Neptune is the eighth planet from the Sun that was first telescopically discovered by the German astronomer Johann Gottfried Galle (1812-1910) on 23rd September 1846, and discovered 65 years 6 months 10 days after William Herschel had found Uranus. In the outer regions of the Solar System, this remote planet lies far from the glowing warmth of the Sun. Neptune’s place as the furthest planet was held until on 18th February 1930, when American Clyde Tombaugh (1905-1997) discovered the even further and remote, Pluto This demotion of the most distant planet was to last until 2006, some seventy-six years later, when the International Astronomical Union (I.A.U.) decided Pluto planetary status was to be withdrawn. So Neptune has again become the holder of the most distant planet in our Solar System.

Const.       Year      Year  
Aquarius    2007/08 — 2022/23
Cetus       2023/24 — 2029/30
Pisces      2030/31 — 2023/24
Aries       2024/25 — 2032/34
Taurus      2034/35 — 2064/65
Orion       2065/66 — 2065/66
Gemini      2066/67 — 2078/79
Cancer      2080/81 — 2087/88
Leo         2089/90 — 2103/04
Virgo       2104/05 — 2123/24
Libra       2124/25 — 2135/36
Scorpius    2136/37 — 2138/39
Ophiuchus   2139/40 — 2147/48
Sagittarius 2148/50 — 2162/63

Neptune is presently (2012) is in the constellation of Aquarius, where it will stay until the planet moves in nearby Cetus for short time during 2023 to 2030. The side table (Table 1) shows what zodiac constellations Neptune can be found until 2160 AD. For southern observers, the planet will continue to become more unfavourably placed for over a century as it slowly crawls into the northern portion of the zodiac. This trend will not be reversed until about 2095 AD, and will be best placed again 2130 AD. Best placement was in Sagittarius last happened in 1930 AD.

Although often stated as bluish, blue-green, or even blue-greenish in colour, Neptune to the eyes of the amateur is certainly much deeper blue than the more greenish coloured Uranus. Colour differences are attributed to the gaseous methane in the upper atmosphere. Images taken of Neptune, like those of Voyager or Hubble, are more azure blue, but these are greatly enhanced. If the truth be known, the amount of sunlight available is much reduced. At mean distance, Neptune sees the Sun shining at -19.4, being about 700 times fainter than from Earth or 570 times brighter than the full moon.

Neptune reaches the maximum visual magnitude of about 7.8v, so it always remains below naked-eye visibility. However, the planet is easily spotted in darkened skies either 7×50 binoculars or small telescopes. In small apertures, Neptune appears like an ordinary ‘star’, and will be only be discerned as a tiny disk in 10cm or 15cm telescopes. The eighth, and now last of the recognised planets, Neptune is not overly difficult to find in the sky. All that is required is knowledge of the general sky positions and having some dedicated star map or standard star atlas to find it.

MYTHOLOGY of NEPTUNE

Ancient Romans knew Neptune as Neptunis being the chief ruling god of water; including streams, rivers, ponds and waterfalls. Only later, he had his realm been greatly extended to preside over the open seas and oceans of the world. No doubt this picture was changed as the people of ancient times found the world more larger than ever imagined. This broader picture is now unclear in modern times, where Neptune is solely constrained to the open sea. Latin name for Neptune derives from the word nare, meaning ‘to swim’, and was only after about 399 B.C. when he was identified with the Greek god Poseidon. In old Roman and Greek mythologies, he became was one of valiant triumvirate of three brothers; being Neptune (Poseidon), Jupiter (Zeus) and Pluto (Hades). Together, in legend, who one time combined forces to dethroned their capricious and tyrannical father Saturn (Cronus). After they had succeeded in their patricidal (or regicidal) plot, they soon divided their own worldly realms into the sky, sea and the underworld (Gods of air, water and the hereafter). According to the ancient Greek writer and poet, Hesiod (c.700 BC), Neptune lives in his 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 main gods of 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, who was 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. His wife was instead the sea goddess (a Nereid), Amphitrite (Greek, Salacis, seawater), both physically representing the sea and its variable or fickle personality. It was only later, in ancient Greek history, that Poseidon was adopted as the main god representing the expansive 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 seafaring nation. Having some real need for such gods was probably more to do with the sometimes inherent dangers when crossing to the many Greek islands by ship or boat, or then 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, and that is 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 grandiose 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 Neptune Fountain in Berlin.

