Neptune is the eighth planet from the Sun that was first telescopically discovered by Johann Gottfried Galle (1812-1910) on the 23rd September 1846. This remote planet lies in the cold outer regions of the Solar System, being far from the warmth of the Sun. Distance averages some 29.9 A.U. or 4.3 billion kilometres from our Sun. From Neptune the Sun appears very bright star at -19 magnitude, whose diameter subtends is around 1.1 arcmin - about 2% to 3% the diameter as seen from Earth.

Neptune is so far from the Sun that the average observed diameter is a tiny 2 arc seconds. We find that that the differences in brightness between each opposition and conjunction are only about 0.2 magnitudes (7%). Neptune takes 164.8 years to completely encircle the Sun, crossing on average each constellation of the zodiac every 14 years. Since first found in 1846, it has yet to complete one whole orbit - soon to be celebrated in 2011AD.

Although often stated as blue-green or blue-greenish in colour, to the amateur’s eyes Neptune is certainly a much deeper blue than the more greenish Uranus. Neptune’s maximum magnitude can only reach about 7.8, so it remains always below naked-eye visibility, but is easily visible in either 7x50 binoculars or small telescopes. In these small apertures Neptune appears like an ordinary ‘field star’, and will be discerned as only a tiny disk in 10cm to 15cm telescopes. The eighth and last planet is not difficult to find. All that is required is knowing the general position and having a good star atlas or dedicated star map to find it.


The Romans knew Neptune as Neptunis being the chief ruling god of water, including streams, rivers, ponds and waterfalls; who later had his realm greatly extended to preside over the open seas and oceans of the world. Neptune’s Latin name derives from the word nare meaning ‘to swim’, and was later identified with the Greek god Poseidon after about 399 B.C. In ancient Roman and Greek mythology, he was one of triumvirate of three brothers; being Neptune (Poseidon), Jupiter (Zeus) and Pluto (Hades) who one time combined forces to dethroned their capricious tyranical father Saturn (Cronus). After they had succeded in their plot, they soon divided their own worldly realms of the sky, sea and the underworld (Gods of air, water and the hereafter) According to the ancient Roman writer Hesoid, Neptune lives in a golden palace under the sea, along with his family. Neptune’ own son is Triton, being the grandson of Saturn, who was born by the pretty Nereid nymph called Amphitrite. [A much fuller discussion of these mythologies see Saturn Part 2.] Like the chief gods Jupiter and Saturn, Neptune was highly venerated by an ancient cult, who mainly celebrated in Ancient Roman city of Paestum, within the famed Doric-styled ‘Temple of Neptune’. His summer Rooman festival was usually held on 23rd July along with various sports and games.

To the Ancient Greeks, Neptune was also known as the god Poseidon, being far more ruthless and more temperamental than his Roman equivalent. As a god, not only could he toss thunderbolts like Zeus, but he could cause violent earthquakes that still frequently occur in Greece. It was only later in ancient Greek history that he was adopted as the main god representing the sea. Much of this mythology is only related to Greece, and possibly Phoenicia, especially among the Athenians, who were once a vibrant sea-faring nation. Having a real need for such a god was probably more to do with the possible inherent dangers when crossing by ship or boat to the many Greek islands or navigating the sometimes unpredictable nature of the Mediterranean sea. Some saw it as very important to worship and praise him so as to have safe sea passage. Today, Neptune continues to be ritually celebrated by sea passengers when crossing the equator. Again this odd tradition was often seen as appeasement towards Neptune who would then allow safe passage across the oceans. Various Neptune statues are quite common in ports throughout the world. Perhaps the most famous of these is placed in the very centre of town in the old English seaport of Bristol.

Neptune is often importantly portrayed in both the arts and literature. He is often shown as an old and slightly portly man, whose head is covered with long wild-hair and matted beard, holding his fisherman‘s staff tipped with a three-pointed trident - sometimes displayed as the special astronomical character symbolising the planet (right). To others he is also draped in a fish net and is seen riding his grand shell-shaped chariot across or under the waves. Neptune (Though I’ve never really worked out why this shell is some kind of grand boat!) Best of the traditional artistic examples is the beautiful representation of Neptune in the wonderful marble sculpture created by Gian Bernini’s. Named Neptune and Triton, this masterwork is placed in the Victorian and Albert Museum in London, and depicts him in the Roman classic story of Aeneas by Publius Vergillus Maro Virgil (70 B.C. - 1 B.C.). Here where Neptune is seen calming the sea through the call of the shell-horn of Triton, as to ensure his safe voyage across the sea. Neptune is today still celebrated through various fountains in Europe, such as the famous Trevi Fountain in Rome, Govanni’s Fountain in Bologna, or the Piazza della Signoria in Florence. Another is the Neptunbrunnen or Neptun Fountain in Berlin.


