This explanation, however, he admitted to be incomplete. There was, and is, outstanding variability, which seems to intimate the presence of an atmosphere and the formation of clouds. But no positive knowledge has yet been gained regarding the physical state of Saturn’s moons. We may, nevertheless, conjecture that, since tidal friction has destroyed the rotation (as regards Saturn) of the remotest member of the family, it has not spared those more exposed to its grinding-down action. All presumably rotate in the same time that they revolve.
The five inner satellites move in approximately circular orbits; the three outer in ellipses about twice as eccentric as the terrestrial path. All, Japetus only excepted, keep strictly to the plane of the rings. And since this makes an angle of 27° with the planet’s orbit, eclipses are much less frequent here than in the Jovian system. They can only occur when Saturn is within a certain distance (different for each) from the node of the satellite-orbit. Even Mimas (No. I), although it wheels round the ring at an interval of only 34,000 miles, often slips outside the obliquely projected shadow-cone. Its distance from Saturn’s centre is 118,000 miles, and it completes a circuit in 22½ hours. Perpetually wrapped in the glare of its magnificent primary, it is a very shy object, only to be caught sight of in its timid excursions by the very finest telescopes. Like all the Saturnian moons, except Titan, and, by a rare conjunction, Japetus, it is far too much contracted to be visible in transit across the disk.
The movements of these bodies have been carefully studied, and their mutual perturbations to some extent unraveled. They have proved exceedingly interesting to students of celestial mechanics. Titan has, in this department, chiefly to be reckoned with. He exercises in the Saturnian system a similar overpowering influence to that wielded by Jupiter in the Solar System.
FOOTNOTES:
[28] A ninth satellite, Phœbe, was discovered in 1904. Its existence had been suspected for many years, and it was discovered at the Arequipa Observatory, Peru, on March 14, 1899, by means of photography. Since that date, it has been several times lost and rediscovered.—E. S.
URANUS AND NEPTUNE.—William F. Denning
While Sir W. Herschel was a musician at Bath he formed the design of making a telescopic survey of the heavens. While engaged in this, he accidentally effected a discovery of great importance, for on the night of March 13, 1781, an object entered the field of his 6.3-inch reflector which ultimately proved to be a new major planet of our system.
The acute eye of Herschel, directly it alighted upon the strange body, recognized it as one of unusual character, for it had a perceptible disk, and could be neither fixed star nor nebula. He afterward found the object to be in motion, and its appearance being “hazy and ill-defined,” with very high powers, he was led to regard it as a comet, and communicated his discovery to the Royal Society at its meeting on April 26, 1781.
The supposed comet soon came under the observation of others, including Maskelyne, the Astronomer Royal, and Messier, the “Comet Ferret,” of Paris. The latter, in a letter to Herschel, said: “Nothing was more difficult than to catch it, and I can not conceive how you could have hit this star or comet several times, for it was absolutely necessary for me to observe it for several days in succession before I could perceive that it was in motion.”
As observations began to accumulate, it was seen that a parabolic orbit failed to accommodate them. Ultimately the secret was revealed. The only orbit to represent the motion of the new body was found to be an approximately circular one situated far outside the path of Saturn, and the inference became irresistible that the supposed “comet” must in reality be a new primary planet revolving on the outskirts of the Solar System. This conclusion was justified by facts of a convincing nature, and its announcement created no small excitement in the scientific world. Every telescope was directed to that part of the firmament which contained the new orb, and its pale blue disk, wrapped in tiny proportions, was viewed again and again with all the delight that so great a novelty could inspire. From the earliest period of ancient history, no discovery of the same kind had been effected. The Chaldeans were acquainted with five major planets, in addition to the earth, and the number had remained constant until the vigilant eye of Herschel enlarged our knowledge, and Saturn was relieved as the sentinel planet going his rounds on the distant frontiers of our system.