On Tuesday, July 12th, Neptune will have completed one full circuit around the Sun since its discovery around midnight on the night of September 23-24, 1846. The great icy world was first pinpointed 164.79 years ago. And as Neptune takes 164.79 Earthly years to circle the sun, it is only now completing its first full orbit since its detection by humans. And there is much to commemorate – for Neptune's discovery marked a turning point in astronomy. Its existence was revealed, not through a serendipitous observation by an astronomer but by the careful work of mathematicians. They calculated that perturbations in the orbit of Uranus, then thought to be the sun's most distant planet, could only be explained by the existence of another, even remoter world whose gravity was affecting Uranus's path.
John Adams and Urbain Le Verrier carried out independent analysis of discrepancies in the observed and calculated positions of Uranus and predicted in 1845 the mass and orbit of the perturbing body. The planet was discovered on September 23, 1846 by Johann Galle and Heinrich d’Arrest from Berlin Observatory very near the predicted position. An international dispute arose between the English and French over priority and the right to name the new planet; they are now jointly credited with Neptune’s discovery.
As to the nature of the planet, this was only revealed in full in 1989 when the US probe Voyager 2 swept past it and sent back images of a seemingly serene blue world – though later analysis revealed dark spots on its surface that are vast cyclonic storms.
Neptune’s composition is probably similar to Uranus: various “ices” and rock with about 15%hydrogen and little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. But there is most likely a small core of rocky material, about the mass of the Earth. Its atmospere is mostly hydrogen and helium with a small amount of methane.
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 chromosphere which gives the clouds their rich blue tint. Like 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.
Neptune also has rings. Earth based observations showed only faint arcs of complete rings, but Voyager 2’s image showed them to be complete rings with bright clumps. 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 named as Liberty, Equality and Fraternity. Next is an unnamed ring then Leverrier and finally the faint but broad Galle.
This is an awkward time to observe Neptune; it doesn't reach a reasonable height above the horizon until the wee hours of the morning. Moreover, it won't appear anywhere near its discovery position with respect to the stars, due to the fact that Earth is in quite a different part of its orbit. But if you want to view Neptune anyway, it's easy to do with decent telescopes and binoculars.
Neptune is well-placed in the early evening sky starting in September, and it will appear extremely close to its discovery position among the stars from mid-October through December.
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