The Solar Theory.
The theory which ascribes a solar origin to meteorites is not of recent date, having been held by Diogenes Laertius and other ancient Greeks. Among the moderns its advocates have been much less numerous than those of the lunar hypothesis. The late Professor Charles W. Hackley, of New York, regarded shooting-stars, aerolites, and even comets, as matter projected with enormous force from the solar surface. The corona seen during total eclipses of the sun he supposed to be the emanations of this matter through the intervals of the luculi.—(See the Proceedings of the American Association for the Advancement of Science, Fourteenth Meeting, 1860.) An ingenious theory, differing in its details from that of Professor Hackley, though somewhat similar in its general features, has lately been advocated by Alexander Wilcocks, M.D., of Philadelphia, in a memoir read before the American Philosophical Society, May 20th, 1864, and published in their Proceedings. In regard to this hypothesis it seems sufficient to remark that it fails to give a satisfactory account of the annual periodicity of meteoric phenomena.
[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.
| Name. | Sidereal Revolution. | Mean Apparent 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.
| Struve. | Encke. | Mean. | In 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 of the period of Enceladus | =10 | 59 | 22 |
| One-half of the period of Mimas | =11 | 18 | 32 |
| One-fourth of the period of 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.
[CHAPTER XIII.]
THE ASTEROID RING BETWEEN MARS AND JUPITER.
The mean distances of the minor planets between Mars and Jupiter vary from 2·20 to 3·49. The breadth of the zone is therefore 20,000,000 miles greater than the distance of the earth from the sun; greater even than the entire interval between the orbits of Mercury and Mars. Moreover, the perihelion distance of some members of the group exceeds the aphelion distance of others by a quantity equal to the whole interval between the orbits of Mars and the earth. The Olbersian hypothesis of the origin of these bodies seems thus to have lost all claim to probability.[30] Professor Alexander's theory of the disruption of a primitive discoidal planet of great equatorial diameter, is less objectionable; still, however, it requires confirmation. But whatever may have been the original constitution of the ring,[31] its existence in its present form for an indefinite period is unquestioned. Let us then consider some of the effects of its secular perturbation by the powerful mass of Jupiter.
Portions of the ring in which the periods of asteroids would be commensurable with that of Jupiter.—The breadth of this zone is such as to contain several portions in which the periods of asteroids would be commensurable with that of Jupiter. As in the case of the perturbation of Saturn's ring by the interior satellites, the tendency of Jupiter's influence would be to form gaps or chasms in the primitive ring.
| The mean distance of an asteroid whose period is 1/2 that of Jupiter | =3·2776 |
| That of one whose period is 1/3 of Jupiter's | =2·5012 |
| That of one whose period is 2/5 of Jupiter's | =2·8245 |
| That of one whose period is 2/7 of Jupiter's | =2·2569 |
| That of one whose period is 3/7 of Jupiter's | =2·9574 |
| That of one whose period is 4/9 of Jupiter's | =3·0299 |
For the purpose of facilitating the comparison of these numbers with the mean distances of the asteroids and of observing whether any order obtains in the distribution of these mean distances in space, we have arranged the minor planets, in the following table, in the consecutive order of their periods:
Periods and Distances of the Asteroids.