N E P T U N E

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 very remote planet lies in the outer regions of the Solar System, lying far from the warmth of the Sun. Since the decision of the International Astronomical Union (I.A.U.) in 2006 of Pluto being demoted as a planet, Neptune has become the most distant planet in our Solar Sytem. The 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 2011 AD.

Although often stated as blue-green or blue-greenish in colour, to the amateur’s eyes Neptune is certainly much deeper colour of blue than the more greenish Uranus. Neptune’s maximum visual magnitude can only reach about 7.8, so it remains always below naked-eye visibility, but is easily visible in either 7×50 binoculars or small telescopes. In these small apertures Neptune appears like an ordinary ‘field star’, and will be only be discerned as a tiny disk in 10cm to 15cm telescopes. The eighth and last planet is not difficult to find. All that is required is knowledge of the general position and having some dedicated star map or star atlas to find it.

NEPTUNE’s MYTHOLOGY

Ancient 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. This broader picture is less realised in modern times, where Neptune is solely constrained to the open sea. Neptune’s Latin name derives from the word nare meaning ‘to swim’, and was only later identified with the Greek god Poseidon after about 399 B.C. In old 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 and tyrannical 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 immediate 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 Roman festival was usually held on 23rd July along with various sports and games.

To the Ancient Greeks, Neptune was also known as the Olympia god Poseidon, being far more ruthless and more temperamental than his Roman equivalent. Not only could he toss thunderbolts like Zeus, 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, but is especially important among the Athenians, who were once a vibrant sea-faring nation. Having some real need for such a god was probably more to do with the sometimes inherent dangers when crossing to the many Greek islands by ship or boat, or hen navigating the sometimes unpredictable nature of the Mediterranean sea. Some saw it as very important ritual to worship and praise him so as to have safe sea passage. Today, Neptune continues to be happily celebrated by sea passengers when crossing the equator. Again this odd tradition was often seen as necessary 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 the three-pointed trident - sometimes displayed as the special astronomical character symbolising the planet (right). To others he is also draped with fishnets and is seen riding his grand shell-shaped chariot across or under the waves. (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.

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 many decades of speculation, the planet’s position was firmly predicted independently by both the Englishman John Couch Adams (1819-1891) and French mathematician Urbain Jean Joseph Le Verrier (1811-1877) (or 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 some Solar System object to have been predicted before the planet’ actual discovery. The central achievement 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 marvellous examples of the deductive and theoretical mathematical astronomy - being the significant scientific milestoneat 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 29.5°, but the resemblance does stop there.

Fig. 2. NEPTUNE OBSERVED by HUBBLE 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. During 2003/04 observations by the 10-meter Hawaiian Keck II telescope had found that this large spot had all but vanished. It was further examined in September 2005 when the Hubble Space Telescope reexamined the planet from afar and found similar circumstances of the missing GDR. Why Neptune has such an variable and active atmosphere so far from the Sun is still not presently fully understood.

A similar Earth-strengthed magnetic field surrounds the planet, which was found to be unusually tilted by some 50° to the rotational axis. The atmospheric daily rotational period of was also discovered to be about 16½ 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, N.A.S.A. 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 “Neptune Triton Orbiter”, N.A.S.A. expects to launch the craft around 2014, whose nuclear-electric propulsion system will be required to get there. 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 PHENOMENA

**************************************************
  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         
****************************************************

Disclaimer

The user applying this data for any purpose forgoes any liability against the author. None of the information should be used for 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 : 2nd September 2008

Southern Astronomical Delights © (2008)

For any problems with this Website or Document please email me.