The translucency of the ring system is also shown by the fact that it can be faintly seen against the sky as a narrow line of light on the occasions—sometimes extending over several weeks near the time of disappearance—when the plane of the rings passes between the sun and the earth. On this line of light two con densations are seen on each side of the planet, corresponding in position with the Cassini division and the Crape ring. They are apparently caused by the larger amount of sunlight transmitted at those places where the ring material is absent or relatively thin. The ring, as a whole, however, is sufficiently dense to cast a strong shadow, which is seen at such times as a narrow black band across the planet's equatorial regions.
The physical constitution of the rings is unlike that of any other object in the solar system. They are not formed of a con tinuous mass of solid or liquid matter, but of discrete particles of unknown minuteness, probably widely separated in proportion to their individual volumes, yet so close as to appear continuous when viewed from the earth. This constitution was first divined by J. Cassini early in the i8th century. But, although the im possibility that a continuous ring could surround a planet with out falling upon it was shown by Laplace, and must have been evident to all investigators in celestial mechanics, Cassini's ex planation was forgotten until 1857. In that year James Clerk Maxwell, in an essay which was the first to gain the newly f ounded Adams prize of the University of Cambridge, made an exhaustive mathematical investigation of the satellite constitu tion, showing that it alone could fulfil the conditions of stability. Although this demonstration placed the subject beyond doubt, it was of great interest when J. E. Keeler, at the Allegheny Ob servatory, proved this constitution by spectroscopic observation in 1895. He found, by measuring the velocity of different parts of the ring to or from the earth, that, as we pass from its outer to its inner regions, the velocity of revolution around the planet increases, each concentric portion having the speed belonging to a satellite revolving in a circular orbit at the same distance from the planet. The relative velocities of different parts of the
system are beautifully shown by the slope of the lines in a spectrogram of Saturn made by V. M. Slipher of the Lowell Observatory.
Satellites.—Saturn is attended, so far as at present known, by nine satellites. A tenth (Themis) was announced by W. H. Pickering in 1905, but its actual existence has not been satisfac torily confirmed.
Details of the satellites are given in the following table: celestial mechanics. Iapetus has the peculiarity of always appear ing brighter when seen to the west than to the east of the planet; this is explained by the supposition that, like our moon, this satellite always presents the same face to the central body.
Phoebe, the outermost satellite, is more than three and one half times as remote from Saturn as Iapetus, and the circum stances of its discovery are interesting. In studying photographs of the neighbourhood of Saturn taken at Arequipa Observatory, Peru, Pickering found on each of three plates a very faint star which was missing on the other two. He concluded that these were the images of a satellite moving around the planet, which was then entering the Milky Way, where minute stars were so numerous that it was not easy to confirm the discovery. When Saturn began to emerge from the Milky Way no difficulty was found in relocating the object and proving that it was a ninth satellite. Its motion, however, was found to be retrograde or in a contrary direction to that of the other satellites. This difference of motion in a single system was, according to the knowledge of that time, a unique phenomenon, for although the satellite of Neptune and those of Uranus were known to have retrograde motions, they are the only satellites of those planets hitherto discovered. But more recently the eighth and ninth satellites of Jupiter have been found, and the motion of these, like that of Phoebe, is retrograde. (T. E. R. P.)