URANUS

URANUS

URANUS, major planet in 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 a diameter of 52,200 km (32,500 mi), and its mean distance from the sun is 2.87 billion km (1.78 billion mi). Uranus takes 84.01 earth years for a single revolution, or orbit, and 17 hr 14 min for a complete rotation about its axis, which is inclined 98° to the plane of the planet’s orbit around the sun. 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 14.5 times greater, a volume 67 times greater, and a gravity 1.17 times greater. Uranus’s magnetic field, however, is only a tenth as strong as earth’s, with an axis tilted 55° from the rotational axis. The density of Uranus is about 1.2.

In 1977, the first nine rings of Uranus were discovered. While recording the occultation of a star behind the planet, the American astronomer James L. Elliot (1943–    ) discovered the presence of five rings encircling the equator of Uranus. Named Alpha, Beta, Gamma, Delta, and Epsilon (starting from the innermost ring), they form a 9400-km- (5840-mi-) wide belt extending to 51,300 km (31,860 mi) from the planet’s center. In January 1986 during the exploratory flight of Voyager 2, these rings were photographed and measured, as were two other new rings and ringlets. Another ring is known as Eta, while the remaining 5 of the 11 known rings of Saturn are not yet named.

In addition to its rings, Uranus has 21 satellites; 15 revolve about its equator and move with the planet in an east-west direction. 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. The next ten small satellites—Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, and Puck—were revealed by Voyager 2 in 1985–86. 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. Two new small moons, Caliban and Sycorax, were discovered by astronomers at the Palomar Observatory in 1997; they were found to have irregular (extremely elliptical or highly inclined) orbits. Three other moons, also irregular, were discovered by astronomers at the Mauna Kea Observatory in 1999. The moons not named yet are identified as 1986 U10, 1999 U1, 1999 U2, and 1999 U3.