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Distance from sun 4,497.07 million km, 30.0611 a.u.
Diameter 49,528 km, 3.81 times earth's diameter
Mass 17.135 times earth's mass
Density 1.64
Gravity 1.122 times earth's gravity, escape velocity 23.3 km/s
Rotation 19.2 hours
Orbit Eccentricity 0.0097
Orbit Inclination 1.774 degrees
Axis Tilt 29.6 degrees
Sidereal Period 164.79 Earth years
Synodic Period 367.49 Earth days

History: Neptune was discovered in 1846 after astronomers were searching for a reason behind irregularities in the motion of Uranus. Its discovery nearly doubled the size of the solar system. Because it was found in the depths of space, it was named after the Roman god of the deep sea. Triton, Neptune's largest satellite, was also found the same year, and another satellite, Nereid, was discovered in 1949.

Description: Neptune is the outer most of the gas giants and like the others, has an atmosphere consisting of mostly hydrogen and helium. Though it is colder than Uranus, its cloud belts are more colorful. Neptune has a very faint set of rings around it, but they are not visible from earth. Neptune's orbit is nearly completely circular, only Venus has a more circular orbit. Neptune is nominally the eighth planet, but because of the high eccentricity of Pluto's orbit, for about 20 years out of each Plutonian orbit, Neptune is actually the furthest away from the sun.

Observation: Neptune is very faint, reaching only eighth magnitude, and has an angular size of about 2.5 minutes. Because of the large size of its orbit, it moves very slowly through the sky, and therefore can be seen in the same constellation for over a decade. It cannot be seen with the naked eye, but may be seen in binoculars. In a small telescope it appears as a small, greenish disk, and Triton is visible under good conditions with a eight inch telescope.

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