Comet of a new species, very like a fixt star; but perhaps there may be more of them.’ This safely covered all the options. He added a pointed postscript: ‘PS I think [Herschel] should give an account of his telescope, and micrometers.’120
The Astronomer Royal was in a dilemma. He had no reason to accept Herschel as a reliable astronomer, and to declare a new planet prematurely might bring himself and the Royal Society into disrepute, and even ridicule. On the other hand, to reject what might be the greatest British astronomical find of the century, especially if the predatory French astronomers accepted it first (and even named it), would be even more damaging. He was also aware that Banks regarded this as a crucial moment in his presidency, and in the fostering of good relations between the Royal Society and the Crown. King George III was particularly fascinated by stars, and particularly keen to outdo the French.
Maskelyne finally chose to act as a man of science: he went back to his own telescopes, and from 6 to 22 April made his own observations. He was, after all, acting precisely according to the motto of the Royal Society itself: Nullius in Verba-‘Nothing upon Another’s Word’. On 23 April he at last wrote directly to ‘Mr William Herschel, Musician, near the Circus, Bath’. He began prudently, but ended firmly.
Greenwich Royal Observatory, April 23, 1781
Sir, I am to acknowledge my obligation to you for the communication of your discovery of the present Comet, or planet.
I don’t know which to call it. It is as likely to be a regular planet moving in an orbit nearly circular round the sun, as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it…
This tipped the argument towards a planet, but was not a decisive opinion. Maskelyne then went into technical details about their respective telescopes-especially the need for ‘very firm stands’-and the difficulties of using micrometers to measure apparent changing diameters (and hence establish a possible planetary orbit): ‘If the light of the small planet is not still, & free from scintillations, it is impossible to prove it to have any other than a spurious diameter that may arise from the faults to which the best telescopes are subject.’ Nonetheless, he praised Herschel for making ‘very good observations’.
Finally, in his last paragraph, he committed himself. ‘On the 6th April I viewed the Comet with my 6 foot reflecting telescope and the greatest power 270, and saw it a very sensible size but not well defined. This however showed it to be a planet and not a fixt star, or of the same kind of fixt stars as to possessing native light with an insensible diameter. I am Sir, etc etc, N. Maskelyne.’121
Herschel had gained an invaluable ally. He immediately sent up a brief, masterly paper which was read at the Royal Society on 26 April. It was entitled simply ‘An Account of a Comet’, and was published in the Philosophical Transactions in June. He stated that ‘between ten and eleven in the evening’ of 13 March 1781 he had at once recognised a new object of ‘uncommon magnitude’ in Gemini, and immediately ‘suspected it to be a comet’. But from the account he then gave of its magnitude, clarity of outline and ‘proper motion’ it was clear that Herschel was now claiming that the ‘comet’ was really a new planet. Though, no doubt advised by Watson, he did not actually say so. To support this, he also claimed that the object remained perfectly round, without the least appearance of comet’s tail, when magnified 270, 460 and 932 times-the latter magnifications being far beyond what even Maskelyne’s Greenwich telescopes could achieve. All this naturally excited even more controversy than his moon paper, and some murmurs of dissent.122
Maskelyne nevertheless stoutly confirmed his opinion to Banks that their dark horse, the ‘musician of Bath’, had made a revolutionary discovery, and had ‘much merit’. Yet he could not suppress a touch of rueful irony. ‘Mr Herschel is undoubtedly the most lucky of Astronomers in looking accidentally at the fixt stars with a 7 foot reflecting telescope magnifying 227 times to discover a comet of only 3’ [seconds of arc] diameter, which if he had magnified only 100 times he could not have known from a fixt star…Perhaps accident may do more for us than design could; and this makes one wish that the number of astronomers was multiplied in order to increase our chance of new discoveries.’123 This suggestion that the discovery had been ‘accidental’, and that he had been ‘lucky’, was to grow increasingly disturbing to Herschel.124
Maskelyne had made public his support of Herschel just in time. On 29 April Messier wrote directly to ‘Monsieur Hertsthel at Bath’ from Paris, congratulating him-‘this discovery does you much honour’-and giving his opinion that this was very likely to be the seventh planet in the solar system. Messier had himself, he said modestly, discovered no fewer than eighteen comets in his lifetime, and this resembled none of them: it was ‘a little planet with a diameter of 4 to 5 seconds, a whitish light like that of Jupiter, and having the appearance when seen with glasses of a star of the 6th magnitude’. He signed ‘with consideration and respect’ as ‘Astronomer to the Navy of France, of the Academy of Sciences, France’.
As Maskelyne and Banks were only too aware, Messier’s congratulations would soon carry the weight of the entire French Académie des Sciences.125 Throughout the spring and summer months of 1781, more and more astronomers-in France, Britain, Germany, Italy and Sweden-observed the tiny moving speck, and took the view that it was indeed a planet circling in a massive ellipse beyond Saturn. These included Jacques Cassini, Henry Cavendish and Pierre Méchain. In October Anders Lexell, the celebrated Russian mathematician, wrote from his observatory far away in St Petersburg, sending a fully computed orbit and adding his congratulations. Using a series of parallax readings, he calculated that the planet was large and unbelievably remote, over sixteen times further from the sun than the earth, and twice as far out as Saturn. The size of the solar system had been doubled. Jérôme Lalande, who also computed the orbit, later said that this was the moment when the Académie des Sciences finally accepted the new planet-seven months after it had been sighted. Lalande himself suggested it should be christened ‘Herschel’.
It is suggestive that it was mathematical calculation, rather than astronomical observation, which finally convinced the scientific community that a seventh planet really did exist. One of the things Lexell’s calculation showed was that Herschel’s vivid impression that the planet was increasing in apparent diameter throughout March and April (and therefore approaching the earth) must have been the product of his growing concentration and excitement, since it was actually getting smaller and moving away. Lexell continued to work patiently for several years on his calculations, and later came up with the revised figure of 18.93 times the distance from the earth, impressively close to the modern computer-generated figure of 19.218. (In fact, as the planet’s orbit is elliptical not circular, the distance varies: at its closest it is 18.376 and at its furthest it is 20.083.)
In May, Watson proudly took Herschel up to London to meet his father Sir William, and to renew his now extremely cordial relations with Nevil Maskelyne. Together with the wealthy Deptford astronomer Alexander Aubert, they all dined with Sir Joseph Banks at the Mitre Club, the tavern much favoured by Dr Johnson. This was Herschel’s first meeting with the inner circle of British astronomers, and it was a great success. There was an air of suppressed triumph and excitement. Banks, in high spirits, seized his hand, congratulated him on ‘the great discovery’, and announced that he was to be elected to the Royal Society and awarded the Copley Gold Medal forthwith-within the next fortnight!126 He claimed it as a decisive British victory over French astronomy, and the eminence of Messier, Pierre Laplace and Lalande, who had hitherto dominated European astronomy.
