The Astronomical Planet
Nearly 4.5 billion kilometers from the Sun, Neptune orbits the Sun once every 165 years, and therefore it has not quite made a full circle around the Sun since it was discovered.
It is invisible to the naked eye because of its extreme distance from Earth. Interestingly, due to Pluto's unusual elliptical orbit, Neptune is actually the farthest planet from the Sun for a 20-year period out of every 248 Earth years.
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Planetary Statistics of Neptune:
Neptune: Eighth Planet from the Sun
Diameter: 30,200 Miles
Distance from the Sun: 2,794.4 Million Miles
Planetary Year: 164.81 Earth years to make one complete revolution around the Sun.
Planetary Day: 19.1 Earth hours to make one complete revolution on its axis.
Temperature: Mean; 48 K (-225 C, -373 F)
Moons: 13 known, 8 named; (Despina, Galatea, Larissa, Naiad, Nereid, Proteus, Thalassa, Triton).
Chronology of Neptune
Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky. When Uranus didn't travel exactly as astronomers expected it to, two mathematicians, working independently of each other, proposed the position and mass of another, as yet unknown planet. The presence of an unknown planet could account for Uranus' orbit. "The establishment" ignored the predictions. However, a young astronomer decided to look for the predicted planet. As a result, Neptune was discovered in 1846. Seventeen days later, its largest moon, Triton, was also discovered.
In 1989, Voyager 2 tracked a large oval dark storm in Neptune's southern hemisphere. This hurricane-like "Great Dark Spot" was large enough to contain the entire Earth; spun counterclockwise; and moved westward at almost 1,200 km per hour. Recent images from the Hubble Space Telescope show no sign of the "Great Dark Spot," although a comparable spot appeared in 1997 in Neptune's northern hemisphere.
Environment of Neptune
Neptune has the smallest diameter of our solar system's giant gas planets. This includes Jupiter, Saturn, and Uranus. They are called gas planets because they have no solid surfaces. Even so, Neptune's volume could hold nearly 60 Earths.
Neptune's atmosphere extends to great depths, gradually merging into water and other "melted ices" over a heavier Earth-sized liquid core. Its is made up of hydrogen, helium, and methane, the last of these giving the planet its blue color (because methane absorbs red light).
Neptune's rotational axis is tilted 30 degrees to the plane of its orbit around the Sun. Its seasons last an incredible 41 years. During the southern summer, the south pole is in constant sunlight for about 41 years, and in northern summer, the north pole is in constant sunlight for about 41 years.
Despite its great distance from the Sun and lower energy input, Neptune's winds are three times stronger than Jupiter's and nine times stronger than Earth's.
The planet has several rings of varying widths, confirmed by Voyager 2's observations in 1989. The outermost ring, Adams, contains five distinct arcs (incomplete rings) named Liberté, Equalité 1, Equalité 2, Fraternité, and Courage. Next is an unnamed ring co-orbital with the moon Galatea, then Le Verrier, Lassell, Arago, and Galle. Neptune's rings are believed to be relatively young and relatively short-lived.
Moons of Neptune
Neptune has 13 known moons, six of which were discovered by Voyager 2. The largest, Triton, orbits Neptune in a direction opposite to the planet's rotation direction, It is gradually getting closer until it will collide with the planet in about 10 to 100 million years. The collision will form vast rings around Neptune that will rival or exceed Saturn's extensive ring system.
Triton is the coldest body yet visited in our solar system; temperatures on its surface are about -235 °C. Despite the deep freeze, Voyager 2 discovered great geysers of gaseous nitrogen on Triton.
Information Courtesy of NASA Jet Propulsion Laboratory at Caltech
Check out these books for more information on Neptune: