Happy birthday Neptune!
Jul. 8th, 2011 08:31 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
beccastareyesSo, I promised you all a story, so hearken to my tale of the discovery of Neptune.
Uranus was discovered by observation -- William Herschel was poking around with his telescope and discovered something that was neither a comet nor part of the fixed stars. Neptune was first predicted by mathematics, then confirmed via observations.
See, after Sir Isaac Newton made his discovery, it made mathematicians very excited because we understood how the planets moved! Sort of. Newton's laws are pretty simple, and you could easily use them to prove that Johannes Kepler's empirical laws describing the motion of the planets were right. (It's an undergrad homework problem nowadays.) But as soon as you toss a third planet in -- say, the Sun, the Moon, and the Earth -- the problem (the 'three body' problem) becomes unsolvable by a nice, simple equation.
However, all is not lost! Most planets orbiting the Sun mostly feel gravity from the Sun. The effects, even from Jupiter (the biggest planet) are usually small, so astronomers could approximate them* in most cases. Now, these equations were not fun -- the first-order equations in my celestial mechanics book took up several pages**, and you could seriously fill books with the more complicated equations. But they worked.
Well, mostly. Even when throwing in Jupiter and Saturn, Uranus refused to behave. You could blame the observers, but that doesn't make sense when the other planets were behaving like they should. Or you could wonder if Newton's Universal Law wasn't as Universal as might be hoped. Or maybe there was another, unknown, planet pulling on it? Considering I've had to do the reverse calculation (find perturbations based on a known planet), the prospect of trying to predict an unknown planet from how it messes up Uranus's orbit fills me with apprehension.
But at least two mathematicians tried. John Couch Adams was an Englishman fresh out of school, and started his attempts in 1843. Pretty much, he'd make an educated guess about what Neptune was doing, calculated what Uranus would do, checked it versus observations, then went back to tweak his Neptune to work better. Adams finally got a satisfactory prediction of 'look here to see this new planet' in fall of 1845, and sent it to some observers... where it apparently vanished into a pile of paperwork.
Meanwhile, in France, Urbain Le Verrier apparently started working on the problem in the fall of 1845, and came up with an answer in summer of 1846. Then of course, the English astronomers remember 'Hey, Adams said the planet would be there too, last year'. Considering England and France were GREAT RIVALS!!!!!111 there was a bit of contention. Meanwhile, Le Verrier asked some German astronomers to take a look at his answer and see if there was, indeed, a planet there. The English astronomer George Airy asked their own people to look for the planet... which failed, because their telescope was small enough that Neptune didn't show a disc, and their star charts were crappy. On the other hand, the German astronomer Johann Gottfried Galle, and his assistant, Heinrich Louis d'Arrest, had nice up-to-date star charts so, once they saw the planet move against the stars, they were all 'Hey, le Verrier, your planet really exists!'.
Which of course, led to more unhappiness from the English. Adams was junior enough that he didn't want to kick up a fuss against senior astronomers lest it imperil his ability to stay employed. And Le Verrier's predictions were the better ones (plus it helps to be confident -- Adams kept updating his numbers, which, until someone found a planet, made it hard to know where to look). Add in someone suggested 'why don't we name planets after their discoverers, since we name comets after their discoverers, and planets are so much cooler?". Answer: 'Look, if we didn't let Herschel call his planet George*** after the guy signing his checks, we won't call this one Le Verrier or Galle or whatever'.
Incidentally, we did end up naming the rings of Neptune after these various historical folks. While Saturn's rings are mostly lettered or named after satellites, Uranus's get Greek letters, and Jupiter's get descriptive names, Neptune has the Galle, Le Verrier, Lassell****, Arago and Adams rings. In a twist of fate, the Adams ring is one of the best studied, being farthest from the planet (making it easier to image from Earth or Earth orbit).
(Also, detailed work turned up plenty of times Neptune was spotted before people figured out it was a planet. There's a couple of 'that's no star' in catalogs that were Neptune masquerading as stars. And Galileo even noticed it, and that it moved, while he was observing Jupiter... but who'd ever heard of a planet that faint? Not him.)
* Well, nowadays we use computers. Still had to go through how it was done in the bad old days, because you get a better understanding of things like resonances.
