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._