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Neptune is the eight planet from the Sun and one of the two planets that cannot be seen with a telescope (the other being Pluto). It is named after Neptune, the God of Sea in Roman mythology. Neptune’s diameter is about 49,528 kilometers at the equator or nearly 4 times that of Earth.

Neptune is 30 times as far from the Sun as Earth. It has an elliptical (oval shaped) orbit around the Sun and takes approximately 165 Earth Years to complete one full revolution. However, its rotation speed about its axis is faster than that of Earth taking 16 hours and 7 minutes to complete one full rotation.
Astronomical Symbol

Neptune’s astronomical symbol is a stylized representation of god's trident.

We know that Uranus' blue-green color is a result of the presence of methane in its atmosphere, but Neptune has a more vivid, brighter blue, so there must be an unknown component that causes the more intense color that we see. The cause of Neptune's bluish trace remains a mystery.


Neptune was the first planet to be discovered by means of mathematics rather the observations of the night sky. Astronomer Galileo had first observedNeptune on December 27, 1612, and again on January 27, 1613. However on both occasions Galileo mistook Neptune for a fixed star when it appeared very near to Jupiter in the night sky.

In 1843, John C. Adams, an English astronomer and mathematician, began working to find the location of the unknown planet nearing Uranus. Adams predicted the planet would be about 1.6 billion kilometers farther from the sun than Uranus. Adams sent his predictions to Sir George B. Airy, the Astronomer Royal of England. Airy however did not have much confidence in Adams and did not look for the unknown planet with a telescope.

When observed, astronomers found that Uranus didn’t travel exactly as they had expected to. French Mathematician Urbain Joseph Le Verrier proposed that the position and mass of another planet could have caused the observed changes to the orbit of Uranus. Le Verrier’s predictions were ignored by the French astronomers saying that his concepts were baseless. He then sent his predictions to Johann Gottfried Galle, the Director of Urania Observatory in Berlin. Galle and his assistant, Heinrich L. d'Arrest successfully located Neptune on their first night of searching on Sept. 23, 1846. Neptune’s largest moon, Triton was also discovered seventeen days later.

Neptune’s discovery led to a nationalistic rivalry between the French and the British over who should get credit for the discovery. However, both Adams and Le Verrier were credited with the discovery of Neptune.

On August 25, 1989, Voyager 2 became the first spacecraft to visit Neptune. It provided close-up views of Neptune, its rings and six of its moons namely Despina, Galatea, Larissa, Naiad, Proteus, and Thalassa.


The atmosphere of Neptune is believed to consist of hydrogen, helium, water, and silicates. Neptune’s winds are the fastest known in the solar system, reaching up to 2000 km/h.

Voyager 2 spacecraft also located a Great Dark Spot on the surface of Neptune, which was about half the diameter of the Great Red Spot of Jupiter. However, Neptune’s great Dark Spot is believed to have been dissipated or is being covered by the other features of Neptune’s atmosphere.

Neptune receives very little heat due to its distance from the Sun. It has a surface temperature of about -218 °C (55 K). Its core consists of molten rock and metal, surrounded by a mixture of rock, water, ammonia, and methane.


Visibility from Earth

Neptune is never visible with the naked eye. With the use of a telescope it appears as a blue-green disk, similar in appearance to Uranus; the blue-green colour comes from the methane in its atmosphere.

The brightness of Neptune is between magnitudes +7.7 and +8.0, so a telescope or binoculars are required to observe it. It can be also photographed as a faint star with a normal camera and high-sensitivity film.

Neptune will soon return to the approximate position where Galle discovered it, on three different dates. These are April 11, 2009, July 17, 2009 and finally for the last time for the next 165 years on February 7, 2010.

Rings of Neptune

Neptune has a faint ring system which was discovered by Voyager 2 during its visit to the planet in 1989. The ring system comprises of the outermost ring called the Adams, also containing three prominent arcs now named Liberty, Equality and Fraternity.
Name of ringRadius (km)
1989 N3R ('Galle')41,900
1989 N2R ('Leverrier') 53,200
1989 N4R ('Lassell')55,400
Arago Ring57,600
Liberté Ring Arc62,900
égalité Ring Arc62,900
Fraternité Ring Arc62,900
Courage Ring Arc62,900
1989 N1R ('Adams')62,930

Neptune Moon’s

Neptune has 13 known moons. The largest by far is Triton, discovered by William Lassell just 17 days after the discovery of Neptune itself.

The following is a list of the moons of Saturn:
  1. Naiad
  2. Thalassa
  3. Despina
  4. Nereid
  5. Galatea
  6. Larissa
  7. Proteus
  8. Triton
  9. S/2002 N1
  10. S/2002 N2
  11. S/2002 N3
  12. S/2002 N4
  13. Psamathe

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