SATURN, in astronomy, the 6th of the major planets in order of distance from the sun, and the outermost known to the ancients. With its ten satellites and wonderful ring system it is to be regarded as the most remarkable body in the solar system, as well as the most beautiful. The ball of the planet itself has a density of only five-sevenths that of water, rotates on its axis (which is inclined about 27° to its orbit plane) in about 10 hours 14 minutes, is about 73,000 miles in mean diameter ; but on account of its huge size and rapid rotation it has an ellipticity of about 0.10, its equatorial diameter be ing about 73,000 miles, and its polar diam eter only about 68,000 miles. Its mean apparent diameter varies with its vary ing distance from the earth between 14" and 20". Its surface is marked by belts parallel to its equator, the equatorial ones being the brightest part of the surface, but they are not so decided as those of JUPITER (q. v.), and they have no perma nent markings. The above rotation time was determined by Prof. Asaph Hall from a bright spot which suddenly appeared on the planet in 1876 and continued visible for several weeks.
The ring system is the most remarkable feature, having an extreme diameter of about 168,000 miles and a width of about 36,000 or 37,000 miles. This width is di vided into three principal parts, the outer ring about 10,000 miles wide, the middle ring about 16,500 miles wide, and the in ner or dusky ring about as wide as the outer one. The division between the outer ones is sharp and permanent, and about 1,600 miles wide. The bright middle ring fades into the dusky one without any sharp boundary line between them. This dusky ring comes within about 9,000 or 10,000 miles of the planet's equator. It was not discovered till November, 1850, and then by G. P. Bond, of Cambridge, Mass., and about two weeks later, inde pendently, by Dawes, of England. It is
not at all prominent at any time, and only visible with a good telescope.
Satellites.—The principal elements of the satellite system of Saturn are as fol lows: Saturn as our moon does toward the earth. Titan is as bright as a 9th magni tude star, and is by far the largest of the satellites of Saturn. It is probably 3,000 or 4,000 miles in diameter, and its mass is estimated at about 1 : 4600 that of Saturn. Its large mass and its nearness to Hype rion cause large perturbations in the orbit of the latter, so much so that Hyperion's motion has been for a long time a puzzle to astronomers, and it is only lately that it is coming to be understood as offering a new case in celestial mechanics, which must be attacked by new methods of an alysis. The perturbations of Titan keep the line of apsides of Hyperion's orbit in the line of conjunction with Titan, forc ing the line to retrograde rapidly, where as it would have a progressive motion according to the usual law of perturba tions. The large outside orbit of Iapetus has an inclination of about to the plane of the rings, but all the other sat ellites move exactly in that plane, and the orbits of the five inner ones are sensibly circular.
The discovery of Phcebe, the 9th satel lite of Saturn, was announced March 18, 1898, by Prof. William H. Pickering, of the Harvard Observatory, Cambridge, Mass. The satellite appears on four plates taken at Harvard's Arequipa station, South America, with the Bruce photo graphic telescope. Saturn's new moon is the only one that has been discovered by means of photography. A tenth satellite was discovered in 1905. As to Saturn's physical condition, it is possible to ad vance at present only the most uncertain conjecture.