DISCOVERY of NEPTUNE

Unlike the rest of the known planets, Neptune discovery was not by chance or lucky but through the application and knowledge of mathematics. As earlier stated in the introduction, Neptune was first visually discovered almost immediately by the German astronomer Johann Gottfried Galle (1812-1910) on 23rd September 1846. Galle’s discovery was to be delivered to him in some correspondence with French mathematician Urbain Jean Joseph Le Verrier (1811-1877) (or Leverrier) on 18th September, 1846, where he was kindly requested that to inspect the region of sky in the constellation of Aquarius. After finding an unlisted and unmapped star, Galle again observed the same object over two nights, whose small movement had told him straight away this was the predicted distant planet that had been suspected to be significantly influencing the calculated orbits of Saturn, and in particular, Uranus. These initial concerns of Neptune were of the apparent slow orbital displacements, known as perturbations from what was predicted by the planet Uranus. By the 1820s, Uranus’ own position had been out by an “intolerable quantity” totally an extraordinary 1 to 2 arc minutes. Astronomers knew that the only possible solution was that Uranus was being affected by another unknown planet further out in the depths of the Solar System.

During 1846, after many decades of speculation, the planetary position was firmly predicted independently by both the Englishman John Couch Adams (1819-1891) and French mathematician Le Verrier. Both had calculated the place of the new planet on the ecliptic, and being within ten degrees (10°) for Adams, and within one degree (1°) by Le Verrier. This became the first example of some Solar System object to have been predicted before it was actual discovered. This brilliant achievement was made by careful computations of the gravitational or perturbing effects by the two outer planets of Saturn and Uranus.

In the mid-19th Century John Adams and Urbain Le Verrier results were considered marvellous examples of deductive and theoretical mathematical astronomy — being held as the significant scientific milestone at the dawn of the the astronomical science known as astrophysics.

The PLANET NEPTUNE

Distances to Neptune from our Sun averages some 29.9 A.U. or 4.3 billion kilometres, being about two-thirds (19.2/29.9 AU) further away than the similar sized planet of Uranus. From the orbit of Neptune, our Sun appears like a very bright star of -19 magnitude, whose very small apparent diameter subtends around 1.1 arcmin. This may varies in size throughout the orbit by only about 2% to 3%. Indeed, Neptune is so far from the Sun that the average observed diameter is merely 2 arcsec or only 1⁄1800th of Earth (1°). We find that that the very small brightness differences between each opposition and conjunction are just 0.2 magnitude (7%.)

Neptune in its long orbit takes some 164.79 years (164 years 10 month 17.0 days) to completely encircle the Sun, crossing on each constellation of the zodiac approximately averaging every fourteen years. Since found in 1846, it has yet only just complete one whole orbit. We recently celebrated this on Sunday 10th July 2011), where the distant planet returned to an almost identical place within the constellation of Aquarius. Exact positions were not exactly the same because of these same small perturbations that was noticed in the original discovery of Neptune have slightly changing the orbit. Its second orbit since discovery will again complete on 24th August 2176.

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, until Voyager 2 had a brief encounter with the planet during August 1989. We knew only that the atmosphere was methane-rich, accounting for its observed blue-green colour. Methane happens to absorbs much of the faint red sunlight, which makes the visual rich blue colour. First impressions sometimes convince novices that its appearance looks very similar images of the watery Earth from space 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 29½°, but here the resemblance stops.

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 but is about one-quarter less dense. Much of the planetary atmosphere was found to be composed of about 84% Hydrogen and 15% Helium, with the Methane content being the remaining about 3%. There had always some suspicion of faint belts across the planetary disk, and as the Voyager 2 drew closer, all was soon revealed.

Fig. 2. (left.) NEPTUNE’s DYNAMIC ATMOSPHERE by HST Image NASA, ESA, STScI (1st September 2005).	Fig. 3. (right) NEPTUNE OBSERVED by HST Image NASA, ESA and the Hubble Heritage Team (STScI/AURA) (12th July 2011)

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-metre Hawaiian Keck II telescope had found that this large spot had vanished. It was further examined in September 2005 when the Hubble Space Telescope (HST) was reexamine the planet from afar. They then found the similar circumstances with the now totally missing GDR, but found new changes. (Fig. 2). On 12th July 2011 another set of images was taken by HST, and this time, they found complicated cloud patterns had developed in the Neptunian atmosphere.

Why Neptune has such an variable and active atmosphere so far from the Sun is still not presently fully understood. It is speculated that Neptune may have seasons that change over 164 years, depending on place in the orbit

A similar magnetic field strengths to Earth surrounds Neptune, being was found to be unusually tilted by some 50° against 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 quite inactive atmosphere of Uranus.

FUTURE NEPTUNE MISSIONS

In December 2004, NASA had announced that it was considering to launch a new exploratory mission to Neptune, to both study and attempt to understand the planetary conditions so far from the Sun. To be called the “Neptune Triton Orbiter”, NASA was expected to launch the craft around 2014, whose nuclear-electric propulsion system would had be required to get there. So far from the Sun means that the avalable solar power does not work in the outer depths of the Solar System. To be captured and then 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 and velocity to place the spacecraft an orbit in 2035. This project and plan has since been scapped for another day.

NEPTUNE PHENOMENA

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  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 these 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 placed solely with the user.

Last Update : 24th April 2012

Southern Astronomical Delights © (2012)

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