SCIENTIFIC PREDICTION--THE DISCOVERY OF NEPTUNE
Under this heading we have to consider a single illustration--the
prediction, and the discovery, in 1846, of the planet Neptune. This
event roused great enthusiasm among scientists as well as in the popular
mind, afforded proof of the reliability of the Newtonian hypothesis, and
demonstrated the precision to which the calculation of celestial motions
had attained. Scientific law appeared not merely as a formulation and
explanation of observed phenomena but as a means for the discovery of
new truths. "Would it not be admirable," wrote Valz to Arago in 1835,
"to arrive thus at a knowledge of the existence of a body which cannot
be perceived?"
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.
Adams started with the assumptions, not improbable, that the orbit of
the unknown planet was a circle, and that its distance from the sun was
twice that of Uranus. This latter assumption was in accord with the
so-called "Bode's Law," which taught that a simple numerical
relationship exists between the planetary distances (4, 7, 10, 16, 28,
52, 100, 196), and that the planets as they lie more remote from the sun
tend to be more nearly double the distance of the next preceding. Adams
was encouraged, by his first attempt, to undertake a more precise
determination.
On his behalf Professor Challis of Cambridge applied to Astronomer Royal
Airy, who furnished the _Reductions of the Planetary Observations_ made
at Greenwich from 1750 till 1830. In his second endeavor Adams assumed
that the unknown planet had an elliptical orbit. He approached the
solution gradually, ever taking into account more terms of the
perturbations. In September, 1845, he gave the results to Challis, who
wrote to Airy on the 22d of that month that Adams sought an opportunity
to submit the solution personally to the Astronomer Royal. On the 21st
of October, 1845, the young mathematician, twice disappointed in his
attempt to meet Airy, left at the Royal Observatory a paper containing
the elements of the new planet. The position assigned to it was within
about one degree of its actual place.
On November 5 Airy wrote to Adams and, among other things, inquired
whether the solution obtained would account for the errors of the
radius vector as well as for those of heliocentric longitude. For Airy
this was a crucial question; but to Adams it seemed unessential, and he
failed to reply.
By this time a formidable rival had entered the field. Leverrier at the
request of Arago had undertaken to investigate the irregularities in the
tables of Uranus. In September of the same year Eugène Bouvard had
presented new tables of that planet. Leverrier acted very promptly and
systematically. His first paper on the problem undertaken appeared in
the _Comptes Rendus_ of the Académie des Sciences November 10, 1845. He
had submitted to rigorous examination the data in reference to the
disturbing influence of Jupiter and of Saturn on the orbit of Uranus. In
his second paper, June 1, 1846, Leverrier reviewed the records of the
ancient and modern observations of Uranus (279 in all), subjected
Bouvard's tables to severe criticism, and decided that there existed in
the orbit of Uranus anomalies that could not be accounted due to errors
of observation. There must exist some extraneous influence, hitherto
unknown to astronomers. Some scientists had thought that the law of
gravitation did not hold at the confines of the solar system (others
that the attractive force of other systems might prove a factor), but
Leverrier rejected this conception. Other theories being likewise
discarded he asked: "Is it possible that the irregularities of Uranus
are due to the action of a disturbing planet, situated in the ecliptic
at a mean distance double that of Uranus? And if so, at what point is
this planet situated? What is its mass? What are the elements of the
orbit which it describes?" The conclusion reached by the calculations
recorded in this second paper was that all the so-called anomalies in
the observations of Uranus could be explained as the perturbation caused
by a planet with a heliocentric longitude of 252° on January 1, 1800.
This would correspond to 325° January 1, 1847.
Airy received Leverrier's second paper on June 23, and was struck by the
fact that the French mathematician assigned the same place to the new
planet as had Adams in the preceding October. He wrote to Leverrier in
reference to the errors of the radius vector and received a satisfactory
and sufficiently compliant reply. At one time the Astronomer Royal had
felt very skeptical about the possibility of the discovery which his own
labors had contributed to advance. He had always, to quote his own
rather nebulous statement, considered the correctness of a distant
mathematical result to be the subject of moral rather than of
mathematical evidence. Now that corroboration of Adams's results had
arrived, he felt it urgent to make a telescopic examination of that part
of the heavens indicated by the theoretical findings of Adams and
Leverrier. He accordingly wrote to Professor Challis, July 9, requesting
him to employ for the purpose the great Northumberland equatorial of the
Cambridge Observatory.
Professor Challis had felt, to use his own language, that it was so
novel a thing to undertake observations in reliance upon merely
theoretical deductions, that, while much labor was certain, success
appeared very doubtful. Nevertheless, having received fresh instructions
from Adams relative to the theoretical place of the new planet, he
began observations July 29. On August 4 in fixing certain reference
points he noted, but mistook for a star, the new planet. On August 12,
having directed the telescope in accordance with Adams's instructions he
again noted the same heavenly body, as a star. Before Challis had
compared the results of the observation of August 12 with the results of
an observation of the same region made on July 30, and arrived at the
inference that the body in question, being absent in the latter
observation, was not a star but a planet, the prize of discovery had
fallen into the hands of another observer.
On August 31 had appeared Leverrier's third paper, in which were stated
the new planet's orbit, mass, distance from the sun, eccentricity, and
longitude. The true heliocentric longitude was given as 326° 32' for
January 1, 1847. This determination placed the planet about 5° to the
east of star δ of Capricorn. Leverrier said it might be recognized by
its disk, which, moreover, would subtend a certain angle.
