Uranus is the seventh planetfrom the Sun. Its name is a reference to the Greek god of the sky,Uranus (Caelus), who, according to Greek mythology, was thegreat-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) andfather of Cronus (Saturn). It has the third-largest planetary radiusand fourth-largest planetary mass in the Solar System. Uranus issimilar in composition to Neptune, and both have bulk chemicalcompositions which differ from that of the larger gas giants Jupiterand Saturn. For this reason, scientists often classify Uranus andNeptune as "ice giants" to distinguish them from theother giant planets.
As with gas giants, ice giants alsolack a well defined "solid surface." Uranus'satmosphere is similar to Jupiter's and Saturn's in its primarycomposition of hydrogen and helium, but it contains more "ices"such as water, ammonia, and methane, along with traces of otherhydrocarbons. It has the coldest planetary atmosphere in the SolarSystem, with a minimum temperature of 49 K (−224 °C; −371 °F),and has a complex, layered cloud structure with water thought to makeup the lowest clouds and methane the uppermost layer of clouds. Theinterior of Uranus is mainly composed of ices and rock.
Like the other giant planets, Uranushas a ring system, a magnetosphere, and numerous moons. The Uraniansystem has a unique configuration because its axis of rotation istilted sideways, nearly into the plane of its solar orbit. Its northand south poles, therefore, lie where most other planets have theirequators. In 1986, images from Voyager 2 showed Uranus as an almostfeatureless planet in visible light, without the cloud bands orstorms associated with the other giant planets. Voyager 2 remains theonly spacecraft to visit the planet. Observations from Earth haveshown seasonal change and increased weather activity as Uranusapproached its equinox in 2007. Wind speeds can reach 250 metres persecond (900 km/h; 560 mph).
History
Like the classical planets, Uranus isvisible to the naked eye, but it was never recognized as a planet byancient observers because of its dimness and slow orbit. Sir WilliamHerschel first observed Uranus on 13 March 1781, leading to itsdiscovery as a planet, expanding the known boundaries of the SolarSystem for the first time in history and making Uranus the firstplanet classified as such with the aid of a telescope.
Discovery
Uranus had been observed on manyoccasions before its recognition as a planet, but it was generallymistaken for a star. Possibly the earliest known observation was byHipparchos, who in 128 BC might have recorded it as a star for hisstar catalogue that was later incorporated into Ptolemy's Almagest.The earliest definite sighting was in 1690, when John Flamsteedobserved it at least six times, cataloging it as 34 Tauri. The Frenchastronomer Pierre Charles Le Monnier observed Uranus at least twelvetimes between 1750 and 1769, including on four consecutive nights.
Sir William Herschel observed Uranus on13 March 1781 from the garden of his house at 19 New King Street inBath, Somerset, England (now the Herschel Museum of Astronomy), andinitially reported it (on 26 April 1781) as a comet. With a homemade6.2-inch reflecting telescope, Herschel "engaged in a seriesof observations on the parallax of the fixed stars."
Herschel recorded in his journal: "Inthe quartile near ζ Tauri ... either Nebulous star or perhaps acomet." On 17 March he noted: "I looked for theComet or Nebulous Star and found that it is a Comet, for it haschanged its place." When he presented his discovery to theRoyal Society, he continued to assert that he had found a comet, butalso implicitly compared it to a planet:
