The main god associated with Neptune is Triton, who is his son and the given name of the odd large Neptunian satellite. Triton is the god who is the messenger of the deep, who commonly portrayed as half man and half-fish, being akin to the modern day mythological merman. From his undersea golden palace, Triton is said to hold a shell-horn or conch shell, whose generated sound calls to create the waves on the shore or a tumulturous sea.

All the moons of Neptune have names adopted from the many water and sea nymphs, who are imagined as either spirits or higher beings than Man, but never as true gods. The word nymph literally translates to mean “the young maiden”, who are generally very shy and aloof from Man, and hide in all the places within the Earth’s natural environment. These can be in either glades, waterfalls, hills, forests or rivers and streams, etc. In the middle-ages they are equivalent to the fairies, and similarly are placed within these many diverse groups. Of the water and sea nymphs, the three main groups associated with Neptune and water are the Naiads, the Nereid and the Oceanides.

(a.) Naiads were said to be associated with rivers, waterfalls and lakes. Although not really directly associated with the sea, they are associated with places of fresh water. They are said to reside near the source of the water supply, and can die, but only when the water source becomes dry. Unlike the gods, they must never live the places they look after. The classical Naiad is Arethusa the spring nymph that causes water to rise from within the Earth. Another is Styx, whose river is the most sacred and holy. Most of the Naiads are view as of lesser importance, whose names are not common in the story of ancient mythologies tolds today. Other Naiads include Minthe and Syrinx.
(b.) Other sea nymphs were the Nereid, who were deemed as sea goddesses and whose main responsiblity was to protected and saved ships from being smashed by rocks during storms in the Aegean Sea. Their father who was born from the sea was Nereus - the righteous and wise, and the proverbial basis of the “old man of the sea - Their daughters totalled fifty in number, whose mother was the Oceanides nymph, Doris, who had long wild-like blond hair. Highest of all the nymphs was beautiful Calypso, the daughter of the Titan, Atlas, who in Homer’ ‘The Odyssey’ held Odysseus prisoner for seven years. These nymphs were also sometimes deemed troublesome, like Siren and some of her accompanying sisters, who were called collectively known as the Sirens. The Sirens are popularly portrayed in the grand Odysseus legend, as the ones to mesmerise of seamen by their sweet seductive voices hauntingly calling them to be destroy on the nearby rocks and shoals. (Odysseus saved himself by being tied to the mast of his ship.) Another famous Nereid is Thetis, the mother of the Greek hero, Achilles. Others include; Galatea, and the latest moon of Neptune, Psamathe. Most of the Nereid eventually became the legends associated with mermaids of the sea.
(c.) The third is the adopted ones of Neptune’s, the Oceanides nymphs. These were the forty daughters of the Titan god, Oceanus. Oldest of the Oceanides was Rhodes, though some later writers also accredit her as the daughter of Neptune (Poseiden) and Amphirite. Examples of other Oceanus nymphs were Calypso, Doris, Methis, Styx and the most celebrated of all, Tyche, the goddess of good fortune.

In Edmund Clarence Stedmean (1833-1908) poem “Andromeda and the Sea-Nymphs”, aptly describes the nymphs as;

Onward they came in their joy, and around them the lamps of the sea nymphs,
Myriad fiery globes, swam panting and heaving; and rainbows,
Crimson and azure and emerald, were broken in star-showers, lighting
Far through the wine-dark depths of the crystal, the gardens of Nereus,
Coral and sea-fan and tangle, the blooms and the palms of the ocean.

[Again see the much fuller discussion of the derivation of these mythologies in Saturn Part 2.]


For many years, only two moons were known to encircle Neptune. After Voyager 2 this number became 7, and the decade or so after its brief encounter with the planet, the Neptunian moon system totalled eight (2001). Detailed Earth-based observations during 2002 and 2003 have brought the total to thirteen (April 2006).

