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Neptune - A distant world
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Neptune orbits the Sun at a distance of almost five thousand 
million kilometres. At such a great distance it takes the 
planet 165 years to travel around the Sun. It has not yet made 
a complete orbit since its discovery in 1846. Although the 
most distant of all the so-called 'gas giants', Neptune shares 
many properties with its warmer relatives, particularly 
Uranus. Both planets are of very similar sizes, and spin at 
similar rates. The interiors of the two planets are thought to 
be similar, a central rocky core surrounded by a liquid mantle 
of hydrogen, helium, methane and ammonia. The outer gaseous 
atmosphere that we observe then surrounds this.

While the discovery of Uranus was made by accident, Neptune's 
location was carefully calculated using Newton's law of 
gravitation. It was noticed that the orbit of Uranus deviated 
slightly from the position predicted by theory. This convinced 
astronomers that another large planet outside the orbit of 
Uranus could be tugging Uranus with its gravity. On the first 
night of the search the planet was found just one degree away 
from the predicted location. This was seen as a tremendous 
triumph for the power of Newtonian science.

Neptune's most obvious feature is its Great Dark Spot, named 
because it resembles Jupiter's Great Red Spot. The size of the 
Earth, this marking is thought to be a region of high pressure 
that lacks the typical atmospheric methane. Although similar in 
many ways, the Great Dark Spot seems to have been formed much 
deeper in the atmosphere than Jupiter's blemish. The Dark Spot 
was thought to be a long-term feature, but observations by 
Voyager 2 and the Hubble space telescope showed the marking 
disappear rapidly and then reappear elsewhere. Apparently 
extremely rapid large-scale changes are common on Neptune.

Neptune has two large and six small moons, most of which are 
extremely dark in colour. Many of the moons have long elongated 
orbits, and one of the larger moons, Triton actually rotates 
backwards around Neptune. The temperature on Triton is a 
chilling -220°C, so cold that even nitrogen is frozen solid. 
Although Neptune has a smaller mean distance from the Sun than 
Pluto does, it has been the most distance object in our solar 
system between January 1979 and March 1999. This is because 
Pluto follows a long elongated path, which periodically loops 
inside Neptune's orbit.

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Images
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1.  The surface of Neptune from space
(http://www.solarsystem.f2s.com/neptune/img1a.jpg)

This picture of Neptune shows one of the great cloud features, 
dubbed the Great Dark Spot by Voyager scientists, can be seen 
toward the center of the image. It circuits Neptune every 18.3 
hours. The bright clouds to the south and east of the Great 
Dark Spot constantly change their appearances in periods as 
short as four hours.

© Calvin J. Hamilton

2.  Clouds moving across the surface of Neptune
(http://www.solarsystem.f2s.com/neptune/img2a.jpg)

This image shows bands of sunlit cirrus-like clouds in 
Neptune's northern hemisphere. These clouds cast shadows on the 
blue cloud deck 50 kilometres below. The white streaky clouds 
are from 48 to 160 kilometres wide and extend for thousands of 
miles. 

© Calvin J. Hamilton

3.  The Great Dark Spot
(http://www.solarsystem.f2s.com/neptune/img3a.jpg)

This close up view of the Great Dark Spot was actually taken at 
from a distance of 590,000 km by Voyager 2’s wide-angle camera. 
It shows the clouds’ structure in the dark regions near the pole 
and the bright clouds east of the Great Dark Spot. Small trails 
of similar clouds trending east to west and large scale structure 
east of the Great Dark Spot all suggest that waves are present 
in the atmosphere and play a large role in the type of clouds 
that are visible. 

© Calvin J. Hamilton

4.  Neptune and Triton
(http://www.solarsystem.f2s.com/neptune/img4a.jpg)

This image was taken by Voyager 2 on 31st August 1989, as it 
departed Neptune and headed out into deep space. It shows a 
beautiful dual-crescent view of Neptune and Triton, one of 
Neptune’s principal satellites. 

© Calvin J. Hamilton