Discovery Of Neptune
Author: Lodge, Sir Oliver
Discovery Of Neptune

1846

Among modern astronomical discoveries none has been regarded as more
important than that of Neptune, the outermost known planet of the solar
system. It was a rich reward to the watchers of the sky when this new planet
swam into their ken. This discovery was hailed by astronomers as "the most
conspicuous triumph of the theory of gravitation." Long after Copernicus even,
the genius of philosophers was slow to grasp the full conception of a
spherical earth and its relations with the heavenly bodies as presented by
him. So it was also with the final acceptance of Newton's demonstration of
the universal law of gravitation (1685), whereby he showed that "the motions
of the solar system were due to the action of a central force directed to the
body at the centre of the system, and varying inversely with the square of the
distance from it." After making this discovery, Newton himself, with the aid
of others, especially of the French mathematician Picard, labored for years to
verify it, and still further verification was necessary before it could be
fully comprehended and accepted by the scientific world. The discovery of the
asteroids or small planets revolving in orbits between those of Mars and
Jupiter, aided in confirming the Newtonian theory, which the discovery of
Uranus, by Sir William Herschel (1781), had done much to establish.

From the time of Sir William Herschel the science of stellar astronomy,
revealing the enormous distances of the stars - none of them really fixed, but
all having real or apparent motions - was rapidly developed. The discovery of
stellar planets, at almost incalculable distances, still further changed the
aspect of the heavens as viewed by astronomers, and when the capital discovery
of Neptune was made those men of science were well prepared for studying its
nature and importance. These matters, as well as the simultaneous calculation
of the place of Neptune by Adams and Leverrier, and its actual discovery by
Galle, are set forth by Sir Oliver Lodge in a manner as charming for
simplicity as it is valuable in its summary of scientific learning.

The explanation by Newton of the observed facts of the motion of the
moon, the way he accounted for precession and nutation and for the tides; the
way in which Laplace explained every detail of the planetary motions - these
achievements may seem to the professional astronomer equally, if not more,
striking and wonderful; but of the facts to be explained in these cases the
general public is necessarily more or less ignorant, and so no beauty or
thoroughness of treatment appeals to it or excites its imagination. But to
predict in the solitude of the study, with no weapons other than pen, ink, and
paper, an unknown and enormously distant world, to calculate its orbit when as
yet it had never been seen, and to be able to say to a practical astronomer,
"Point your telescope in such a direction at such a time, and you will see a
new planet hitherto unknown to man" - this must always appeal to the
imagination with dramatic intensity, and must awaken some interest in the
dullest.

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 inches. By 1840 it was as
much as 90 inches, 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 Mecanique
Celeste.

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 3/4 degrees 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 labelling 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.

Was it likely that a young and unknown man should have solved so
extremely difficult a problem? It was altogether unlikely. Still, he should
be tested: he should be asked for explanations concerning some of the
perturbations which Professor Airy had noticed, and see whether he could
explain these also by his hypothesis. If he could, there might be something
in his theory. If he failed - well, there was an end of it. The questions
were not difficult. They concerned the error of the radius vector. Adams
could have answered them with perfect ease; but sad to say, though a brilliant
mathematician, he was not a man of business. He did not answer Professor
Airy's letter.

It may seem a pity to many that the Greenwich equatorial was not pointed
at the place, just to see whether any foreign object did happen to be in that
neighborhood; but it is no light matter to derange the work of an observatory,
and alter the plans laid out for the staff, into a sudden sweep for a new
planet on the strength of a mathematical investigation just received by post.
If observatories were conducted on these unsystematic and spasmodic principles
they would not be the calm, accurate, satisfactory places they are.

Of course, if anyone had known that a new planet was to be found for the
looking, any course would have been justified; but no one could know this. I
do not suppose that Adams himself felt an absolute confidence in his attempted
prediction. So there the matter dropped. Adam's communication was
pigeonholed, and remained in seclusion eight or nine months.

