URANUS, major planetin the solar system, equivalent in brightness to a sixth-magnitude star. It ranks seventh in order of distance from the sun, revolving outside the orbit of Saturn and inside the orbit of Neptune. Uranus was accidentally discovered in 1781 by the British astronomer Sir William Herschel and was originally named the Georgium Sidus (Star of George) in honor of his royal patron King George III of Great Britain. The planet was later, for a time, called Herschel in honor of its discoverer. The name Uranus, which was first proposed by the German astronomer Johann Elert Bode (1747-1826), was in use by the late 19th century.

Uranus has an equatorial radius of 25,559 km (15,882 mi), and its average distance from the sun is 2,870,972,200 km (1,783,939,400 mi). Uranus takes 84.02 earth years for a single revolution, or orbit, around the sun and 17.24 hr to complete a rotation about its axis; the direction of rotation is retrograde (opposite to that of earth). Uranus's atmosphere consists largely of hydrogen and helium, with a trace of methane. Through a telescope the planet appears as a small, bluish-green disk with a faint green periphery. Compared to the earth, Uranus has a mass 14.371 times greater and a volume 63.1 times greater. The magnetic field of Uranus has its axis tilted 59° from the rotational axis and appears to exhibit other significant anomalies.

Uranus has 11 rings, known (in increasing order of distance from the planet) as 1986U2R, 6, 5, 4, Alpha, Beta, Eta, Gamma, Delta, Lambda, and Epsilon. These rings form a belt with a width of about 13,150 km (about 8170 mi), extending from about 38,000 km (23,610 mi) to 51,150 km (31,780 mi) from the planet's center. Nine of these rings were discovered in 1977. While recording the occultation of a star behind the planet, the American astronomer James L. Elliot (1943-    ), working with a team from the Kuiper Airborne Observatory of the National Aeronautics and Space Administration (NASA), discovered the presence of what were eventually identified as six rings encircling the equator of Uranus; three more rings were identified by a team at the Perth Observatory in Australia. In January 1986, during the exploratory flight of NASA's Voyager 2, these nine rings were photographed and measured, as were two new ones.

In addition to its rings, Uranus has at least 27 satellites. The two largest moons, Oberon and Titania, were discovered by Herschel in 1787. The next two, Umbriel and Ariel, were found in 1851 by the British astronomer William Lassell (1799-1880). Miranda, thought before Voyager 2's flyby in 1986 to be the innermost moon, was discovered in 1948 by the American astronomer Gerard Peter Kuiper. Ten smaller satellites-Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, and Puck-were revealed by Voyager 2 in 1985-86, along with a much smaller moon provisionally identified as S/1986 U10. The spacecraft also recorded surface details of the large, previously discovered moons, including Oberon's cratered surface, Titania's rifts and fractures, Miranda's grooved markings, and evidence of flow on Ariel. Eleven additional satellites were identified by astronomers from 1997 through 2003.

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