "somewhat too small."[29] This remark is confirmed by the measurements of Encke, whose results are given in column third. The fourth contains the _mean_ of Struve's and Encke's measurements; and the fifth, the same, expressed in equatorial radii of Saturn.

TABLE II.--THE RINGS OF SATURN.

+---------------------+---------+---------+----------+------------+ | | | | | IN | | | STRUVE. | ENCKE. | MEAN. | SEMI-DIAM. | | | | | | OF SATURN. | +---------------------+---------+---------+----------+------------+ | Equatorial radius | ´´ | ´´ | ´´ | | | of the planet | 8·9955 | | | | | Ext. semi-diameter | | | | | | of exterior ring | 20·047 | 20·2225 | 20·13475 | 2·23830 | | Int. semi-diameter | | | | | | of exterior ring | 17·644 | 18·0190 | 17·83150 | 1·98230 | | Ext. semi-diameter | | | | | | of interior ring | 17·237 | 17·3745 | 17·30575 | 1·92380 | | Int. semi diameter | | | | | | of interior ring | 13·334 | 13·3780 | 13·35600 | 1·48470 | | Breadth of interval | 00·407 | 00·6445 | 00·52575 | 0·05844 | +---------------------+---------+---------+----------+------------+

The period of a satellite revolving at the distance, 1·9238, the interior limit of the interval =10h. 50m. 16s. One-sixth of the period of Dione =10 56 53 One-third " Enceladus =10 59 22 One-half " Mimas =11 18 32 One-fourth " Tethys =11 19 36 And the period of a satellite at the distance, 1·9823, the exterior limit of the interval =11 28 3

The interval, therefore, occupies precisely the space in which the periods would be commensurable with those of the four members of the system immediately exterior. Particles occupying this portion of the _primitive_ ring would always come into conjunction with one of these satellites in the same parts of their orbits. Such orbits would become more and more eccentric until the matter moving in them would unite near one of the apsides with other portions of the ring. _We have thus a physical cause for the existence of this remarkable interval._