The Original Known Moons

Triton is the largest moon, which was discovered on 10th October 1846 by English amateur astronomer William Lassell, merely 17 days after Neptune. Overall it is one-third the Moon’s mass and 25% about smaller. Oddly the circular orbit moves in the opposite direction from all the others in just 5 days 21 hours.

by Voyager 2

Courtesy NASA / ESA / STScI

Voyager 2 discovered much of what we know about Triton. This certainly is an interesting world, whose surface looks like a rockmelon or cantaloupe. One of the first observations is the very cold temperatures, which hover c. -230o to -240oC or c.30o above absolute zero. You would expect such a body in the depths of the solar system would be inert and quite uninteresting, but amazingly this moon that has many dark spewing volcanoes, that eject a combination of dust particles, and ices of nitrogen and methane expelled from the interior. These plumes fire material 8 to 10 kilometres above Triton’s surface, only to fall back as dark snow-like particles, as they slowly drift in the thin zephyr breezes in the upper nitrogen-rich upper atmosphere. A source for the energy to make Triton so active in not understood, but the culprit could be the tidal gravitational forces tugging between Triton and Neptune or internal heat sources. Some other surface spots are dark where it seems ice and rock are mixed and flow out onto the surface. Others areas near the moon’s equator show greenish tinges whose origin and composition is unknown.

From Neptune cloud-tops, Triton appears almost the same diameter as the Earth’ Moon and varies little in apparent diameter. The near circular orbit is retrograde and because of the high inclination of 156.8oto the ecliptic, and always shows phases from Neptune and can never reaches ‘Full Triton’. With an orbital period of 5.88 days (5 days 19 hours), the changes in phases would be quite noticeable each hour or so. Much discussion continues about the origin of Triton, whose highly inclined orbit suggests it was independently captured by Neptune sometime in the past. It could also explain the mess of the outer orbits of Neptune. Almost contradicting to this, however, is the near circular orbit and it lock-in synchronous rotation. Since the new outer moons have been discovered there could addition theories supporting or deposing this idea.

Triton can be seen in 25cm (10-inch) telescope when at maximum distance from the planet,and is roughly about 13.5 magnitude at opposition.

Nereid is a small satellite that orbits Neptune in almost exactly one year. Discovered by Gerard Kuiper in 1949, this odd moon has the highest orbital eccentric (0.751) of any of the other planets or moons within the Solar System. At 19.2 magnitude the moon is very faint and beyond the reach of amateur telescopes. Even from Neptune, at opposition-full moon, Nereid appears only as a variable 1st magnitude star, whose mean diameter is just less than 1 arcmin. Unlike Triton, Nereid follows the same plane as the inner satellites. There is little known about this moon, being roughly estimated as 340 km sized moon, which was found as an oddly flattened or cigar-shaped by Voyager 2 when viewed at some distance.

Inner Satellites

All the six inner satellites (II to VII) were discovered by Voyager 2, being close to the planet and associated with the rings. Voyager found these were found 1989, and were originally labelled as S/1989 N1 through N6. Largest is Proteus, then followed in decreasing order of size as we approach the planetary surface. Four satellites; Naiad (III), Thalassa (IV), Despina (V) and Galatea (VI), are associated with the rings, and like them, all these moon all have near circular orbits. Despina probably has the most significant influence of most of Neptune’s rings. However, Larissa (VII) lies beyond these rings, and arguably could be considered the boundary to the middle or traditional satellites of Triton and Neried.

Most of these satellites remain invisible even in the largest of telescopes mainly becuse of the proximity to the planet versus, the considerable distance from the sun, and the weakness of solar light. Magnitudes range from 20th for Larissa (VII) to 24.1 for the inner-most moon Naiad (III).

Outer Satellites

Detailed Earth-based observations during 2002 and 2003 have discovered five more moon that extend well past the orbit of Nereid. These were found using deep images of the surrounding area around Neptune. Most of these are about 40km to 60km in size. The last named moon was the tenth moon, Psamathe was previously designated S/2003 N1. The other are yet to be again optically verified and named by the IAU.

Inner most is S/2002 N1, whose orbit is which highly elliptical and is high inclination (134.1o), thus making the orbit as retrograde. The next two are S/2002 N2 & N3, which are very similar many respects, including the distance from Neptune. Both are moderately inclined to the ecliptic at 48.5o and 34.7o, respectively, differing from all the other moons.

