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Airy also wrote to Leverrier, but he made no mention of Adams. Instead, he asked for the same information for which he had asked Adams the previous year. Did the analysis account for the irregularities in orbital distance? Airy was clearly doubtful on this point, but Leverrier replied immediately and confirmed that the orbital radius did not represent a problem. Airy was again impressed by Leverrier's confident and clear explanations, and became convinced that the theoretical calculations he had in his possession were to be taken seriously. He decided that the search for the new planet should now begin.

Airy decided that the best way to search for the planet was to map the area of sky near the predicted position, then to repeat the exercise a few weeks later, and a few weeks after that. The data from the three surveys could then be compared,

58 For more details of Leverrier's procedure (see, for example, Grant (1966), pp. 179-83).

59 Letter to William Whewell, 25 June 1846. Quoted from Standage (2000), p. 91.

and if one of the 'stars' was found to have moved, the quarry would have been found. He decided also that the most appropriate telescope to use for the search was the Northumberland telescope at the Cambridge Observatory, rather than the smaller instruments at the Royal Observatory.

Airy asked Challis to conduct a search of an area of sky measuring 30° in longitude by 10° in latitude, centred on the position predicted by Adams and Leverrier. This represented an enormous amount of work and was, in fact, an extremely inefficient method, given that a quick result was desired. Nevertheless, Challis began his search on 29 July 1846 and even decided to record the positions of stars much fainter than the new planet was predicted to be. Had Adams' predictions been well known, many amateur astronomers might have pointed their telescopes at the appropriate area of the heavens and the chances of discovery would have been much improved, but it would appear that Airy and Challis were both keen for the honour of discovering a new planet to go to Cambridge. Leverrier's prediction did appear in the British press in August.

In France, a search mounted by the Paris Observatory in early August was soon terminated. Tempting as the possibility of finding a new planet must have been, there was no guarantee of success, and it was hard to justify utilizing valuable resources on what might turn out to be a wild-goose chase. On 31 August, Leverrier's third paper on Uranus was presented to the Academy. In it, he treated the value of the longitude at epoch, e', which he had calculated previously, and the semi-major axis, a', which he had assumed previously, as approximations. He thus introduced two new unknowns into the problem, (i.e. the differences between the true values, and the values previously used) and assumed that these were small quantities. Lengthy calculations produced orbital elements for the disturbing planet as follows:60

Eccentricity 0.107 61

Longitude of perihelion 284° 45'

Mass 1/9300

True heliocentric longitude, 1 January 1847 326° 32'

Leverrier included also an estimate of the size of the new planet, suggesting that it should have a disc measuring about 3.3' across. Provided one had a sufficiently powerful telescope, searching for a 3'' disc was much easier than trying to detect a very slow motion between successive observations.

During 1846, Adams was also busy refining his calculations. In September, he sent the results of his most recent attempt at the problem of Uranus to

60 Quoted from Grosser (1962).

the Royal Observatory. His initial calculations had predicted an eccentricity that was rather large in comparison to the other planets, and by reducing the assumed semi-major axis he was able to derive an orbit with a more reasonable eccentricity as well as one that slightly better fitted the data. His revised orbital elements were:61

Eccentricity 0.120 62

Longitude of perihelion 299° 11'

Mass 1/6666

True heliocentric longitude, 1 January 1847 329° 57'

Adams expressed also the opinion that by decreasing the semi-major axis still further, an even better fit with observation might be achieved, and he suggested that a/a' = 0.57 (corresponding to about 33.7 AU) would probably deliver the best results. As it happened, Airy was away in Germany and Adams' letter was not forwarded to him.

Neptune

Neither Adams nor Leverrier was having much luck with their respective astronomical establishments. Adams could, at least, be thankful that Challis was at last searching for the new planet, but Leverrier was having no success in getting his fellow countrymen to scan the heavens in search of something that might not exist. On 18 September, Leverrier wrote asking for help to an assistant at the Berlin Observatory - Johann Gottfried Galle - who had sent him a copy of his doctoral dissertation the previous year. On 23 September, the same day he received the letter, Galle and an astronomy student, Heinrich d'Arrest, turned the Fraunhofer telescope to the place indicated by Leverrier. They decided to compare their observations, as they made them, with the Berlin Academy's new star atlas, and after a few hours found a 'star' within 1° of the predicted position that was not in the atlas. The following day (accompanied now by the Observatory's director, Encke) they observed the 'star' again. It had moved by exactly the amount predicted by the calculations! One can only imagine the excitement that these men must have felt. Once they knew they were looking at a planet, it became easier to discern a disc (as Encke put it, 'the disc can be

Adams actually supplied the longitude values for 1 October 1846; the figure for 1 January 1847

is included here for easy comparison with Leverrier's data. The values are from Grosser (1962).

recognized only when one knows that it exists'62) and they managed to measure its size as between 2.2 and 3.2". Galle wrote to Leverrier on 25 September:

The planet whose position you have pointed out actually exists. The same day that I received your letter, I found a star of the eighth magnitude which was not shown on the excellent chart... published by the Royal Academy of Berlin. The observations made the following day determined that this was the sought-for planet.

Despite the magnitude of the scientific achievement represented by Galle's discovery, there were a few people in England who perhaps were a little disappointed. The discovery of a trans-Uranian planet was a triumph for the power of the theory of gravitation, but having the discovery taken from under your nose must have been a blow. Worse still, Challis looked back over his observations and found he had seen the new planet twice during his search! Subsequent investigations revealed other 'old' sightings. Adams himself soon put any disappointment to one side and decided instead to use the now known position of the planet to calculate a more accurate orbit. The official spokesman for

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