SATURN. The name of one of the old planets, the largest of all the bodies of the solar system, except the Sun and Jupiter. It is encompassed by three rings, unconnected with the planet, but revolv ing around it. It is also accompanied by eight satellites.
The apparent semi-diameter of Saturn, at the mean distance of the planet front the earth, is about 16'4. The real diameter, that of the earth being represented by unity, is about 79,000 miles. The mean density is about of that of the Sun, or ith of that of the Earth, and the mass of the planet is about the Sun's mass being repre sented by unity. It revolves on its axis in 10 hours, 29 minutes; its equator is inclined to the ecliptic at an angle of 31° 19'. Its light and heat are to the light and heat received by the earth as 1I to 1000. The following are the elements of its orbit : Epoch 1801, January 1, I2th hour, mean astronomical time at Greenwich. Semi-axis major, 9.5387861, that of the Earth being represented by unity.
Excentrieity, -0561505 ; its secular diminution (diminution in 100 'years) -000,312,402.
fnelination of the orbit to the ecliptic, 2° 29' 35'7; its secular dimi nution, 15'5.
Longitudes from the mean equinox of the epoch : (1) of the ascend ing node, 11I° 56' 37'4; its secular increase (combined with the pre cession), 3070" ; (2) of the perihelion, 89° 9' 294 ; its secular increase (combined with the precession) 6950"; (3) of the planet (mean), 135° 20' Mean sidereal revolution in 3651 days, 43996"•13; sidereal revolu tion, 10759:2198174 mean solar days.
The discovery of the ellipticity of Saturn is due to Sir William Herschel, who concluded, from his observations, that the polar is to the equatorial diameter as 10 to 11. On a subsequent occasion he was led to suspect an irregularity in the figure of the planet, but the researches of Besse!, and also those of Mr. Main,* late first assistant at the Royal Observatory, Greenwich, have proved beyond all doubt that the form of the planet is strictly spheroidal.
When viewed in the telescope the planet Saturn appears to be diversified with belts extending across his disc in a direction parallel to his equator. It was by watching the changes in the appearance of these belts, that Sir William Herschel succeeded in discovering the motion of the planet on his axis, and in determining the time of a complete rotation.
The satellite of Saturn which was first perceived is the sixth in the order of distance from the primary. Its discovery was effected by Huyghens in 1655. Four other satellites were subsequently discovered by Cassini in the same centtfry. These five satellites were named the first, second, third, fourth, and fifth, reckoning according to their distance from the primary. In 1789, two additional satellites were discovered by Sir William Herschel. Both of these satellites were found to revolve within the orbit of the first of the five satellites pre viously discovered. They ought therefore in aceonLance with the prevailing nomenclature, to be designated as the first and second satellites, lint this would have rendered a revision of the names of the earlier satellites indispensable. The Ilerschehian satellites however were usually styled the sixth and seventh, reckoning inward in the order of distance from the primary. To remedy the imperfection which thus arose from confounding the order of discovery with the order of distance, Sir John Herschel has recently proposed to apply to the satellites the names of the Titanian divinities,—Japetus, Titan, Rhea, Diem, Tethys, Encelsidus, Mimas, commencing with the most distant satellite. This nomenclature has been generally adopted by astronomers.
On the 19th of September, 1818, an eighth satellite of Saturn was discovered by Mr. Bond, at the Observatory of Harvard College, Cam bridge, U.S.; and also independently, on the same evening, by Mr. Lassen at his Observatory near Liverpool. In conformity with Sir John lierschel'e nomenclature it has been called Hyperion. It ranks the seventh in the order of distance from the central body. The following table will be found useful :— The periods and mean distances inserted in the foregoing table have been taken from Sir John Herschel's ' Outlines of Astronomy.' Tho period and mean distance of Hyperion have not yet been determined with sufficient precision.