SATELLITES, OR MOONS. 



83 



wrn, which were discovered by Sir William 

 Herschel, in 1789, with his great 40 foot tele- 

 scope, and which have been again seen by Sir 

 John Herschel at the Cape, by Vico at Rome, 

 and by Lamont at Munich, belong, in common 

 with the satellites of Uranus, to the smallest of 

 the visible bodies that enter into the constitu- 

 tion of our solar system. These satellites, in- 

 deed, are only to be seen under peculiarly fa- 

 vourable circumstances, and with the most 

 powerful telescopes. All determinations of 

 the true diameters of satellites, deductions of 

 these from measurements of the apparent mag- 

 nitudes of small discs, are exposed to many 

 optical difficulties ; and physical astronomy, 

 wliich calculates before-hand, and with such 

 admirable precision, the motions of the heav- 

 enly bodies, as they are exhibited from our 

 place of observation, the earth, is more con- 

 cerned about motion and mass, than volume. 



The absolute distance of a satellite from its 

 primary, is greatest in the case of the outer- 

 most or seventh satellite of Saturn, which is 

 half a million of geographical miles* remote, 

 or ten times as far as the distance of our moon 

 from the earth. In reference to Jupiter, the 

 outermost or fourth satellite is ifc more than 

 260,000 geographical miles* from the planet ; 

 the fifth satellite of Uranus, however, if it actu- 

 ally exist, must be at the distance of 340,000 

 miles. 



On comparing, in each of these subordinate 

 systems, the volume of the primary planet, with 

 the distance of the farthest orbit in which a sat- 

 ellite has been formed, w^e discover totally dis- 

 similar numerical relations. Expressed in sem- 

 idiameters of the principal planets, the dis- 

 tances of the farthest satellites of Uranus, Sat- 

 urn, and Jupiter, are as 91, 64, and 27. Sat- 

 urn's outermost satelMte, therefore, is but a 

 very little (y'^th) more remote from the centre 

 of the primary than our moon is from the earth. 

 The satellite that approaches its primary most 

 closely, is undoubtedly the first or innermost 

 of Saturn, which, in addition, presents the only 

 instance of a revolution in less than 24 hours. 

 The distance of this satellite from Saturn's cen- 

 tre, according to Madler and Beer, expressed in 

 semidiameters of the primary, is only 2-47, or 

 20,022 geographical miles.* This' satellite can- 

 not, therefore, be distant from the surface of 

 its primary more than 11,870 g. miles; and 

 from the outer adge of the ring, only 1,229 g. 

 miles. One who has been a traveller readily 

 forms an idea of so short a distance, the more 

 so when he thinks of that bold seaman. Captain 

 Beechey, having sailed over 18,200 geographi- 

 cal miles in the course of three years. Recur- 

 ring to semidiameters of the primary as meas- 

 ures of distance, we find that the first or inner- 

 most satellite of Jupiter is no more than six 

 semidiameters of the planet from his centre ; 

 our moon, on the contrary, is 60^ semidiame- 

 ters of the earth from its centre. The first sat- 

 ellite of Jupiter is, nevertheless, 6,500 miles far- 

 ther from his centre, than our moon from the 

 centre of the earth. 



In the subordinate systems of the satellites, 

 m other respects, all the laws of gravitation are 

 reflected that have been established in connec- 



* The miles are always German geographical miles, 15 

 to a degree of the Equator.— Tbanslatob. 



E 



tion with the sun and the primaries which re- 

 volve around him. The twelve satellites of 

 Saturn, Jupiter, and the Earth, all revolve, like 

 the primary planets, from west to east, and in 

 elliptical orbits, which differ but little from cir- 

 cles. It is only the moon, and probably the 

 first, or innermost satellite of Saturn (0 068), 

 which have orbits, whose eccentricity surpass- 

 es that of Jupiter. Bessel's very accurate ob- 

 servations on the 6th satellite of Saturn show- 

 that the excentricity here (0 029), exceeds that 

 of the Earth. 



It is only in connection with the satellites of 

 Uranus, on the extreme limit of the planetary 

 system, at nineteen times the distance of the 

 earth from the sun, and where his central force 

 must be notably diminished, that we find any 

 thing like contrasts to admitted laws. Instead, 

 of moving, like all the other satellites, in or- 

 bits but little inclined to the ecliptic, and from 

 west to east, (the ring of Saturn, a kind of fused 

 or undivided satellite, not excepted), the moons 

 of Uranus revolve in planes nearly perpendic- 

 ular to the ecliptic, and, as Sir John Herschel 

 has found, after many years of observation, in 

 retrograde courses from east to west. If the 

 primary and secondary planets of our system 

 have actually been formed out of rotating rings 

 of vapour, by condensations of former solar 

 and planetary atmospheres, there must have 

 been strange, and to us altogakher inconceiva- 

 ble conditions of retardation or counteraction 

 among the vaporous rings that revolved around 

 Uranus, to have brought about such a singular 

 opposition to the motions of the central body 

 as we observe in his 2d and 3d satellites. 



It is highly probable, that the period of rota- 

 tion of all the satellites is the same as their pe- 

 riod of revolution, so that they still keep the 

 same side turned towards their primaries. In- 

 equalities, as a consequence of slight variations 

 in the revolution, nevertheless, occasion oscil- 

 lations of from 6 to 8 degrees — an apparent li- 

 bration — both in longitude and latitude. We 

 therefore actually see, in succession, more than 

 one half of the surface of the moon ; at one 

 time more of her eastern and northern, at an- 

 other more of her western and southern limb. 

 By the libration(") the annular mountain Mal- 

 apert, which the south pole of the moon covers 

 at times, is made more visible to us, and then 

 we obtain a better view of the arctic landscape 

 around the mountain-crater, Gioja, as also of 

 the extensive grey level nearEndymion, which 

 surpasses the Mare vaporum in superficial ex- 

 tent. In spite of all this, however, three-sev- 

 enths of the moon's surface remain, and, un- 

 less some new and unexpected cause of pertur- 

 bation interferes, will ever remain withdrawn 

 from our eyes. These cosmic relations remind 

 us, involuntarily, of a nearly similar position 

 of things in the intellectual world, in the prod- 

 ucts of thought, where, in the deep investiga- 

 tion of the dark elaboratory of nature and the 

 prime creative power, there are alsa regions 

 turned from our ken, and that seem unattaina- 

 ble, though, in the course of thousands of years, 

 mankind have, from time to time, caught a 

 glimpse of some narrow stripe or margin, now 

 in a true and steady, now in a more false and 

 flickering light. 



We have hitherto regarded the principal plan- 