Discoverer : Johann Gottfried Galle 23 Sept. 1846
Predictions : John Adams and Urbain Leverrier
Satellites : 13 (2006)
Equatorial : 49 532 ± 30 km.
Polar : 48 684 ± 60 km.
Flattening : 0.0171
Polar Tilt : 29.56o
Period (P) : 164.791 32 years
Synodic Period : 367.49 days
Orbital Velocity 5.48 kms-1
Eccentricity (e) : 0.0113
Inclination (i) : 1.769 17o
Mass : 1.024 x1026 kg.
Mean Density : 1.638 g.cm-3
Mean Distance : 4.347 31 x 109 km
Sidereal Rotation : 16.7±1.4 hr.
Mean Sidereal Rotation : 16.11±0.01 hr.
(magnetic) Day Length : 16.11 hr.
Maximum Diameter : 2.4″ (arcsec)
Minimum Diameter : 2.2″ (arcsec)
Maximum Magnitude : +7.84

The Planet Neptune

Unlike the rest of the known planets, Neptune was discovered not by chance or lucky but through the knowledge of mathematics. Much of the initial concerns was the apparent slow displacement from what was predicted by the planet Uranus over time, whose position by the 1820’s was out “intolerable quantity” of 1 to 2 arc minutes. The only possible solution was that Uranus was being affected by another planet further out in the Solar System. During 1846, after decades of speculation, the planet’s position was predicted independently by Englishman John Couch Adams (1819-1891) and the French mathematician Urbain Jean Joseph Le Verrier (1811-1877) (or sometimes written just as Leverrier). Both calculated Neptune’s place on the ecliptic - within ten degrees by Adams and within one degree by Le Verrier. This became the first example of an object in the Solar System to have been predicted before its discovery. The central achievement in discovering Neptune, was made by careful computations of the gravitational or perturbing effects by the two outer planets of Saturn and Uranus. During the mid-19th Century such results were considered a marvellous example of the deductive and theoretical mathematical astronomy - being the significant scientific milestone at the dawn of the science of astrophysics.

We will probably celebrate this event on 10th August 2011, when Neptune will have completed its first circuit of the Solar System since discovery. Here the planet will once again return almost to the identical place in the constellation of Aquarius. The exact position will, however, not be exactly the same because of these same small perturbations slightly changing the orbit.

For many years our knowledge was thwarted by even the largest telescopes discovering any of Neptune’s secrets - including properties as the diameter, composition, and nature. Little was known about Neptune, that is, until Voyager 2’ brief encounter with the planet in August 1989. We knew only that it had a methane-rich atmosphere, which accounts for its blue-green colour. Methane absorbs much of the faint red-light received from the Sun which accounts for the rich blue colour. First impressions sometimes convince novices that its appearance looks very similar images from space of Earth. In fact, there is no true water or any ocean-like familiarity here. Perhaps the only thing in common with Earth is the axial tilt of 29.5o, but the resemblance does stop there.

Neptune HST

Courtesy NASA / ESA / STScI
Sept. 2005.

Neptune’s equatorial diameter is now known to be 44 000 kilometres. This makes it just a fraction smaller than Uranus, but it is still four times the Earth diameter. By mass it is about 17 times heavier than Earth and but is about one-quarter less dense. Much of the planet’s atmosphere was found to be composed of about 84% Hydrogen and 15% Helium, with the Methane content being the remaining 3%. There had always some suspicion of faint belts across the planet’s disk, and as the spacecraft drew closer, all was soon revealed.

Here many new discoveries were made. Perhaps one of the most startling discovery was the aptly named Great Dark Spot (GDR) on the cloud-base, and faint ‘cirrus-like’ clouds of methane appearing above Neptune’ cloud-based surface. In 2003 and 2004 observations by the 10-meter Hawaiian Keck II telescope had found that this large spot had all but vanished from view. The was further examined in September 2005 when the Hubble Space Telescope reexamined the planet from afar and found a similar circumstance of the missing GDR. Why Neptune has such an variable and active atmosphere so far from the Sun is still not fully understood.

A similar Earth-strengthed magnetic field surrounds the planet, which was found to be unusually tilted by some 50o to the rotational axis. The atmospheric daily rotational period of was also discovered to be about 16.5 hours, being slightly slower than first thought. Intriguingly, the planet was found to be more active than ever imagined - surprising after Voyager 2 visit to the then apparently inactive Uranus.

Future Neptune Missions

In December 2004, NASA announced that it was considering to launch a new exploratory mission to Neptune, to study the understand the planetary conditions so far from the Sun. Called the &#8220Neptune Triton Orbiter”, NASA expects a launch around 2014, whose nuclear-electric propulsion system will be required. So far from the Sun means that solar power doesn’t work in the outer Solar System. To orbit Neptune, this spacecraft would need to travel through the solar system helped by many gravity assists from the inner planets and Jupiter, before finally reaching the right trajectory to place the spacecraft in orbit in 2035.

