Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth and slightly larger than Neptune. Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 astronomical units (4.50×109 km). It is named after the Roman god of the sea and has the astronomical symbol ♆, a stylised version of the god Neptune’s trident. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the planet’s remaining known 13 moons were located telescopically until the 20th century.
The planet’s distance from Earth gives it a very small apparent size, making it challenging to study with Earth-based telescopes. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989.
The cosmic voice of Neptune
(NASA Voyager recording)
Shortly after its discovery, Neptune was referred to simply as “the planet exterior to Uranus” or as “Le Verrier’s planet“. The first suggestion for a name came from Galle, who proposed the name Janus. In England, Challis put forward the name Oceanus. Claiming the right to name his discovery, Le Verrier quickly proposed the name Neptune for this new planet, though falsely stating that this had been officially approved by the French Bureau des Longitudes. In October, he sought to name the planet Le Verrier, after himself, and he had loyal support in this from the observatory director, François Arago. This suggestion met with stiff resistance outside France.
The etymology of Latin Neptunus is unclear and disputed. The ancient grammarian Varro derived the name from nuptus i.e. “covering” (opertio), with a more or less explicit allusion to the nuptiae, “marriage of Heaven and Earth“.
Among modern scholars Paul Kretschmer proposed a derivation from neptu – “moist substance”. Similarly Raymond Bloch supposed it might be an adjectival form in no from nuptu, meaning “he who is moist“. Georges Dumézil though remarked words deriving root nep are not attested in languages other than Vedic and Avestan. He proposed an etymology that brings together Neptunus with Vedic and Avestan theonyms Apam Napat, Apam Napá and Old Irish theonym Nechtan, all meaning descendant of the waters. By using the comparative approach the Indo-Iranian, Avestan and Irish figures would show common features with the Roman historicised legends about Neptune. Dumézil thence proposed to derive the nouns from root nepot – “descendant, sister’s son“.
Portrait of Andrea Doria as Neptune
Neptune is one of the only four Roman gods to whom it was appropriate to sacrifice bulls, the other three being Apollo, Mars and Jupiter. The wrong offering would require a piaculum, if due to inadvertency or necessity. The type of the offering implies a stricter connection between the deity and the worldly realm.
Nethuns is the Etruscan name of the god. In the past it has been believed that the Roman theonym derived from Etruscan but more recently this view has been rejected. Nethuns was certainly an important god for the Etruscans. His name is to be found on two cases of the Piacenza Liver, namely case 7 on the outer rim and case 28 on the gall-bladder, (plus once in case 22 along with Tinia). This last location tallies with Pliny the Elder’s testimony that the gall-bladder is sacred to Neptune. Theonym Nethuns recurs eight times on columns VIII, IX and XI of the Liber Linteus (flere, flerchva Nethunsl), requiring offerings of wine.
The mariner of old said to Neptune in a great tempest, “O God! thou mayest save me if thou wilt, and if thou wilt thou mayest destroy me; but whether or no, I will steer my rudder true.