Water, Ammonium hydrosulfide(NH4SH),MethaneNeptune is the eighth and farthest planet from the Sun in our Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter andthe third-largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twinUranus, which is 15 Earth masses and not as dense. On average, Neptune orbits the Sun at a distance of 30. 1 AU, approximately 30 times the Earth-Sundistance. Its astronomical symbol is ♆, a stylized version of the god Neptune's trident. Discovered on September 23, 1846, Neptune was the first planet found by mathematicalprediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbitwas subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed by Johann Galle within a degree of the position predictedby Urbain Le Verrier, and its largest moon, Triton, was discovered shortly thereafter,though none of the planet's remaining 12 moons were located telescopically until the 20thcentury. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August25, 1989. Neptune is similar in composition to Uranus, and both have compositions which differ fromthose of the larger gas giants Jupiter and Saturn. Neptune's atmosphere, while similar to Jupiter's and Saturn's in that it is composed primarilyof hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, containsa higher proportion of "ices" such as water, ammonia and methane. Astronomers sometimes categorize Uranus and Neptune as "ice giants" in order to emphasizethese distinctions. The interior of Neptune, like that of Uranus, is primarily composed of ices and rock. Traces of methane in the outermost regions in part account for the planet's blue appearance. In contrast to the relatively featureless atmosphere of Uranus, Neptune's atmosphereis notable for its active and visible weather patterns. At the time of the 1989 Voyager 2 flyby, for example, the planet's southern hemispherepossessed a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the SolarSystem, with recorded wind speeds as high as 2,100 km/h. Because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldestplaces in the Solar System, with temperatures at its cloud tops approaching −218 °C(55 K). Temperatures at the planet's centre, however, are approximately 5,400 Kelvin (5,000 °C). Neptune has a faint and fragmented ring system, which may have been detected during the 1960sbut was only indisputably confirmed in 1989 by Voyager 2. -~-DiscoveryGalileo's drawings show that he first observed Neptune on December 28, 1612, and again onJanuary 27, 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 inthe 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 outerplanet. -~-Neptune is 17 times the mass of Earth and is slightly more massive than its near-twinUranus, which is 15 Earth masses and not as dense. On average, Neptune orbits the Sun at a distance of 30. 1 AU, approximately 30 times the Earth-Sundistance. Its astronomical symbol is ♆, a stylized version of the god Neptune's trident. Discovered on September 23, 1846, Neptune was the first planet found by mathematicalprediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbitwas subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed by Johann Galle within a degree of the position predictedby Urbain Le Verrier, and its largest moon, Triton, was discovered shortly thereafter,though none of the planet's remaining 12 moons were located telescopically until the 20thcentury. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August25, 1989. Neptune is similar in composition to Uranus, and both have compositions which differ fromthose of the larger gas giants Jupiter and Saturn. Neptune's atmosphere, while similar to Jupiter's and Saturn's in that it is composed primarilyof hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, containsa higher proportion of "ices" such as water, ammonia and methane. Astronomers sometimes categorize Uranus and Neptune as "ice giants" in order to emphasizethese distinctions. The interior of Neptune, like that of Uranus, is primarily composed of ices and rock. Traces of methane in the outermost regions in part account for the planet's blue appearance. In contrast to the relatively featureless atmosphere of Uranus, Neptune's atmosphereis notable for its active and visible weather patterns. At the time of the 1989 Voyager 2 flyby, for example, the planet's southern hemispherepossessed a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the SolarSystem, with recorded wind speeds as high as 2,100 km/h. Because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldestplaces in the Solar System, with temperatures at its cloud tops approaching −218 °C(55 K). Temperatures at the planet's centre, however, are approximately 5,400 Kelvin (5,000 °C). Neptune has a faint and fragmented ring system, which may have been detected during the 1960sbut was only indisputably confirmed in 1989 by Voyager 2. -~-DiscoveryGalileo's drawings show that he first observed Neptune on December 28, 1612, and again onJanuary 27, 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 inthe 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 outerplanet. -~-Since Neptune was only beginning its yearly retrograde cycle, the motion of the planetwas far too slight to be detected with Galileo's small telescope.
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