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Neptune   Discovered by Urbain Le Verrier* John Couch Adams* Johann Galle Discovery date September 23, 1846 Pronunciation i/'n?ptu?n/ Adjective Neptunian Epoch J2000 Aphelion 4,553,946,490 km 30.44125206 AU Perihelion 4,452,940,833 km 29.76607095 AU Semi-major axis 4,503,443,661 km 30.10366151 AU Eccentricity 0.011214269 Orbital period 60,190 days 164.79 years 89,666 Neptune solar days Synodic period 367.49 day Average orbital speed 5.43 km/s Mean anomaly 267.767281° Inclination 1.767975° to Ecliptic 6.43° to Sun’s equator 0.72° to Invariable plane Longitude of ascending node 131.794310° Argument of perihelion 265.646853° Satellites 13 Equatorial radius 24,764 ± 15 km 3.883 Earths Polar radius 24,341 ± 30 km 3.829 Earths Flattening 0.0171 ± 0.0013 Surface area 7.6183×109 km2 14.98 Earths Volume 6.254×1013 km3 57.74 Earths Mass 1.0243×1026 kg 17.147 Earths Mean density 1.638 g/cm3 Equatorial surface gravity 11.15 m/s2 1.14 g Escape velocity 23.5 km/s Sidereal rotation period 0.6713 day 16 h 6 min 36 s Equatorial rotation velocity 2.68 km/s 9,660 km/h Axial tilt 28.32° North pole right ascension 19h 57m 20s 299.3° North pole declination 42.950° Albedo 0.290 (bond) 0.41 (geom.) Surface temp.    1 bar level    0.1 bar (10 kPa) min mean max 72 K 55 K Apparent magnitude 8.02 to 7.78 Angular diameter 2.2–2.4? Scale height 19.7 ± 0.6 km Composition 80±3.2% hydrogen (H2) 19±3.2% helium 1.5±0.5% methane ~0.019% hydrogen deuteride (HD) ~0.00015% ethane Ices: ammonia water ammonium hydrosulfide(NH4SH) methane (?) Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly 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. 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 mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed by Johann Galle within a degree of the position predicted by 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 20th century. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.Neptune is similar in composition to Uranus, and both have compositions which differ from those of the larger gas giants, Jupiter and Saturn. Neptune's atmosphere, while similar to Jupiter's and Saturn's in that it is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, contains a higher proportion of "ices" such as water, ammonia and methane. Astronomers sometimes categorize Uranus and Neptune as "ice giants" in order to emphasize these 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 atmosphere is notable for its active and visible weather patterns. 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 Spot on 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 km/h. 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, which may have been detected during the 1960s but was only indisputably confirmed in 1989 by Voyager 2. Cite error: There are tags on this page, but the references will not show without a {{Reflist}} template or a tag; see the help page.
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