September 23 – Neptune

Officially, Neptune was discovered visually on September 23rd 1846 by Johann Galle, but its existence had been proposed by Alexis Bouvard years before, and its position was predicted by Urban Le Verrier on August 31st 1846, and by John Couch Adams a couple of days later.  James Challis at the University of Cambridge was also in the running, and observed Neptune twice before the discovery was announced, but failed to realise what was going on.  And Galileo had seen and noted Neptune himself, but even he had no idea what it was.

Neptune from Voyager 1 (image credit: NASA)

Neptune from Voyager 1 (image credit: NASA)

The first suggestion for a name for this new planet came from Galle, who thought Janus might be a good idea, after the two-faced Roman god of beginnings, from whom we get the name of the first month of the year.  Challis, no doubt anxious to make up for being beaten to the finishing tape, suggested Oceanus (whose children we have been meeting all year in the guise of asteroids).

Le Verrier was the first to suggest naming the new planet Neptune, the Roman god of the sea, and appropriately a brother of Jupiter and Pluto (he also suggested, modestly, Le Verrier, but this idea didn’t gain much support outside France).  The name is used almost universally, although as Neptune was the Roman equivalent to the Greek Poseidon, Greece is, as far as I’m aware, the only western country to use that name instead.

Neptune is a small giant (yes, I know what I just said).  It is 17 times more massive than Earth, but only about 5% as massive as Jupiter.  It is similar in composition to Uranus, and the two of them, while still coming under the “gas giant” umbrella, are sometimes also referred to as “ice giants”.

As with the other giants, Neptune has a ring system, but not one that’s going to be used as a backdrop to an episode of Star Trek anytime soon.  The rings are fairly thin, and few enough in number to have been named after some of the players in Neptune’s discovery (the Adams, Le Verrier, Lassell, Galle and Arago rings).

Neptune's Rings (image credit: NASA)

Neptune’s Rings (image credit: NASA)

At last count Neptune was known to have fourteen moons, all named after water deities.  I’ll just briefly mention that the biggest are Triton (way bigger than the rest at 2,700+ km diameter), Proteus, Nereid, Larissa, Galatea and Despina, but as all fourteen will probably be turning up in these pages over the next twelve months, we will leave it at that for now.

Just as an aside, it has been speculated that in the dim and distant past of the solar system there might have been a fifth gas giant, which was flung out of orbit by a strong gravitational kick from Jupiter or Saturn.


ALSO TODAY . . . .

1791  –  Birth, in Hamburg, of Johann Franz Encke, comet hunter, and expert at predicting when they were going to return. Encke also has a gap in Saturn’s rings named after him, in recognition of his observations of that planet.