Saturn's train of satellites is the most numerous and remarkable in our system. As already mentioned, Huygens, the discoverer of the true form of the ring, discovered also the first and brightest satellite, Titan, which is a body somewhat larger than our own moon, having a diameter of 2,720 miles. A few years later came Cassini's discoveries of four other satellites, beginning in 1671 and ending in 1684. For more than 100 years discovery paused there, and it was not until August and September, 1789, that Sir William Herschel added the sixth and seventh to our knowledge of the Saturnian system.
In 1848 Bond in America and Lassell in England made independently the discovery of the eighth satellite—another of the coincidences which marked the progress of research upon Saturn, and in both of which Bond was concerned. Then followed another pause of fifty years broken by the discovery, in 1898, by Professor Pickering, of a ninth, whose existence was not completely confirmed till 1904. The motion of this satellite has proved to be retrograde, unlike that of the earlier discovered members of the family, so that its discovery has introduced us to a new and abnormal feature of the Saturnian system. The discoverer of Phœbe, as the ninth satellite has been named, has followed up his success by the discovery of a tenth member of Saturn's retinue, known provisionally as Themis. Accordingly the system, as at present known, consists of a triple ring and ten satellites. The last discovered moons are very small bodies, the diameter of Phœbe, for instance, being estimated at 150 miles; while its distance from Saturn is 8,000,000 miles. From the surface of the planet Phœbe would appear like a star of fifth or sixth magnitude; to observers on our own earth its magnitude is fifteenth or sixteenth. The ten satellites have been named as follows: 1, Titan, discovered by Huygens; 2, Japetus; 3, Rhea; 4, Dione; 5, Tethys, all discovered by Cassini; 6, Enceladus; and 7, Mimas, Sir William Herschel; 8, Hyperion, Bond and Lassell; 9, Phœbe; and 10, Themis, W. H. Pickering. Titan, the largest satellite, has been found to be considerably denser than Saturn himself.
The most of these little moons are, of course, beyond the power of small glasses; but a 2-inch will show Titan perfectly well. Japetus also is not a difficult object, but is much easier at his western than at his eastern elongation, a fact which probably points to a surface of unequal reflective power. Rhea, Dione, and Tethys are much more difficult. Kitchiner states that a friend of his saw them with 27⁄10-inch aperture, the planet being hidden; but probably his friend had been amusing himself at the quaint old gentleman's expense. Noble concludes that with a first-class 3-inch and under favourable circumstances four, or as a bare possibility even five, satellites may be seen; and I have repeatedly seen all the five with 3⅞-inches. The only particular advantages of seeing them are the test which they afford of the instrument used, and the accompanying practice of the eye in picking up minute points of light. There is a considerable interest in watching the gradual disappearance of the brilliant disc of Saturn behind the edge of the field, or of the thick wire which may be placed in the eye-piece to hide the planet, and then catching the sudden flash up of the tiny dots of light which were previously lost in the glare of the larger body. For purposes of identification, recourse must be had to the 'Companion to the Observatory,' which prints lists of the elongations of the various satellites and a diagram of their orbits which renders it an easy matter to identify any particular satellite seen. Transits are, with the exception of that of Titan, beyond the powers of such instruments as we are contemplating. The shadow of Titan has, however, been seen in transit with a telescope of only 2⅞-inch aperture.
[*] The plane of the rings passes through the earth on April 13, and through the sun on July 27, 1907, at which periods it is probable that the rings will altogether disappear.
CHAPTER XII
URANUS AND NEPTUNE
Hitherto we have been dealing with bodies which, from time immemorial, have been known to man as planets. There must have been a period when one by one the various members of our system known to the ancients were discriminated from the fixed stars by unknown but patient and skilful observers; but, from the dawn of historical astronomy, up to the night of March 13, 1781, there had been no addition to the number of those five primary planets the story of whose discovery is lost in the mists of antiquity.
It may be questioned whether any one man, Kepler and Newton being possible exceptions, has ever done so much for the science of astronomy as was accomplished by Sir William Herschel. Certainly no single observer has ever done so much, or, which is almost more important than the actual amount of his achievement, has so completely revolutionized methods and ideas in observing.