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During the 19th century observations of the positions of the planet Uranus were seen to be in disagreement with the predicted ephemeris. Two mathematicians, a Frenchman Urbain Leverrier and an Englishman John Couch Adams, analysed these small departures from the predicted positions assuming that they were due to the gravitational pull of another, unknown planet. Adams and Leverrier worked independently and both predicted the presence of a new planet in essentially the same place in the sky.
Leverrier had the good fortune to communicate his predictions to Galle at Berlin who searched and discovered Neptune in 1846. Adams had attempted to interest the Astronomer Royal, Airy, in his calculations but, due to a clash of personalities, Airy did not consider Adams's work important. He suggested that Adams should ask Challis, at Cambridge, to undertake a search. Challis used the Northumberland telescope, which is still in Cambridge, to search for the new planet. In fact Challis did observe Neptune but, as he was engaged in a systematic search of a large area of sky and looking for changes in the position of one of the objects he had charted, he missed the fact that one of the brightest objects in the search field showed a small disc and was indeed Neptune.
Initially Leverrier was credited with the prediction and it was only some years later that Adams was given joint credit for the first predicted discovery of a new planet in the solar system.
Neptune's atmosphere was seen to have a banded structure similar to that found on the other gas giant planets. A giant hurricane (the Great Dark Spot - see right)), with a diameter equal to the Earth's, was seen together with other cloud formations including some extensive, and very beautiful cirrus clouds high (50 km) above the main clouds. Neptune was shown to have a very active cloud system, possibly even more active than that of Jupiter with wind speeds of up to 2000 km/h.
In 1994, observations made by Hubble Space Telescope showed that the Great Dark Spot had disappeared and that a new spot had appeared in the northern hemisphere.
Observations from the Earth had shown that Neptune possessed some ring arcs; the Voyager pictures showed that there are at least 4 rings and many dust particles were detected in the plane of the rings. The supposed arcs were found to be brighter clumps in each complete ring.
Neptune has a magnetic field which is not aligned with the rotation axis. It is believed that this field is generated in a spherical shell near the surface of the planet. Aurorae were seen associated with this magnetic field.
The most surprising discovery was of volcanoes. These are quite different from the volcanoes on the Earth. Instead of red-hot magma being ejected it is nitrogen gas, evaporated from its liquid state that is being vented and is carrying with it darker carbon compounds from below the satellite's surface.
The picture above shows a variety of these features, although in the reproduction they may be hard to see. The road-like fissures can easily be seen. Two volcanoes show up as dark plumes of carbonaceous material.
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