Since William Herschel's discovery of Uranus in 1781 the planet's orbit had been observed to differ from the one calculated according to Newton's principle of universal gravitation. There could be only one of two explanations, either Newton's laws were wrong, or there was another planet even further away from the Sun than Uranus which was attracting it and so causing the orbital disturbances. Confidence in Newton's laws was very strong, indeed any idea that they were incorrect would have had almost frightening implications, yet while there was obviously the possibility of the existence of another planet, the problem of working out details about it was of such complexity that mathematicians shrank from the task and some were even of the opinion that it was insoluble. Matters came to a head in I846 from two different quarters.

In England John Couch Adams, a young Cambridge mathematician who, like John Herschel, had passed through his mathematical training with flying colours, had decided to set himself the problem as soon as he had taken his degree. This he did and after a prodigious amount of work starting in I842 he actually came to a solution in September I845. The next step was to have the solution tested by observation and this was an even more difficult task for Adams. Young, unknown except to Peacock and his own immediate circle in Cambridge, he nevertheless sent his solution to the Astronomer Royal, at that time another ex- Cambridge mathematician, George Airy. Airy was by nature an autocrat and a somewhat arrogant one into the bargain. He received Adams' note about the position of the unknown planet but did not bother particularly about it. Adams called on him but as he was at dinner at the time he refused to see the young man, instead sending him a note asking a purely technical question which Adams, for his part, did not answer for the simple reason that he claimed the question was superfluous. The director of the University Observatory, James Challis also showed no enthusiasm about searching for the planet and with a promise to look into the matter seems to have ignored the problem. The truth of it all appears to be that neither Airy nor Challis really believed that the mathematics involved could be handled to provide a definite solution, least of all by a young man of twenty-six. Here matters might have stagnated had not the problem been taken up by a brilliant French mathematician Urbain Leverrier who, eight years older than Adams, had already built up an enviable reputation for himself.

Leverriex tackled the problem very carefully and extremely thoroughly and also came to a solution. This he reached in June I846 and published his results, but even so there was no immediate response from continental observers. In England, however, Airy and Challis did notice that Leverrier's result gave a position very close to that provided by Adams and the search was on. Unfortunately the planet was dim (this both Adams and Leverrier had predicted) and seeking it meant sweeping a certain small area of the sky and plotting all the stars visible in the telescope then, a few nights later, doing the same again, and finally examining the results to see if one of the stars had moved. It was a tedious task and took time, especially as Challis allowed himself to be side-tracked on to other matters. But if Adams was willing to wait, Leverrier was far from being agreeable to sitting patiently by and when no continental astronomer offered to do anything, he himself wrote to Johann Galle, director of the Berlin Observatory, asking him to initiate a search. It was Galle's good fortune that he had recently made a chart of that area of the sky which Leverrier asked him to observe, and on the 3rd September I846, a year after Adams had completed his solution, Galle was able to report to Leverrier that he had actually seen the planet, which came to be known as Neptune.

For a time there were rival claims about priority of discovery, and it was even suggested by some partisans that Adams had been conjured up by the Astronomer Royal and primed to claim something he had never done. However John Herschel was able to lend his authority and international reputation to help calm what might have become an unfortunate scientific contretemps, so that posterity has been able to give due credit both to Adams and Leverrier, at least on the English side of the Channel. The actual effect of the discovery was astounding. To the popular imagination and to the scientific as well, the discovery of a totally unknown and unsuspected planet by dint of nothing more nor less than years of laborious calculation seemed almost miraculous, and showed forcibly to everyone the power of the new science which had been growing steadily since the Renaissance. In addition it was a glorious vindication of Newton's law of universal gravitation.