An Introduction to the History of Science

by Walter Libby

The prediction and discovery of Neptune, to which many minds contributed, and which has been described with a show of justice as a movement of the times, arose from the previous discovery of the planet Uranus by Sir William Herschel in 1781. After that event Bode suggested that it was possible other astronomers had observed Uranus before, without recognizing it as a planet. By a study of the star catalogues this conjecture was soon verified. It was found that Flamsteed had made, in 1690, the first observation of the heavenly body now called Uranus. Ultimately it was shown that there were at least seventeen similar observations prior to 1781.

It might naturally be supposed that these so-called ancient observations would lead to a ready determination of the planet's orbit, mass, mean distance, longitude with reference to the sun, etc. The contrary, however, seemed to be the case. When Alexis Bouvard, the associate of Laplace, prepared in 1821 tables of Uranus, Jupiter, and Saturn on the principles of the Mécanique Céleste, he was unable to fix an orbit for Uranus which would harmonize with the data of ancient and modern observations, that is, those antecedent and subsequent to Herschel's discovery in 1781. If he computed an orbit from the two sets of data combined, the requirements of the earlier observations were fairly well met, but the later observations were not represented with sufficient precision. If on the other hand only the modern data were taken into account, tables could be constructed meeting all the observations subsequent to 1781, but failing to satisfy those prior to that date. A consistent result could be obtained only by sacrificing the modern or the ancient observations. "I have thought it preferable," says Bouvard, "to abide by the second [alternative], as being that which combines the greater number of probabilities in favor of the truth, and I leave it to the future to make known whether the difficulty of reconciling the two systems result from the inaccuracy of ancient observations, or whether it depend upon some extraneous and unknown influence, which has acted on the planet." It was not till three years after the death of Alexis Bouvard that the extraneous influence, of which he thus gave in 1821 some indication, became fully known.

Almost immediately, however, after the publication of the tables, fresh discrepancies arose between computation and observation. At the first meeting of the British Association in 1832 Professor Airy in a paper on the Progress of Astronomy showed that observational data in reference to the planet Uranus diverged widely from the tables of 1821. In 1833 through his influence the "reduction of all the planetary observations made at Greenwich from 1750" was undertaken. Airy became Astronomer Royal in 1835, and continued to take special interest in Uranus, laying particular emphasis on the fact that the radius vector assigned in the tables to this planet was much too small.

In 1834 the Reverend T. J. Hussey, an amateur astronomer, had written to Airy in reference to the irregularities in the orbit of Uranus: "The apparently inexplicable discrepancies between the ancient and modern observations suggested to me the possibility of some disturbing body beyond Uranus, not taken into account because unknown.... Subsequently, in conversation with Bouvard, I inquired if the above might not be the case." Bouvard answered that the idea had occurred to him; indeed, he had had some correspondence in reference to it in 1829 with Hansen, an authority on planetary perturbations.

In the following year Nicolai (as well as Valz) was interested in the problem of an ultra-Uranian planet in connection with the orbit of Halley's comet (itself the subject of a striking scientific prediction fulfilled in 1758), now reappearing, and under the disturbing influence of Jupiter. In fact, the probability of the approaching discovery of a new planet soon found expression in popular treatises on astronomy. Mrs. Somerville in her book on The Connection of the Physical Sciences (1836) said that the discrepancies in the records of Uranus might reveal the existence and even "the mass and orbit of a body placed for ever beyond the sphere of vision." Similarly Mädler in his Popular Astronomy (1841) took the view that Uranus might have been predicted by study of the perturbations it produced in the orbit of Saturn. Applying this conclusion to a body beyond Uranus we, he continued, "may, indeed, express the hope that analysis will one day or other solemnize this, her highest, triumph, making discoveries with the mind's eye in regions where, in our actual state, we are unable to penetrate."

One should not pass over in this account the labors of Eugène Bouvard, the nephew of Alexis, who continued to note anomalies in the orbit of Uranus and to construct new planetary tables till the very eve of the discovery of Neptune. In 1837 he wrote to Airy that the differences between the observations of Uranus and the calculation were large and were becoming continually larger: "Is that owing to a perturbation brought about in this planet by some body situated beyond it? I don't know, but that's my uncle's opinion."

In 1840 the distinguished astronomer Bessel declared that attempts to explain the discrepancies "must be based on the endeavor to discover an orbit and a mass for some unknown planet, of such a nature, that the resulting perturbations of Uranus may reconcile the present want of harmony in the observations." Two years later he undertook researches in reference to the new planet of whose existence he felt certain. His labors, however, were interrupted by the death of his assistant Flemming, and by his own illness, which proved fatal in 1846, a few months before the actual discovery of Neptune. It is evident that the quest of the new planet had become general. The error of Uranus still amounted to less than two minutes. This deviation from the computed place is not appreciable by the naked eye, yet it was felt, by the scientific world, to challenge the validity of the Newtonian theory, or to foreshadow the addition of still another planet to our solar system.

In July, 1841, John Couch Adams, a young undergraduate of St. John's College, Cambridge, whose interest had been aroused by reading Airy's paper on the Progress of Astronomy, made note of his resolution to attempt, after completing his college course, the solution of the problem then forming in so many minds. After achieving the B.A. as senior wrangler at the beginning of 1843, Adams undertook to "find the most probable orbit and mass of the disturbing body which has acted on Uranus." The ordinary problem in planetary perturbations calls for the determination of the effect on a known orbit exerted by a body of known mass and motion. This was an inverse problem; the perturbation being given, it was required to find the position, mass, and orbit of the disturbing planet. The data were further equivocal in that the elements of the given planet Uranus were themselves in doubt; the unreliability of its planetary tables, in fact, being the occasion of the investigation now undertaken. That thirteen unknown quantities were involved indicates sufficiently the difficulty of the problem.