Of this planet’s eight satellites,[28] the largest, Titan (No. VI), was discovered first (by Huygens in 1655), and the smallest, Hyperion (No. VII), last (by Lassell and Bond in 1848). The five others were detected by J. D. Cassini and William Herschel. Titan, alone of the entire group, equals our moon in size. It measures, according to Professor Barnard, 2,720 miles across. Its period of revolution is nearly sixteen days, its distance from Saturn’s centre, 771,000 miles. The orbit of Japetus (No. VIII) is the largest, and its period the longest of any secondary body in the Solar System. It circulates in 79⅓ days at a distance of 2,225,000 miles, equal to 59½ of Saturn’s equatorial radii. Hence its path is of about the same _proportional_ dimensions as that of our moon. Japetus is remarkable for its variability in light. It is capable of tripling or quadrupling its minimum lustre. Sir William Herschel noticed that these maxima coincided with a position on the western side of the planet, and inferred rotation of the lunar kind. “From the changes in this body,” he argued in 1792, “we may conclude that some part of its surface, and this by far the largest, reflects much less light than the rest; and that neither the darkest nor the brightest side is turned toward the planet, but partly one and partly the other, though probably less of the bright side.” 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.