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Summary Article: Uranus from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Seventh planet from the Sun, discovered by German-born British astronomer William Herschel in 1781. It is twice as far out as the sixth planet, Saturn. Uranus has a mass 14.5 times that of Earth. The spin axis of Uranus is tilted at 98°, so that one pole points towards the Sun, giving extreme seasons.

Mean distance from the Sun 2.9 billion km/1.8 billion mi

Equatorial diameter 50,800 km/31,600 mi

Rotation period 17 hours 12 minutes

Year 84 Earth years

Atmosphere deep atmosphere composed mainly of hydrogen and helium

Surface composed primarily of rock and various ices with only about 15% hydrogen and helium, but may also contain heavier elements, which might account for Uranus's mean density being higher than that of Saturn

Satellites 27 moons were known by the end of 2011

Rings By early 2012, 13 rings were known to circle Uranus. Eleven were detected by the US space probe Voyager 2 in 1977, although they were not identified until 1978 and 1986. Two more were found by the Hubble Space Telescope in 2005. The rings are charcoal black and lie in the plane of the planet's equator. Some may be debris of former ‘moonlets’ that have broken up, while the outermost, which has nearly twice the diameter of the inner ring system, is constantly replenished by the satellite Mab, which orbits within the ring. The brightest ring is Epsilon, the furthest from the planet's centre (51,150 km/31,800 mi) and 100 km/60 mi at its widest point. In 1995, US astronomers determined that the ring particles contained long-chain hydrocarbons. Looking at the brightest region of Epsilon, they were also able to calculate the precession of Uranus as 264 days, the fastest known precession in the Solar System

Uranus has a peculiar magnetic field, in that it is tilted at 60° to the axis of spin, and its centre is displaced about a third of the way from the planet's centre to the surface. Uranus spins from east to west, the opposite of the other planets, with the exception of Venus and possibly the dwarf planet Pluto. The rotation rate of the atmosphere varies with latitude, from about 16 hours in mid-southern latitudes to longer than 17 hours at the equator.

Titania, the largest moon, has a diameter of 1,580 km/980 mi. Voyager 2 flew by Uranus in January 1986, and revealed that Uranus is covered with a cloud layer under which a hot ocean of superheated water exists. The pressure caused by the thick atmosphere keeps the water from boiling away and the heat keeps the pressure from solidifying the water. This has led to suggestions that the planet may have formed from the coalescence of comets. Other discoveries were that the pole facing the Sun is no hotter than the pole facing away, and the equator is hotter than the poles; this is because strong winds transport heat from the Sunward pole around the planet. Winds near the equator flow in the direction opposite to the planet's rotation.

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