Neptune is the eighth and farthest planet from the Sun in the Solar System. It is the fourth-largest planet by diameter and the third-largest by mass. Neptune is 17 times the mass of Earth and is somewhat more massive than its near-twin Uranus, which is 15 times the mass of Earth but not as dense. On average, Neptune orbits the Sun at a distance of 30.1 AU, approximately 30 times the Earth–Sun distance. Named for the Roman god of the sea, its astronomical symbol is ♆, a stylised version of the god Neptune's trident.
Neptune was the first planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus ledAlexis Bouvard to deduce that its orbit was subject to gravitational
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For example, at the time of the 1989 Voyager 2 flyby, the planet's southern hemisphere possessed a Great Dark Spot comparable to the Great Red Spoton Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 kilometres per hour (1,300 mph). Because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching −218 °C (55 K). Temperatures at the planet's centre are approximately 5,400 K(5,000 °C). Neptune has a faint and fragmented ring system (labeled 'arcs'), which may have been detected during the 1960s but was only indisputably confirmed in 1989 by Voyager 2.
Main article: Discovery of Neptune
Galileo's drawings show that he first observed Neptune on 28 December 1612, and again on 27 January 1613. On both occasions, Galileo mistook Neptune for a fixed star when it appeared very close—in conjunction—to Jupiter in the night sky; hence, he is not credited with Neptune's discovery. During the period of his first observation in December 1612, Neptune was stationary in the sky because it had just turned retrograde that very day. This apparent backward motion is created when the orbit of the Earth takes it past an outer planet. Since Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to