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NEPTUNE
Neptune is the fourth and last of the "gas giant" planets in proximity to the Sun. Overall, Neptune is the eight planet in our solar system, at an average distance of 2.8 billion miles from the Sun. For relatively "brief" periods of time Neptune crosses Pluto's highly elliptical orbit, making Pluto the 8th planet from the Sun and Neptune the furthest. Like the other gas giants, its rotation is fast, at about 19 hours. Neptune takes 165 years to make one orbit around the Sun and has eight known satellites.
Atmosphere and Weather: Like Uranus, Neptune has a dynamic atmosphere with a shroud of cold clouds. The average cloud temperature ranges from about -240 degrees to -330 degrees F. Voyager II found the clouds to be ever changing and rapidly circling the planet. Scientists were amazed at how much changeable weather Voyager II observed on Neptune. A couple of interesting features found from Voyager II's fly-by were very bright, white cirrus clouds circling the globe rapidly. One cirrus cloud, nicknamed "scooter" was observed to move across the planet every 16 hours! These very high altitude clouds are made of methane ice crystals. The other interesting feature was dubbed "the great dark spot."
The great dark spot was very similar to Jupiter's "great Red Spot", in that it was an oval shaped, counter-clockwise moving storm about the size of Earth, moving west at 700 mph. You may notice the past tense word usage in reference to the great dark spot. After the Voyager II mission, the Hubble Telescope found that the great dark spot had disappeared, but a new oval shaped dark spot had formed in a different location. This proves that Neptune does indeed have an ever dynamic atmosphere, with changes in temperature and extremely fast wind speeds. In fact, the highest winds observed in the solar system have been measured on Neptune with speeds near 1,200 mph near the "older" great dark spot. The close up view photograph (above right) shows Neptune's "great dark spot" (shown slightly left of center) with "scooter" the bright cloud patch, just underneath.
Neptune's thick atmosphere is mostly hydrogen, with smaller amounts of helium and methane. It is the absorption of red light by methane which gives Neptune its very blue coloration. The average temperature on Neptune is a brutally cold -373 degrees F. Like Jupiter and Saturn, Neptune radiates more heat than it receives. Therefore, it has an immense internal heat source.
Neptune is smaller in diameter but larger in mass than Uranus.
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In Roman mythology Neptune (Greek: Poseidon) was the god of the Sea.
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After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws. It was therefore predicted that another more distant planet must be perturbing Uranus' orbit. Neptune was first observed by Galle and d'Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus. An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune's discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.
More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune's motion would have been obvious to him. But, alas, cloudy skies prevented obsevations on those few critical days.
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Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Much of we know about Neptune comes from this single encounter. But fortunately, recent ground-based and HST observations have added a great deal, too.
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Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour.
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Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.
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Neptune also has rings. Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2's images showed them to be complete rings with bright clumps. One of the rings appears to have a curious twisted structure.
Like Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown.
Neptune's rings have been given names: the outermost is Adams (which contains three prominent arcs now named Liberty, Equality and Fraternity), next is an unnamed ring co-orbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle.
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Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers.
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Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk. There are several Web sites that show the current position of Neptune (and the other planets) in the sky, but much more detailed charts will be required to actually find it.
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Planet
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Diameter: 49528 km
Minimum Distance from Sun: 4456 million km
Maximum Distance from Sun: 4537 million km
Name in Roman/Greek Mythology: Neptune/Poisidon
Rotation Period around Axis: 16.123 hours
Revolution Period: 248.54 years
Tilt of Axis: 57.5°
Average Surface Temperature: -220° C
Number of Satellites: 1
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