THE PLANETS. 



31 



know, through actual observation, as a central 

 body in reference to the conglobated masses 

 of matter, in the shape of planets, comets, and 

 aerolitic asteroids, which revolve around, and 

 immediately depend upon him. Among the 

 multiple or double stars or suns, in so far as 

 their nature has yet been studied, there does 

 ' not appear to reign the same planetary depend- 

 ence, in respect of relative motion and illumi- 

 nation, which characterizes our solar system. 

 Two or more self-luminous stars, whose plan- 

 ets and moons — if any such exist — escape our 

 present telescopic powers of vision, revolve 

 unquestionably around a common centre of 

 gravity; but this centre falls in a space that 

 perchance is filled with unaggregated matter 

 (world-mist), whilst with our sun it is always 

 situated in the inner confines of a visible cen- 

 tral body. When we consider our sun and 

 earth, or our earth and moon, as double stars, 

 and our whole planetary system as a multiple 

 group of stars, the analogy with the proper 

 multiple or double fixed stars, which such a 

 designation presents to the mind, extends no 

 farther than to motions connected with sys- 

 tems of attraction of different orders, quite in- 

 dependently of light evolving processes, and 

 kinds of illumination. 



In this generalization of cosmic views, which 

 befits the sketch of a Picture of Nature or the 

 Universe, the solar system to which our earth 

 belongs may be considered in a two-fold rela- 

 tionship : immediately, to the several classes 

 of individualized conglomerate matter — to the 

 magnitude, the fashion, the density, and the dis- 

 tance of the bodies of the system ; and, next, in 

 its relations to other parts of our astral system, 

 to the sun's change of place within the same. 



The solar system, in other words, the very 

 variously fashioned matter which circulates 

 about the sun, consists, according to our pres- 

 ent knowledge, of eleven principal planets, of 

 eighteen moons or satellites, and of myriads 

 of comets, three of which, called planetary 

 comets, never quit the limited spheres of the 

 proper planets. We may further, with no 

 slight show of propriety, reckon as falling with- 

 m in the empire of our sun, as included within the 

 sphere of his central force — 1st, a ring of va- 

 porous matter, revolving, in all probability, be- 

 twixt the orbits of Venus and Mars, certainly 

 extending beyond the orbit of the earth('), which 

 is visible to us in a pyramidal form, and is 

 known under the name of the zodiacal light ; 

 2d, a host of very small asteroids, whose or- 

 bits either intersect the orbit of the earth, or 

 approach it very nearly, and give occasion to 

 the phenomena of aerolites and falling stars. 

 When we direct our attention to the complexi- 

 ty of formations which circulate about the sun 

 in orbits more or less excentric, unless, with 

 the immortal author of the " Mechanique Ce- 

 leste," we regard the greater number of com- 

 ets as nebulous stars which sweep from one 

 central system to another(^), we must confess, 

 that the planetary system, strictly so called — 

 the group of bodies which revolve, with their 

 attendant satellites, in but slightly excentric 

 orbits round the sun — constitutes but a small 

 portion of the entire system, when the number, 

 not the mass, of the individuals is made the 

 basis of consideration. I 



The telescopic planets, Vesta, Juno, Ceres, 

 and Pallas, with their mutually intersecting, 

 much inclined, and more excentric orbits, have 

 been viewed as constituting a kind of zone of 

 separation between two divisions of our plan- 

 etary system, and as forming in themselves a 

 middle group. According to this view, the in- 

 ner planetary group, comprising Mercury, Ve- 

 nus, the Earth, and Mars, presents several re- 

 markable points of contrast with the outer 

 group, consisting of Jupiter, Saturn, and Ura- 

 nus(^). The inner planets, nearer to the sun, 

 are of moderate dimensions, of greater density, 

 turn more slowly upon their axes, and very 

 nearly in the same period of time (twenty-four 

 hours), are flattened towards their poles in a 

 less degree, and, with one exception, are un- 

 accompanied by moons. The outer, and, from 

 the sun, more distant planets, are vastly larger, 

 of but one-fifth of the density, more than twice 

 as rapid in their periods of rotation about their 

 axes, flattened towards their poles in a much 

 greater degree, and attended by a far larger 

 number of moons ; in the ratio of 17 to 1, if 

 Uranus have actually so many as six satellites. 



These general observations on certain char- 

 acteristic peculiarities of the two great groups, 

 are not, however, precisely or in all respects 

 applicable to the particular planets of each 

 group ; for example, to the ratios of their ab- 

 solute magnitudes, to their distances from the 

 central body, to their densities, to the times 

 of their rotations on their axes, to their excen- 

 tricities, and to the inclinations of their orbits 

 and of their axes. We know as yet of no in- 

 timate necessity, of no mechanical natural 

 law, like the beautiful law which connects the 

 squares of the times of revolution with the 

 cubes of the greater axes, which makes the 

 six elements of the planets just indicated, and 

 the form of their orbits, dependent on one an- 

 other, or on their mean distances. Mars, more 

 remote from the sun, is smaller than the Earth 

 or Venus ; he approaches Mercury — the near- 

 est of all the known planets to the sun — most 

 closely in his diameter ; Saturn, again, is small- 

 er than Jupiter, yet much larger than Uranus. 

 The zone of the telescopic planets, so insignif- 

 icant in point of volume, lies, in a series of 

 distances setting out from the sun, immediate- 

 ly before Jupiter, the most considerable of all 

 the planetary bodies ; and yet these asteroids, 

 several of whose discs can scarcely be meas- 

 ured, are barely one half more in their super- 

 ficies than France, or Madagascar, or Borneo. 

 Again, however remarkable the very small 

 density of all the colossal planets that lie far- 

 thest from the sun, there is still nothing like a 

 regular sequence among them(8). Uranus ap- 

 pears to be more dense than Saturn, even when 

 Lament's smaller mass, ^y^o J' ^s adopted ; and 

 although the differences "in point of density of 

 the inner group of planets (^), are insignificant, 

 we still find Venus and Mars, on either side of 

 the Earth, of less density than itself The 

 time of rotation decreases, it is true, with the 

 distance from the sun ; but for Mars it is rel- 

 atively greater than for the Earth, and for Sat- 

 urn it is greater than for Jupiter. The greatest 

 excentricities in the eUiptical orbits of any of 

 the planets, occur in those of Juno, Pallas, and 

 Mercury ; the least in those of Venus and the 



