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.
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.
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 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
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
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 have 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.
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
Neptune was the name that ancient Romans gave to the Greek god of the sea and earthquakes,
Poseidon. Often depicted as a bearded men 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 cause 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 Dolphinus. Neptune
and Amphitrite had several children, among whom was Triton whose name was later given to the principal moon of the planet
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.