In 1821, Alexis Bouvard published astronomical tables of the orbit of Neptune's neighbour Uranus. In 2009, a study suggested that Galileo was at least aware that the "star" he had observed had moved relative to the fixed stars. Because Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with Galileo's small telescope. This apparent backward motion is created when Earth's orbit takes it past an outer planet. At his first observation in December 1612, Neptune was almost stationary in the sky because it had just turned retrograde that day. Hence, he is not credited with Neptune's discovery. On both occasions, Galileo seems to have mistaken Neptune for a fixed star when it appeared close-in conjunction-to Jupiter in the night sky. Some of the earliest recorded observations ever made through a telescope, Galileo's drawings on 28 December 1612 and 27 January 1613 contain plotted points that match up with what is now known to have been the positions of Neptune on those dates. Neptune has a faint and fragmented ring system (labelled "arcs"), which was discovered in 1984, then later confirmed by Voyager 2. Temperatures at the planet's centre are approximately 5,400 K (5,100 ☌ 9,300 ☏). 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 55 K (−218 ☌ −361 ☏). In addition, 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 (580 m/s 1,300 mph). More recently, in 2018, a newer main dark spot and smaller dark spot were identified and studied. For example, at the time of the Voyager 2 flyby in 1989, the planet's southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptune's atmosphere has active and visible weather patterns. Newest data from the Gemini observatory shows the blue color is more saturated than the one present on Uranus due to thinner haze of Neptune's more active atmosphere. Along with Rayleigh scattering, traces of methane in the outermost regions in part account for the planet's blue appearance. However, similar to Uranus, its interior is primarily composed of ices and rock Uranus and Neptune are normally considered " ice giants" to emphasise this distinction. Like Jupiter and Saturn, Neptune's atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, though it contains a higher proportion of ices such as water, ammonia and methane. The advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989 Voyager 2 remains the only spacecraft to have visited Neptune. The planet's distance from Earth gives it a very small apparent size, making it challenging to study with Earth-based telescopes. Its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 13 known moons were located telescopically until the 20th century. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the position predicted by Le Verrier. After Bouvard's death, the position of Neptune was predicted from his observations, independently, by John Couch Adams and Urbain Le Verrier. Unexpected changes in the orbit of Uranus led Alexis Bouvard to hypothesise that its orbit was subject to gravitational perturbation by an unknown planet. Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. It is named after the Roman god of the sea and has the astronomical symbol, representing Neptune's trident. The planet orbits the Sun once every 164.8 years at an average distance of 30.1 AU (4.5 billion km 2.8 billion mi). Being composed primarily of gases and liquids, it has no well-defined "solid surface". It is referred to as one of the solar system's two ice giant planets (the other one being Uranus). Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. It is 17 times the mass of Earth, and slightly more massive than its near-twin Uranus. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is the eighth planet from the Sun and the farthest known solar planet.
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