** Found this page if you want to get an idea about the math.
*** Well, Georgium Sidus -- 'George's Star', after King George III. The same King George from the American revolution.
**** William Lassell discovered Triton, only two weeks and change after Neptune was first spotted.
Uranus was discovered by observation -- William Herschel was poking around with his telescope and discovered something that was neither a comet nor part of the fixed stars. Neptune was first predicted by mathematics, then confirmed via observations.
See, after Sir Isaac Newton made his discovery, it made mathematicians very excited because we understood how the planets moved! Sort of. Newton's laws are pretty simple, and you could easily use them to prove that Johannes Kepler's empirical laws describing the motion of the planets were right. (It's an undergrad homework problem nowadays.) But as soon as you toss a third planet in -- say, the Sun, the Moon, and the Earth -- the problem (the 'three body' problem) becomes unsolvable by a nice, simple equation.
However, all is not lost! Most planets orbiting the Sun mostly feel gravity from the Sun. The effects, even from Jupiter (the biggest planet) are usually small, so astronomers could approximate them* in most cases. Now, these equations were not fun -- the first-order equations in my celestial mechanics book took up several pages**, and you could seriously fill books with the more complicated equations. But they worked.
Well, mostly. Even when throwing in Jupiter and Saturn, Uranus refused to behave. You could blame the observers, but that doesn't make sense when the other planets were behaving like they should. Or you could wonder if Newton's Universal Law wasn't as Universal as might be hoped. Or maybe there was another, unknown, planet pulling on it? Considering I've had to do the reverse calculation (find perturbations based on a known planet), the prospect of trying to predict an unknown planet from how it messes up Uranus's orbit fills me with apprehension.
But at least two mathematicians tried. John Couch Adams was an Englishman fresh out of school, and started his attempts in 1843. Pretty much, he'd make an educated guess about what Neptune was doing, calculated what Uranus would do, checked it versus observations, then went back to tweak his Neptune to work better. Adams finally got a satisfactory prediction of 'look here to see this new planet' in fall of 1845, and sent it to some observers... where it apparently vanished into a pile of paperwork.
Meanwhile, in France, Urbain Le Verrier apparently started working on the problem in the fall of 1845, and came up with an answer in summer of 1846. Then of course, the English astronomers remember 'Hey, Adams said the planet would be there too, last year'. Considering England and France were GREAT RIVALS!!!!!111 there was a bit of contention. Meanwhile, Le Verrier asked some German astronomers to take a look at his answer and see if there was, indeed, a planet there. The English astronomer George Airy asked their own people to look for the planet... which failed, because their telescope was small enough that Neptune didn't show a disc, and their star charts were crappy. On the other hand, the German astronomer Johann Gottfried Galle, and his assistant, Heinrich Louis d'Arrest, had nice up-to-date star charts so, once they saw the planet move against the stars, they were all 'Hey, le Verrier, your planet really exists!'.
Which of course, led to more unhappiness from the English. Adams was junior enough that he didn't want to kick up a fuss against senior astronomers lest it imperil his ability to stay employed. And Le Verrier's predictions were the better ones (plus it helps to be confident -- Adams kept updating his numbers, which, until someone found a planet, made it hard to know where to look). Add in someone suggested 'why don't we name planets after their discoverers, since we name comets after their discoverers, and planets are so much cooler?". Answer: 'Look, if we didn't let Herschel call his planet George*** after the guy signing his checks, we won't call this one Le Verrier or Galle or whatever'.
Incidentally, we did end up naming the rings of Neptune after these various historical folks. While Saturn's rings are mostly lettered or named after satellites, Uranus's get Greek letters, and Jupiter's get descriptive names, Neptune has the Galle, Le Verrier, Lassell****, Arago and Adams rings. In a twist of fate, the Adams ring is one of the best studied, being farthest from the planet (making it easier to image from Earth or Earth orbit).
(Also, detailed work turned up plenty of times Neptune was spotted before people figured out it was a planet. There's a couple of 'that's no star' in catalogs that were Neptune masquerading as stars. And Galileo even noticed it, and that it moved, while he was observing Jupiter... but who'd ever heard of a planet that faint? Not him.)
* Well, nowadays we use computers. Still had to go through how it was done in the bad old days, because you get a better understanding of things like resonances.
** Found this page if you want to get an idea about the math.
*** Well, Georgium Sidus -- 'George's Star', after King George III. The same King George from the American revolution.
**** William Lassell discovered Triton, only two weeks and change after Neptune was first spotted.
