Uranus (astronomy)

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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 2005

Rings
11 rings, composed of rock and dust, around the planet's equator, were detected by the US space probe Voyager 2 in 1977. The rings are charcoal black and may be debris of former ‘moonlets’ that have broken up. The ring furthest from the planet centre (51,000 km/31,800 mi), Epsilon, is 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 is displaced about a third of the way from the planet's centre to its 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.

The triangle represents equality and the rainbow signifies peace. The arms may be replaced by the motto Dios, Union, Libertad, ‘God, Union, Liberty’. Effective date: September 1972. >>