Saturn, next in position, is about 735 times larger in volume, though only about 100 times greater iu mass than the earth. its apparent diameter when in opposition is 18", and there is a considerable toward the poles. Its surface is traversed by dusky belts much less diStinctly marked than those of Jupiter, doubtless in great part to its inferior brightness: its general color is a dull white or yellowish, but the shaded por tions, when seen distinctly, are of a glaucous color. The most remarkable peculiarity of Saturn is its ring, or series of concentric rings, each one parallel and in the same plane with the others and with the planet's equator; the rings are at present sappoSed to be three in number, the two outermost are bright like the planet itself, while the innermost is of a purplish color, and is only discernible through a powerful telescope. The rings are not always visible when Saturn is in the " opposite" half of its orbit, for when the plane of the rings is intermediate between that of the earth's orbit and of the ecliptic, their dark surface is turned toward us, and when the sun is in their plane only the nar row edge is illumined; in both of these cases the ring is invisible from the earth. Its plane at an angle of 28° to the ecliptic, we see the two surfaces of the ring periods of 15 years at a time; and at the middle of each period, the rings attain their maximum obliquity to the ecliptic, and are then best seen from the earth. It is hardly necessary to remark that at the end of each period they become invisible. Saturn has also no less than eight satellites, seven of which revolve round it in orbits little removed front the place of the ring, while the eighth, which is the second in size, is considerably- inclined to it. Two of the satellites were discovered by Herschel in 1787 and 1789, four by Cassini in 1672-84, one by HuyghenS in 1655, one by Mr. Lassell in England and prof. Bond in America in 1848. The satellites are all situated outside of the ring, and the largest of them is nearly equal to the planet Mars in size.
Uranus, the next planet in position, was discovered accidentally by the elder Her schel on Mar. 13, 1781, and was named " the Gcorgium Sidus " and "Herschel," but these names soon fell into disuse. It is about 56 (some astronomers say 82) times greater than the earth in volume, and 20 (according to others, 15) times in mass; but though so large, its distance is so much greater in proportion that astronomers have been unable to gain much information concerning it. No spots or belts have hitherto been discovered
ou its surface, and consequently its time of rotation and the position of its axis are unknown. It is attended by a number of satellites, but so minute do these bodies appear, that astronomers hitherto have been unable to agree as to their exact number; sir Wil liam Herschel reckoned six, while other astronomers believe in the existence of four, five, and eight respectively. That there are at least four is without doubt.
Neptune is the next and outermost member of the solar system, and, at a distance of nearly 3.000 millions of miles from the center of the system, slowly performs its revolu tion round the sun, accomplishing the complete circuit in about 165 solar years. It is about 84 times larger than the earth, but from its extreme remoteness is of almost inap preciable magnitude when seen through an ordinary telescope. It was the disturbance in the motion of Uranus caused by the attractive force of this planet which led Leverrier and Adams to a calculation of its size and position, on the supposition of its existence; and the directions which were given by the former to Dr. Galle of Berlin, specifying its exact position in the heavens. led that astronomer to its discovery on Sept. 23, 1846. Mr. Lassell of Liverpool has discovered that Neptune is attended by one satellite. The satellites of Uranus and Neptune differ from the other planets, primary and secondary, in the direction of their motion, which is from e. to w. and in the case of the former, in planes nearly perpendicular to the.ecliptic. Both dranus and Neptune were observed long before the times of Herschel and Leverrier, but they were always supposed to be stars. Uranus is known to have been observed by Flamsteed between 1690 and 1715, and Neptune by Lalande in 1795.
In astronomical tables, almanacs, etc., the planets are for convenience denoted by symbols instead of their names, as follows: Mercury, ; Venus, 7 ; earth, 6 ; Mars, s ; the planetoids, in the order of their discovery, ®, etc.; Jupiter, 24; Saturn, T2 or ; Uranus, ht: Neptune, ; the sun, O ; the moon, a .
For a TABLE of the periods, distances, size, density, etc., of the planets, see SOLAR SYSTEM.