Neptune

Neptune is the eighth planet, and the third most massive. Occasionally, Pluto's highly elliptic orbit passes inside that of Neptune, making Neptune the furthest planet from the sun during these times.

Neptune is God of the Oceans and Poseiden is his Greek counterpart. The planet is no doubt so named due to its blue colour.

Like Saturn, Neptune has rings, but they are dark so that we can't easily see them.

Semimajor axis (km) 4495.06x106 Perihelion (km) 4444.45x106 Aphelion (km) 4545.67x106 Mean orbital speed (km s-1) 5.43 Sidereal orbit period (days) 60,189.0 Tropical orbit period (days) 59,799.9 Synodic period (days) 367.49 Length of day (hours) 16.11 Sidereal rotation period (hours) 16.11 Obliquity to orbit (deg) 28.32

Semimajor axis (AU) 30.06896348 Eccentricity 0.00858587 Inclination (deg) 1.76917 Longitude of ascending node (deg) 131.72169 Longitude of perihelion (deg) 44.97135 Mean longitude (deg) 304.88003 Discoverer Johann Gottfried Galle Date of Discovery 23 Sep 1846 J2000 Data from NSSDC

Only one spacecraft has been to Neptune, Voyager 2 in August 1989, discovering six more moons and two new rings. It also showed Neptune to have strong weather. Notably, the Great Dark Spot was observed, which was a anticyclone the size of the Earth. The Hubble Space Telescope has since shown it to have disappeared, so Neptune's weather alters quickly. Winds can reach 2000 kmph. The atmosphere is largely composed of hydrogen and helium, plus small amounts of methane (which is responsible for the blue hue because it absorbs red light).

Neptune probably has a rocky core, but composition is unknown.

Mass (kg)	1.0243x1026
Equatorial radius [1 bar] (km)	24764
Polar radius [1 bar] (km)	24341
Mean density (kg m-3)	1638
Equatorial gravity [1 bar] (m s-2)	11.15
Escape Velocity (km s-1)	23.5
Black-body temp (K)	46.6
Solar intensity (W m2)	1.51
Natural satellites	13

Of the thirteen moons, seven are named (from closest to furthest); Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, Triton and Nereid. Triton is far and away the most massive at 2.14x10²² kg and is often compared with Pluto, given what is known. Triton is also in a retrograde orbit, the largest moon in the solar system to do so.

Neptune's discovery was prompted by discrepancies between the position predicted by Newton's laws and the real position of Uranus. It was observed in 1846 following the predicted locations of John Adams and Urbain Le Verrier. These two, an Englishman and a Frenchman respectively, worked independently and there was much dispute over credit at the time. They are now jointly accredited.