Uranus, the seventh planet in order of distance from the sun, and the outermost but one of all the known members of the planetary system. Uranus travels at a mean distance of 1,753,869,000 m. from the sun, his greatest distance being 1,835,561,000 m., his least 1,672,177,000 m. Hence it will be perceived that the eccentricity of his orbit is considerable, the greatest exceeding the least distance by 163,384,000 m., or by a greater distance than that which separates Venus from the earth when these two planets are furthest from each other. The eccentricity of Uranus's orbit is in fact 0046578. As a consequence of this eccentricity, his apparent brightness in different oppositions varies considerably. It is not merely that his distance from the earth varies, but his distance from the sun varies also, and consequently the amount of light he receives from it. "When at opposition near to the perihelion of his orbit his distance from the earth amounts to 1,581,700,000 m., and when in opposition near aphelion it amounts to 1,744,100,000 m.
As his apparent size varies inversely as the square of the distance from the earth, while his apparent intrinsic brightness varies as the square of the distance from the sun, it follows that his apparent absolute brightness is greater when he is at opposition near perihelion than when he is at opposition near aphelion in the proportion of (17,441)2 x (1,835,561)2 to (15,817)2 x (1,672,177)2, or as 14,651 to 10,000; that is, nearly as 3 to 2. His orbit is only inclined 46½' to the ecliptic. His mean diameter is about 33,000 m., the compression of his globe unknown. In volume he exceeds the earth about 74 times; but his density being barely 17/100 of hers, his mass exceeds the earth's only about 12 times. Some astronomers assert that marks can be recognized on the surface of Uranus, and that by noting these his rotation period can be determined; but very little reliance can be placed on the assigned period of 9½ hours. The sidereal revolution of Uranus is accomplished in 84 years 6½ days, or 30,686.8208 days; his mean synodical period is 369.5 days, or only 4¼ days more than a tropical' year. - Uranus was discovered by Sir W. Herschel on March 13, 1781, when he was examining the constellation Gemini (near tj) for double stars.
Observing that a star in the telescopic field looked larger than the rest, he suspected it to be a comet. Using higher powers, he found the disk enlarged, which would not happen with a star. He thereupon announced the discovery of a comet. But after a short time it was found that the supposed comet was travelling on a nearly circular orbit around the sun; and it was presently recognized as a member of the sun's family of planets. Herschel called it the Georgium Sidus, and foreign astronomers called it Herschel; but the name Uranus, assigned to it by Bode of Berlin, is now always used. In 1787 Herschel discovered two satellites attending on Uranus, and he afterward supposed he had discovered four others; but there is every reason to believe that he had mistaken in most cases small stars for satellites. It appears probable, however, from an inquiry recently instituted by Prof. Holden of Washington, that among his scattered observations of supposed satellites are some really relating to the two inner satellites discovered later by Lassell.
The following are the elements of these bodies according to Mr. Hind, superintendent of the English "Nautical Almanac:"
Mean distance in radii of H.
2d 12b. 28m
4 3 27
8 16 55
13 11 6
It will be observed that the inclination here assigned is greater than a right angle. What is meant is, that the satellites travel on retrograde paths at an inclination of 79° 26', the complement of that here assigned. It was found, soon after the discovery of Uranus, that the planet had often been observed as a supposed star by Flamsteed, Bradley, Lemonnier, and Mayer. Lemonnier indeed had observed it 12 times. For the interesting result of these researches see Neptune. - The spectroscope has revealed nothing very satisfactory respecting Uranus, though Huggins suspects the presence of large quantities of hydrogen in the planet's atmosphere.