DISCOVERY 



65 



G. H. Darwin's researches indicated, was possibly 

 vinique ; while the satellites of Mars are bodies so 

 closely resembling the asteroids that it is by no means 

 improbable that Mars was at one time moonless, and 

 that the little attendants were originally asteroids 

 which came too near to the red planet at some epoch 

 in the distant past. But the satellite-systems of the 

 outer planets are of a different order. The four large 

 satellites of Jupiter are in point of size but slightly 

 inferior to the inner planets, and only their great 

 distance from us renders them more or less enigmatical 

 objects. The largest telescopes in the world have been 

 employed on these satellites from time to time, but 

 authentic information concerning them is scanty. 

 Polar caps, dusky markings, and canaliform appear- 

 ances have been glimpsed, but the evidence is con- 

 flicting. More certain is the fact — which Herschel, 

 with his quick insight, perceived over a century ago — 

 that each of the four moons completes its rotation in 

 the same period as its revolution and turns the same 

 face constantly to its primary, just as the Moon does 

 to the Earth and Mercury does to the Sun. 



Probably no discovery in recent years was so totally 

 unlooked-for as that of the tiny fifth satellite of Jupiter 

 by Barnard at the Lick Observatory in September 1892. 

 For nearly three hundred years the Jovian system had 

 been regarded as complete — as a perfectly harmonious 

 and sjTnmetrical sj'stem. The tiny moon, no more 

 than 100 miles in diameter, whirling round its primary 

 in a period of 11 hours 57 minutes at a distance of 

 112,000 miles seemed strangely out of place among 

 the larger satellites, and the suggestion was hazarded 

 by Sir Robert Ball that it was only the first of a number 

 of small satellites. Four other moons have since been 

 detected, two by Perrine at the Lick Observatory in 

 1905, one by Mellotte at Greenwich in 1908, and 

 another by Nicholson at the Lick Observatory in 1914. 

 These objects — so faint as to be detected only by their 

 images on the photographic plate — are akin to the 

 fifth satellite in size, but not in distance ; for the sixth 

 and seventh moons are about seven mUlion mUes from 

 Jupiter, and the eighth and ninth — which revolve in 

 a retrograde direction — about eighteen million. The 

 idea is at once suggested that these tiny Jovian satel- 

 Utes are captured asteroids, and that the four large 

 moons are the only original members of the system. 

 Be this as it may, there can be no doubt that the 

 sateUites of Jupiter fall into two well-defined groups — 

 giant and dwarf moons. 



The same cannot be said of the Saturnian system. 

 The ten satellites of which that system is composed are 

 of various degrees of brilliance and presumably of 

 different sizes. Titan, the largest, is of the same order 

 of size as the Jovian moons ; it was discovered by 

 Huyghens as long ago as 1655. Of the others, four 



were detected by Cassini in the seventeenth century, 

 and two by Herschel in the eighteenth. In 1848 came 

 the discovery of Hyperion, the eighth ; while Professor 

 W. H. Pickering detected Phoebe in 1898, and Themis 

 in 1905 by means of the photographic plate. Like 

 the distant satellites of Jupiter, Phoebe moves round 

 its primary in a retrograde direction. Apparently 

 most of Saturn's satellites turn the same face to their 

 primary. 



That LTranus and Neptune possess other sateUites 

 than those at present known is highly probable. The 

 four Uranian and the one Neptunian moon are relatively 

 large satellites ; and it is not improbable that both 

 these worlds possess smaller and fainter moons, which 

 will be revealed in the future by the aid of the camera. 



The system of Saturn is unique in the existence of 

 the wonderful rings. The ring-system was one of 

 the earliest telescopic discoveries. Huyghens in 1656 



THE PI..VNET SATURN .4MD ITS RINGS. 



detected the curious formation which baffled Galileo ; 

 and Cassini, one of the keenest observers who have ever 

 lived — discovered the existence of the division in the 

 ring known by his name — in other words, found that 

 there existed not one ring, but two ; while a third, the 

 " dusky " ring, was discovered in 1850 by Bond, of 

 Cambridge, U.S.A., and independently by the English 

 astronomer Dawes. 



The earlier astronomers proceeded on the assump- 

 tion that the rings were exactly what they seemed to 

 be — solid structures, and it was not till the middle of 

 the last century that the mathematical analysis of 

 Roche of Montpelier, and later of Clerk-Maxwell, 

 demonstrated that they were neither solid nor fluid, but 

 composed of myriads of meteorites, or " brickbats," 

 as the latter mathematician designated them. The 

 meteoritic theory of the rings received a triumphant 

 verification when in 1895 Keeler, by means of Doppler's 

 principle, succeeded in measuring the rates at which 

 the inner and outer rings revolved round Saturn. 



The late Professor Percival Lowell commenced, early 

 in the present century, an exhaustive study of the ring- 

 system. His scrutiny was rewarded when in 1907 he 

 made the highly important discovery of the knots or 



