Neptune Neptune is the fourth largest planet in the Solar System and, for most of the time, is the eighth planet from the Sun.
The ninth planet, Pluto, has an orbit so eccentric that it sometimes crosses that of Neptune, making Neptune the most distant planet from the Sun for a period of 20 years out of each 248 years. This last happened from January 1979 until February 1999 and will next happen in September 2226.

Neptune is never visible to the naked eye. The brightness of Neptune varies between +7.7 and +8.0 magnitude, so a telescope or binoculars are required to observe it. Through a telescope, it appears as a small blue-green disk.

Astronomy of Neptune

Neptune has an equatorial diameter of 49,500 kilometres and could contain nearly 60 Earths. It is about 30 times further from the Sun than Earth is. At its furthest, at aphelion, Neptune is 4,546,000,000 kilometres from the Sun. At perihelion, closest to the Sun, Neptune`s distance is 4,456,000,000 kilometres. Neptune’s rotational axis is tilted 30 degrees to the plane of its orbit around the Sun (a few degrees more than the Earth), bringing seasons to the planet. Each season lasts 40 years; the poles are in constant darkness or sunlight for 40 years at a time. Temperatures of –210ºC have been measured at Neptune`s cloud-top, whilst 482ºC was measured within the stratosphere.

Neptune’s gravitational pull slightly changes Uranus’ orbit, which is how it was discovered in 1846. Neptune`s day, the time it takes to revolve around its axis is equal to 16.67 Earth hours. Its year, the time it takes to orbit the Sun is equal to 165 Earth years. Since its discovery, Neptune has not yet completed a single revolution around the Sun.

Composition of Neptune

Neptune is the outermost of the gas giants. The innermost two-thirds of Neptune is composed of a mixture of molten rock, water, liquid ammonia and methane. The outer third is a mixture of heated gases comprised of hydrogen, helium, water and methane. It is largely the methane, absorbing red light, that gives Neptune its blue cloud colour. Methane preferentially absorbs the longer wavelengths of sunlight (those near the red end of the spectrum), leaving the colours at the blue end of the spectrum to be reflected.

Neptune is a dynamic planet with several large, dark spots reminiscent of Jupiter’s storms. The largest observed, known as the Great Dark Spot, was about the size of the Earth. Since its observation by Voyager II, it has either dissipated or is being masked by other atmospheric aspects. Neptune has the strongest winds measured on any planet with most blowing westward, opposite to the rotation of the planet. Near the Great Dark Spot, winds were measured at up to 2,000 kilometres an hour.

Neptune’s Rings

Neptune has a set of four dark, narrow and faint rings made up of dust particles thought to have been caused by small meteorites smashing into Neptune’s moons. Ground based telescopes show the rings appearing as arcs, but from Voyager II the arcs were discovered to be bright spots within the ring system. The rings are at distances of 41,900 to 62,930 kilometres from the planet, whilst its moons are from 48,227 to 46,738,000 kilometres distance.

Neptune’s Moons

Neptune’s largest moon, Triton, was discovered at the same time as the planet itself and another satellite, Nereid, was found in 1949. A further six were spotted by Voyager II during its flyby in 1989, another four in 2002 and an additional one in 2003. Research undertaken on Triton shows evidence that life may have existed there at one time.


Triton orbits Neptune in a tilted, circular, retrograde orbit (opposite to the direction of the planet’s rotation), completing an orbit in 5.875 days at an average distance of 330,000 kilometres above Neptune’s cloud tops. Reflecting 60-95% of the sunlight that strikes it, Triton is a very bright moon (compared to 11% with Earth`s Moon). It has a thin atmosphere that extends up to 800 kilometres above the surface.

Triton is one of only three objects in the Solar System known to have a nitrogen-dominated atmosphere (the others are Earth and Saturn’s giant moon, Titan). It has the coldest surface known anywhere in the Solar System (about –235ºC) and is so cold that most of Triton’s nitrogen is condensed as frost, making it the only satellite in the Solar System known to have a surface made mainly of nitrogen ice. The pinkish deposits constitute a vast south polar cap believed to contain methane ice, which has reacted under sunlight to form pink or red compounds. It is thought there may be water beneath the ice, with the possibility of life within that water. Dark streaks overlying these pink ices are believed to be an icy and perhaps carbonaceous dust deposited from huge geyser-like plumes up to 8 kilometres high, some of which were found to be active during the Voyager II flyby. The bluish-green band visible in this image extends all the way around Triton near the equator; it may consist of relatively fresh nitrogen frost deposits. The greenish areas include the cataloupe terrain, origin unknown, and a set of ‘cryovolcanic’ landscapes apparently produced by icy-cold liquids (now frozen) erupted from Triton’s interior.

Neptune Missions

Launched in 1977 NASA’s Voyager II flew past Neptune in August 1989. The spacecraft passed about 4,950 kilometres above Neptune’s north pole, and made its closest approach to any planet since leaving Earth 12 years earlier. Five hours later, it passed Triton at about 40,000 kilometres. 9,000 images of Neptune and its moons and rings were sent back. Before this visit, virtually nothing was known about Neptune. In 2003 there was a proposal to NASA`s Vision Mission Studies to implement an orbiter with probes.


Neptune was the name that ancient Romans gave to the Greek god of the sea and earthquakes, Poseidon. Often depicted as a bearded man holding a trident and seated in a seashell drawn by sea-horses, he was the brother of Jupiter (Zeus) and of Pluto (Hades). After the defeat of their father Saturn (Cronos), the three brothers divided the world into three parts, each ruled by one brother. Jupiter took the sky, Neptune the sea and Pluto the underworld. Neptune had the reputation for having a violent temper, with tempests and earthquakes a reflection of his furious rage.

Neptune fell in love with the water nymph Amphitrite when he saw her dancing on the island of Naxos. He asked her to marry him but was refused. Not discouraged, Neptune sent a dolphin to look for her. The dolphin pleaded Neptune’s case so persuasively that she changed her mind. As a reward for finding and returning Amphitrite to him, Neptune immortalized the dolphin by placing it in the heavens as the constellation Delphinus. Neptune and Amphitrite had several children, among whom was Triton whose name was later given to the principal moon of the planet Neptune.


In terms of astrology, Neptune is the planet of dreams and imagination. Expressed in its positive form, Neptune endows a person with creative genius, artistic vision, spiritual wisdom, romanticism and intuition. These influences manifest themselves through creation of beautiful art and music, architectural masterpieces, great literature and the rise of spiritual leaders. In its negative form, Neptune’s influence results in excessive escapism, deception, an inability to set goals and a naive nature which may lead them to associate with destructive people. The effect of this can lead to abuse of drugs and alcohol and sometimes imprisonment.

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