Prediction is no novelty in science; and in astronomy least of all is it a novelty. Thousands of years ago Thales, and others whose very names we have forgotten, could predict eclipses, but not without a certain degree of inaccuracy. And many other phenomena were capable of prediction by accumulated experience. A gap between Mars and Jupiter caused a missing planet to be suspected and looked for, and to be found in a hundred pieces. The abnormal proper-motion of Sirius suggested to Bessel the existence of an unseen companion. And these last instances seem to approach very near the same class of prediction as that of the discovery of Neptune. Wherein, then, lies the difference? How comes it that some classes of prediction—such as that if you put your finger in fire it will be burned—are childishly easy and commonplace, while others excite in the keenest intellects the highest feelings of admiration? Mainly, the difference lies, first, in the grounds on which the prediction is based; second, in the difficulty of the investigation whereby it is accomplished; third, in the completeness and the accuracy with which it can be verified. In all these points, the discovery of Neptune stands out as one among the many verified predictions of science, and the circumstances surrounding it are of singular interest.
Three distinct observations suffice to determine the orbit of a planet completely, but it is well to have the three observations as far apart as possible so as to minimize the effects of minute but necessary errors of observation. When Uranus was found old records of stellar observations were ransacked with the object of discovering whether it had ever been unwittingly seen before. If seen, it had been thought, of course, to be a star—for it shines like a star of the sixth magnitude, and can therefore be just seen without a telescope if one knows precisely where to look for it and if one has good sight—but if it had been seen and catalogued as a star it would have moved from its place, and the catalogue would by that entry be wrong. The thing to do, therefore, was to examine all the catalogues for errors, to see whether the stars entered there actually existed, or whether any were missing. If a wrong entry were discovered, it might of course have been due to some clerical error, though that is hardly probable considering the care spent in making these records, or it might have been a tailless comet, or possibly the newly found planet.
The next thing to do was to calculate backward, to see whether by any possibility the planet could have been in that place at that time. Examined in this way the tabulated observations of Flamsteed showed that he had unwittingly observed Uranus five distinct times; the first time in 1690, nearly a century before Herschel discovered its true nature. But more remarkable still, Le Monnier, of Paris, had observed it eight times in one month, cataloguing it each time as a different star. If only he had reduced and compared his observations, he would have anticipated Herschel by twelve years. As it was, he missed it. It was seen once by Bradley also. Altogether it had been seen twenty times.
These old observations of Flamsteed and those of Le Monnier, combined with those made after Herschel's discovery, were very useful in determining an exact orbit for the new planet, and its motion was considered thoroughly known. For a time Uranus seemed to travel regularly, and as expected, in the orbit which had been calculated for it; but early in the present century it began to be slightly refractory, and by 1820 its actual place showed quite a distinct discrepancy from its position as calculated with the aid of the old observations. It was thought at first that this discrepancy must be due to inaccuracies in the older observations, and they were accordingly rejected, and tables prepared for the planet based on the newer and more accurate observations only. But by 1830 it became apparent that it did not coincide with even these. The error amounted to about 20". By 1840 it was as much as 90", or a minute and a half. This discrepancy is quite distinct, but still it is very small; and had two objects been in the heavens at once, the actual Uranus and the theoretical Uranus, no unaided eye could possibly have distinguished them or detected that they were other than a single star.
The errors of Uranus, though small, were enormously greater than other things which had certainly been observed; there was an unmistakable discrepancy between theory and observation. Some cause was evidently at work on this distant planet, causing it to disagree with its motion as calculated according to the law of gravitation. If the law of gravitation held exactly at so great a distance from the sun, there must be some perturbing force acting on it besides all the known forces that had been fully taken into account. Could it be an outer planet? The question occurred to several, and one or two tried to solve the problem, but were soon stopped by the tremendous difficulties of calculation.
The ordinary problem of perturbation is difficult enough: Given a disturbing planet in such and such a position, to find the perturbations it produces. This was the problem that Laplace worked out in the Mécanique Céleste.
But the inverse problem—given the perturbations, to find the planet that causes them—such a problem had never yet been attacked, and by only a few had its possibility been conceived. Friedrich Bessel made preparations for solving this mystery in 1840, but he was prevented by fatal illness.
In 1841 the difficulties of the problem presented by these residual perturbations of Uranus excited the imagination of a young student, an undergraduate of Cambridge—John Couch Adams by name—and he determined to make a study of them as soon as he was through his tripos. In January, 1843, he was graduated as senior wrangler, and shortly afterward he set to work. In less than two years he reached a definite conclusion; and in October, 1845, he wrote to the astronomer-royal, at Greenwich, Professor Airy, saying that the perturbations of Uranus could be explained by assuming the existence of an outer planet, which he reckoned was now situated in a specified latitude and longitude.
We know now that had the astronomer-royal put sufficient faith in this result to point his big telescope at the spot indicated and begin sweeping for a planet, he would have detected it within 1-3/4º of the place assigned to it by Adams. But anyone in the situation of the astronomer-royal knows that almost every post brings absurd letters from ambitious correspondents, some of them having just discovered perpetual motion, or squared the circle, or proved the earth flat, or discovered the constitution of the moon or of ether or of electricity; and in this mass of rubbish it requires great skill and patience to detect such gems of value as may exist.
Now this letter of Adams's was indeed a jewel of the first water, and no doubt bore on its face a very different appearance from the chaff of which I have spoken; but still Adams was unknown: he had been graduated as senior wrangler, it is true, but somebody must be graduated as senior wrangler every year, and a first-rate mathematician is not produced every year. Those behind the scenes—as Professor Airy of course was, having been a senior wrangler himself—knew perfectly well that the labeling of a young man on his taking his degree is much more worthless as a testimony to his genius and ability than the general public is apt to suppose.