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Neptune (copied verbatim from N Series Talk)

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1166428.  Sat Jan 02, 2016 7:07 pm Reply with quote

The discovery of Neptune was a triumph for theory over observation. After Uranus was first discovered, it soon became clear that it was not behaving as expected. Various hypotheses were put forward as to why; one, suggested in 1834 to the Astronomer Royal George Airy* by the Rev TJ Hussey, was that there was another planet, further out, affecting the orbit of Uranus. Airy summarily rebuffed this idea, and Hussey did not pursue it any further. Three years later, Airy confirmed his opinion on the hypothesis in a letter in which he said that, even if such a planet were to exist, "it would be nearly impossible ever to find out its place".

Step forward John Couch Adams. Having just achieved the position of Senior Wrangler in 1843 (reportedly with twice the score of the second-highest achieving student), he decided to take on the problem of finding this trans-Uranian planet. His problem was this: he knew the effects that one body was having on another, and from that he needed to deduce the mass and orbit that the body needed to have to produce those effects.

Amazingly, by the middle of 1845 he had a solution that told him where to look to see Neptune. Unfortunately, what he didn't have was a telescope. He needed an astronomer to help him - it was just his bad luck that the highest astronomer available was one George Airy.

Adams wrote to Airy; Airy ignored him. Adams called on Airy in the Royal Observatory; the first time he went, Airy was away, and the second, Airy was eating dinner and Adams was dismissed without a meeting. Adams decided to leave a letter detailing his findings, and in November he got a reply. The findings were satisfactory, Airy said, but could they predict the observed irregularities in the orbital radius of Uranus? Adams did not reply to this question; he was busy, and found the question trivial. And so, were it not for events across the channel, the story ends with Adams's major new result gathering dust on a desk in Greenwich.

However, in the same year, at the behest of the director of the Paris Observatory, a theoretician called Le Verrier was working on the same problem. He wrote two memoirs, the second of which produced results almost identical to Adams's. Airy wrote to him, asking the same question he had posed Adams, and Le Verrier replied with a quick and confident yes. Airy finally asked James Challis, the director of the Cambridge observatory, to start looking for a planet in the area predicted by Adams and Le Verrier.

Back in France, however, Le Verrier was having no luck getting the Paris Observatory to actually look for his predicted planet. Frustrated, he wrote to an assistant at the Berlin Observatory, Johann Gottfried Galle, who, within an hour of looking found a 'star' that fit the bill. They checked again the next day, this time with the director of the Observatory, and it had moved by exactly the amount predicted. Success! (And to make matters worse for Challis, he realised after the announcement that he had seen, and missed, the planet twice during his search!)

Adams decided not to hold a grudge at being written out of the discovery, but Sir John Herschel, by then aware of Adams's work, decided that some record of his contribution should be made. In October 1846 he wrote a letter praising Le Verrier's work and pointing out that Adams had arrived independently at the same conclusions.

When the French saw this, they were outraged. It was seen as attempt by the English, and particularly Airy, Challis and Herschel, to steal the glory of Neptune's discovery. Le Verrier was furious with Airy for not mentioning Adams's results earlier, and decided the new planet should be named after himself rather than his earlier suggestion of "Neptune". In England, Adams was seen as brilliant, and robbed of his discovery by the incompetent Challis. A poem circulated Cambridge describing his bad luck:

When Airy was told, he wouldn't believe it;
When Challis saw, he couldn't perceive it.

Eventually things died down, and the consensus - outside France - was that the new planet should be called Neptune. Adams and Le Verrier eventually became friends soon after meeting - despite neither speaking the other's language!

Although we might think both men had their fair share of bad luck, there was one piece of good fortune that overshadows all their setbacks. Neptune and Uranus were on the same side of the sun in the early 19th century - had they been on opposite sides, the effect on Uranus would have been almost invisible, and the search for Neptune would surely have been delayed for decades.

So who discovered Neptune? The records show Le Verrier, but Adams surely deserves some credit. However, there is one footnote - in 1610, Galileo was making observations of the four large satellites of Jupiter, and he recorded the nearby stars as well. One, he noted, had shifted position. He seems not to have placed much importance on this.

There is no doubt that this "star" was, in fact, Neptune.

* though he became Astronomer Royal in 1835, in fact.

From Eudoxus to Einstein: A History of Mathematical Astronomy - C. M. Linton;
Patrick Moore's New Guide to the Planets - Patrick Moore


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