The systematic and conclusive character of Leverrier's research,
submitted to one of the greatest academies of science, carried
conviction to the minds of astronomers. The learned world felt itself on
the eve of a great discovery. Sir John Herschel, in an address before
the British Association on September 10, said that the year past had
given prospect of a new planet. "We see it as Columbus saw America from
the shores of Spain. Its movements have been felt trembling along the
far-reaching line of our analysis with a certainty hardly inferior to
ocular demonstration."
On September 18 Leverrier sent a letter to Dr. Galle, of the Berlin
Observatory, which was provided with a set of star maps, prepared at the
instance of Bessel. Galle replied one week later. "The planet, of the
position of which you gave the indication, really exists. The same day
that I received your letter [September 23] I found a star of the eighth
magnitude, which was not inscribed in the excellent map (prepared by Dr.
Bremiker) belonging to the collection of star maps of the Royal Academy
of Berlin. The observation of the following day showed decisively that
it was the planet sought." It was only 57' from the point predicted.
Arago said that the discovery made by Leverrier was one of the most
brilliant manifestations of the precision of modern astronomic science.
It would encourage the best geometers to seek with renewed ardor the
eternal truths which, in Pliny's phrase, are latent in the majesty of
theory.
Professor Challis received Leverrier's third paper on September 29, and
in the evening turned his magnificent refractor to the part of the
heavens that Leverrier had so definitely and so confidently indicated.
Among the three hundred stars observed Challis was struck by the
appearance of one which presented a disk and shone with the brightness
of a star of the eighth magnitude. This proved to be the planet. On
October 1 Challis heard that the German observer had anticipated him.
Arago, while recognizing the excellent work done by Adams in his
calculations, thought that the fact that the young mathematician had
failed to publish his results should deprive him of any share whatever
in the glory of the discovery of the new planet, and that history would
confirm this definite judgment. Arago named the new planet after the
French discoverer, but soon acquiesced in the name Neptune, which has
since prevailed.
Airy, in whose possession Adams's results had remained for months
unpublished and unheeded, wrote Leverrier: "You are to be recognized
beyond doubt as the predictor of the planet's place." A vigorous
official himself, Airy was deeply impressed by the calm decisiveness and
definite directions of the French mathematician. "It is here, if I
mistake not, that we see a character far superior to that of the able,
or enterprising, or industrious mathematician; it is here that we see
the philosopher." This explains, if anything could, his view that a
distant mathematical result is the subject of ethical rather than of
mathematical evidence.
Adams's friends felt that he had not received from either of the
astronomers, to whom he confided his results, the kind of help or advice
he should have received. Challis was kindly, but wanting in initiative.
Although he had command of the great Northumberland telescope, he had no
thought of commencing the search in 1845, for, without mistrusting the
evidence which the theory gave of the _existence_ of the planet, it
might be reasonable to suppose that its position was determined but
roughly, and that a search for it must necessarily be long and
laborious. In the view of Simon Newcomb,[3] Adams's results, which were
delivered at the Greenwich Observatory October 21, 1845, were so near to
the mark that a few hours' close search could not have failed to make
the planet known.
Both Adams and Leverrier had assumed as a rough approximation at
starting that the orbit of the new planet was circular and that, in
accordance with Bode's Law, its distance was twice that of Uranus. S. C.
Walker, of the Smithsonian Institution, Washington, was able to
determine the elements of the orbit of Neptune accurately in 1847. In
February of that year he had found (as had Petersen of Altona about the
same time) that Lalande had in May, 1795, observed Neptune and mistaken
it for a fixed star. When Lalande's records in Paris were studied, it
was found that he had made two observations of Neptune on May 8 and 10.
Their failure to agree caused the observer to reject one and mark the
other as doubtful. Had he repeated the observation, he might have noted
that the _star_ moved, and was in reality a planet.
Neptune's orbit is more nearly circular than that of any of the major
planets except Venus. Its distance is thirty times that of the earth
from the sun instead of thirty-nine times, as Bode's Law would require.
That generalization was a presupposition of the calculations leading to
the discovery. It was then rejected like a discredited ladder. Man's
conception of the universe is widened at the thought that the outmost
known planet of our solar system is about 2,796,000,000 miles from the
sun and requires about 165 years for one revolution.
Professor Peirce, of Harvard University, pointing to the difference
between the calculations of Leverrier and the facts, put forward the
view that the discovery made by Galle must be regarded as a happy
accident. This view, however, has not been sustained.
REFERENCES
Sir Robert Ball, Neptune's Jubilee Year, _Scientific American_,
Supplement, Oct. 10, 1896.
Sir Robert Ball, _The Story of the Heavens_, chap. XV.
B. A. Gould, _Report on the History of the Discovery of Neptune,
Smithsonian Contributions to Knowledge_, 1850.
Robert Grant, _History of Physical Astronomy_.
Simon Newcomb, _Popular Astronomy_.
Benjamin Peirce, _Proceedings of the American Academy of Arts and
Sciences_, vol. I, pp. 57-68, 144, 285, 338-41, etc.
FOOTNOTES:
[3] See article "Neptune," _Encyc. Brit._