Neptune in 2006

Neptune can be easily found in either binoculars or a small telescope in Capricornus during 2006. The planet presently lies c.1.2oN in the centre of the ‘sea-goat’ back, from the three equatorially aligned stars, being (east to west); θ Cap, ι Cap and γ Cap. Neptune is quite poorly placed for the first three months of the year as it lies close to the Sun. From January to Neptune’s conjunction with the Sun on 16th February, the planet is an evening object, before switching to the morning sky. It would be probably best to look for Neptune after about the mid to the end of March, when the planet is sufficiently far from the sun and high enough above the horizon. By the end of May, Neptune rises about midnight, and becomes an evening object. Opposition occurs on 11th August, when the planet rises with the setting sun and available throughout the whole night. On 25th May and 29th October reaches its two apparent stationary points, which are separated by 0.8o of sky. Quadrature occurs on 10th May and the 19th November, which is the generally the best time to observe as Neptune appears more than 90o from the sun’s sky position.

Neptune presently trails Uranus by an average of 24.3o - close enough to observe on the same night. Over the last decade or so, the separation between the oppositions of both planets is slowly increasing by roughly 1.8o per year. Both were closest three times during 1993, when they were in Sagittarius and the minimum separation was about 20 arc minutes.

For most of 2006. it is probably best to use 4.3v magnitude, ι Cap / Iota Capricornii / 23 Cap, in seeking out Neptune, whose apparent position deviates over the whole year only slightly. Neptune makes no close approaches to bright stars except with an 8th magnitude star on 8th April, missing by about 5 arc seconds.


       CONJUNCTION : 06 February 09h AEST           
       OPPOSITION  : 11 August 08h AEST             
OhUT      RA      Dec.  Vis Diam Tilt Dis Elon. Con
Date    hh mm.m   o  ′  mag  ″     o  A.U    o     
14 Jan  21 15.7 -16 04  8.0 2.21 -29  30.96 023 Cap
28 Jan  21 17.8 -15 55  8.0 2.20 -29  31.02 009 Cap
11 Feb  21 19.9 -15 45  8.0 2.20 -29  31.04 005 Cap

25 Feb  21 21.9 -15 36  8.0 2.20 -29  30.99 018 Cap
11 Mar  21 23.9 -15 27  8.0 2.21 -29  30.89 032 Cap
25 Mar  21 25.6 -15 19  8.0 2.22 -29  30.74 045 Cap
08 Apr  21 27.1 -15 12  8.0 2.23 -29  30.56 059 Cap
22 Apr  21 28.2 -15 07  7.9 2.25 -29  30.34 072 Cap

06 May  21 28.9 -15 04  7.9 2.27 -29  30.11 086 Cap
20 May  21 29.2 -15 03  7.9 2.29 -29  29.87 099 Cap
03 Jun  21 29.0 -15 04  7.9 2.30 -29  29.65 113 Cap
17 Jun  21 28.5 -15 06  7.9 2.32 -29  29.44 126 Cap
01 Jul  21 27.6 -15 11  7.9 2.33 -29  29.27 140 Cap

15 Jul  21 26.5 -15 17  7.8 2.34 -29  29.14 153 Cap
29 Jul  21 25.1 -15 24  7.8 2.35 -29  29.06 167 Cap
12 Aug  21 23.6 -15 31  7.8 2.35 -29  29.04 179 Cap
26 Aug  21 22.1 -15 38  7.8 2.35 -29  29.07 165 Cap
09 Sep  21 20.7 -15 45  7.8 2.34 -29  29.16 151 Cap

23 Sep  21 19.6 -15 50  7.9 2.33 -29  29.30 138 Cap
07 Oct  21 18.7 -15 54  7.9 2.32 -29  29.48 124 Cap
21 Oct  21 18.3 -15 56  7.9 2.30 -29  29.70 110 Cap
04 Nov  21 18.2 -15 56  7.9 2.28 -29  29.93 096 Cap
18 Nov  21 18.6 -15 55  7.9 2.26 -29  30.17 082 Cap

02 Dec  21 19.5 -15 54  7.9 2.25 -29  30.41 068 Cap
16 Dec  21 20.7 -15 45  8.0 2.23 -29  30.62 054 Cap
30 Dec  21 22.2 -15 38  8.0 2.22 -29  30.79 040 Cap


     Conjunction     |        Opposition    
dd mon year  UT    AEST |  dd mon year  UT    AEST
05 Feb 2006  23h   09h* |  10 Aug 2006  22h   08h*
08 Feb 2007  10h   20h  |  13 Aug 2007  11h   21h 
10 Feb 2008  20h   06h* |  15 Aug 2008  01h   11h 
12 Feb 2009  07h   17h  |  17 Aug 2009  14h   00h*
14 Feb 2010  17h   03h* |  20 Aug 2010  03h   13h 

17 Feb 2011  20h   06h* |  22 Aug 2011  22h   08h*
19 Feb 2012  08h   18h  |  24 Aug 2012  11h   21h 
21 Feb 2013  04h   14h  |  27 Aug 2013  00h   10h 
23 Feb 2014  15h   01h* |  29 Aug 2014  13h   23h 
26 Feb 2015  13h   23h  |  01 Sep 2015  13h   02h 
28 Feb 2016  14h   00h* |                         
No Neptune Aphelion / Perihelion occurs between dates
      * Event Occurs in AEST the Next Day         


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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