Apparent Orbits of the Five Inner Satellites of Saturn, as seen in an Inverting Telescope.
(The arrows in the diagram show the direction of the motion of the satellites. The figures indicate the interval, in hours, from the time of last East elongation.)
Titan is by far the largest satellite, being equal to a star of the 8th mag. and visible in any small telescope. Iapetus ranks next, ordinarily about 9th mag., but there are variations at different parts of the orbit similar to the variations which affect the satellites of Jupiter; a variegated surface, and the effects of rotation, originate the changes observed and give strong support to the inference that this satellite rotates in the same period that it revolves round its primary. Tethys, Dione, and Rhea are fainter, and the difficulty of seeing them is intensified by their proximity to the planet; but a good 4-inch refractor will reveal them on a clear dark night. The others are objects for powerful instruments and pellucid skies; but Enceladus is sometimes seen with moderate aperture. The planet being usually much inclined, his satellites are dispersed round about the rings, and are not easy of identification. Minute stars lying near the path of Saturn are very liable to be mistaken for them. But the ephemerides drawn up by Mr. Marth, and published annually by the Royal Astronomical Society, are of the utmost service to amateurs engaged in these observations. By simple reference they may readily identify the individual satellites on any night; and these ephemerides are additionally useful as giving the times of conjunctions of some of the satellites with the ends of the ring and N. and S. points of the ball.
When the thin side of the rings is presented to the Earth, transits and other phenomena may be observed in connection with the Saturnian moons; but they appear to have been rarely recorded. Sir W. Herschel describes a “beautiful observation of the transit of the shadow of Titan over the disk in 1789, November 2.” It was also seen in 1833 and 1862. The late Mr. Capron re-observed it on Dec. 10, 1877, with a 8-1/4-inch reflector, power 144, and made the following sketch:—
These shadow-transits admit of easy observation with appliances of very moderate capacity. Mr. Banks witnessed a phenomenon of the kind with a refractor of only 2-7/8 inches, and says it was watched with the same facility and ease as the shadow of Sat. I. on Jupiter.
In looking for Iapetus it must be remembered that it is commonly situated at a great distance from the planet. Titan is relatively much nearer, and will always be recognized without trouble. Enceladus, Tethys, Dione, and Rhea hover near the outskirts of the ring; while Mimas is extremely close to it.