Uranus, major planet, seventh in order of distance from the Sun, revolving outside the orbit of Saturn and inside the orbit of Neptune (see Solar System). It is of the sixth magnitude, so that it is just visible to the naked eye. Uranus was accidentally discovered in 1781 by the British astronomer William Herschel and was originally named Georgium Sidus (Star of George) in honour of his royal patron, George III. The planet was later, for a time, called Herschel in honour of its discoverer. The name Uranus, which was first proposed by the German astronomer Johann Elert Bode, was in use by the late 19th century. Uranus has a diameter of 51,120 km (31,771 mi), and its mean distance from the Sun is 2.87 billion km (1.78 billion mi). Uranus takes 84 years for a single revolution, or orbit, and 17 hr 15 min for a complete rotation about its axis. The axis is inclined at an angle of only 8° to the plane of the planet's orbit around the Sun. In other words, the planet's axis is “lying down” in relation to its orbit. The consequence is that each pole faces the Sun for 42 years (half the “year” of Uranus) and then is in darkness for 42 years. Furthermore, the north pole (the one that rotates anticlockwise when viewed from above) is “below” the plane of the orbit, so that the rotation is technically retrograde, or in the opposite sense to that of the Earth and most of the other planets. 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 disc 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 the Earth's, with an axis tilted 55° from the rotational axis. The density of Uranus is approximately 1.2 times that of water.
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