File:Discovery (20339007574).jpg

Title: Discovery
Identifier: discovery0304londuoft (find matches)
Year: [1] (s)
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Subjects: Science
Publisher: London (etc. ) Professional and Industrial Pub. Co. (etc. )
Contributing Library: Gerstein - University of Toronto
Digitizing Sponsor: University of Toronto

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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
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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

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