Perhaps the most extraordinary are the more recently discovered Psamathe (X) and S/2002 N4. Both lie almost 100 times further than the Moon does from Earth, whose periods average 25.0 and 25.7 years, respectively. Both have retrograde orbits, and share very similar inclinations. This also is slightly different from Triton, but shares almost the same inclination as S/2002 N1.

Most of these five newly discovered satellites are amazingly faint, lying between 25th and 26th magnitude. None are telescopically visible from Earth, and can only be found using deep-sky imaging.

There is much debate on the nature of the Neptunian satellite system, because the outer orbits are so different in inclination. Seemingly, they defy what would be expected, making it difficult to understand how these bodies were formed.


                  Distance Period   Mag.  Size                      
No.  Satellite     (km)    (days)   (v.)  (km)  Discover. Date
III  Naiad          48 227  0.294   24.1    58  Voyager 2 1989
IV   Thalassa       50 075  0.311   23.4    80  Voyager 2 1989
V    Despina        52 526  0.335   22.0   148  Voyager 2 1989
VI   Galatea        61 593  0.429   22.0   158  Voyager 2 1989
VII  Larissa        73 548  0.555   21.5   192  Voyager 2 1989
VIII Proteus       117 647  1.122   20.0   416  Voyager 2 1989

I    Triton        354 800    5.88  13.0  2706  Lassell   1846
II   Nereid      5 513 400   360.1  19.2   340  Kuiper    1949
     S/2002 N1  15 728 000  1879.7  24.5    61  Holman    2002
     S/2002 N2  22 422 000  2914.1  25.4    40  Holman    2002
     S/2002 N3  23 571 000  3167.9  25.4    40  Holman    2002
X    Psamathe   46 695 000  9115.9  25.6    38  Jewitt    2003
     S/2002 N4  48 387 000  9374.0  24.6    60  Holman    2002


The story of Neptune’s mmon begins when the planet was discovered. William Lassell at the same time that he found Triton, reported that he though he saw rings orbiting the planet. His observations seemed quite peculiar, as he could confirm the direction or size of them. After much debate and discussion, especially as other could not confirm Lassell’s view, the problem was resolved by problems with his telescope having gross distortions. From what we know from Voyager 2, it would have been impossible to see them. This story remains today as a unusual observational curiosity.

The rings are unusually named after Neptune astronomers and even the partial ring. The nomenclature deviates significantly from the naming systems used for the rings of Jupiter and Saturn, and the Greek letters for used Uranus. Some rings even have names after the French ideals of their republic!

Neptune’ rings vary between 41 900 and 62 900 kilometres, being some 0.5 to 0.7 times of the planet’s diameter. Like Uranus, these rings are mostly dark material. From increasing distance from the planet, the Voyager 2 rings are named Galle, Le Verrier, Lassell, Arago and Adams. (Arego being named after François Arago, the French astronomer who first seriously began the hunt for Neptune. The outer ring at the distance of 62 900 km, is named Courage, with the bright portions of the partial ring arcs being Liberté, Egalité and Fraternité*; (liberty, equality and fraternity). These fragments being in the order of leading, equidistant and following, respectively.

All the Neptunian rings have some peculiarities which are different and darker than the other gaseous planets with rings. Most of Neptune’s are faint and narrow, showing either axis-symmetric, partial or distorted rings. Other like Lassell are brighter on the outer edge of the ring. Adams structure is the most complex, showing areas of the spiral or radial features and contains arcs, likely caused by the moon Galatea, which also trails its own ring of dust

Neptune’s Rings

 Name                  Dist.  Width
                       (km)    (km)
1989 N3R (Galle)      41 900   15 
1989 N2R (Leverrier)  53 200   15 
Lassell Ring          55 400   -  
Arago Ring            57 600  >50 
1989 N1R (Adams)      62 930   -  
Liberté Ring Arc      62 900   -  
Egalité Ring Arc      62 900   -  
Fraternité Ring Arc   62 900   -  
Courage Ring Arc      62 900    - 


The user applying this data for any purpose forgoes any liability against the author. None of the information should be used for regarding either legal or medical purposes. Although the data is accurate as possible some errors might be present. The onus of its use is place solely with the user.


Last Update : 29th April 2006

Southern Astronomical Delights © (2006)


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