Meanwhile, and quite independently, something of the same sort was going
on in France. A brilliant young mathematician, Urbain Jean Joseph Leverrier,
born in Normandy in 1811, held the post of astronomical professor at the Ecole
Polytechnique, founded by Napoleon. His first published papers directed
attention to his wonderful powers; and the official head of astronomy in
France, the famous Arago, suggested to him the unexplained perturbations of
Uranus as a worthy object for his fresh and well-armed vigor. At once he set
to work in a thorough and systematic way. He first considered whether the
discrepancies could be due to errors in the tables or errors in the old
observations. He discussed them with minute care, and came to the conclusion
that they were not thus to be explained away. This part of the work he
published in November, 1845.

He then set to work to consider the perturbations produced by Jupiter and
Saturn to see whether they had been accurately allowed for, or whether some
minute improvements could be made sufficient to destroy the irregularities.
He introduced several fresh terms into these calculations, but none of them of
sufficient importance to do more than partly explain the mysterious
perturbations. He next examined the various hypotheses that had been
suggested to account for them. Were they caused by a failure in the law of
gravitation or by the presence of a resisting medium? Were they due to some
large but unseen satellite or to a collision with some comet?

All these theories he examined and dismissed for various reasons. The
perturbations were due to some continuous cause - for instance, some unknown
planet. Could this planet be inside the orbit of Uranus? No, for then it
would perturb Saturn and Jupiter also, and they were not perturbed by it. It
must, therefore, be some planet outside the orbit of Uranus, and in all
probability, according to Bode's empirical law, at nearly double the distance
from the sun that Uranus is. Finally he proceeded to determine where this
planet was, and what its orbit must be to produce the observed disturbances.

Not without failures and disheartening complications was this part of the
process completed. This was, after all, the real tug of war. Many unknown
quantities existed: its mass, its distance, its eccentricity, the obliquity of
its orbit, its position - nothing was known, in fact, about the planet except
the microscopic disturbance it caused in Uranus, several thousand million
miles away from it. Without going into further detail, suffice it to say that
in June, 1846, he published his last paper, and in it announced to the world
his theory as to the situation of the planet.

Professor Airy received a copy of this paper before the end of the month,
and was astonished to find that Leverrier's theoretical place for the planet
was within 1 degree of the place Adams had assigned to it eight months before.
So striking a coincidence seemed sufficient to justify a Herschelian sweep for
a week or two. But a sweep for so distant a planet would be no easy matter.
When seen through a large telescope it would still only look like a star, and
it would require considerable labor and watching to sift it out from the other
stars surrounding it. We know that Uranus had been seen twenty times, and
thought to be a star, before its true nature was discovered by Herschel; and
Uranus is only about half as far away as Neptune.

Neither at Paris nor at Greenwich was any optical search undertaken; but
Professor Airy wrote to ask M. Leverrier the same old question that he had
fruitlessly put to Adams: Did the new theory explain the errors of the radius
vector or not? The reply of Leverrier was both prompt and satisfactory -
these errors were explained, as well as all the others. The existence of the
object was then for the first time officially believed in. The British
Association met that year at Southampton, and Sir John Herschel was one of its
sectional presidents. In his inaugural address, on September 10, 1846, he
called attention to the researches of Leverrier and Adams in these memorable
words:

"The past year has given to us the new [minor] planet Astraea; it has
done more - it has given us the probable prospect of another. 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."

It was nearly time to begin to look for it. So the astronomer-royal
thought on reading Leverrier's paper. But as the national telescope at
Greenwich was otherwise occupied, he wrote to Professor Challis, at Cambridge,
to know whether he would permit a search to be made for it with the
Northumberland equatorial, the largetelescope at Cambridge University,
presented to it by one of the Dukes ofNorthumberland.

Professor Challis said he would conduct the search himself, and shortly
began a leisurely and dignified series of sweeps around the place designated
by theory, cataloguing all the stars he observed, intending afterward to sort
out his observations, compare one with another, and find out whether any one
star had changed its position; because if it had it must be the planet. Thus,
without giving an excessive time to the business, he accumulated a host of
observations.

Professor Challis thus actually saw the planet twice - on August 4 and
August 12, 1846 - without knowing it. If he had had a map of the heavens
containing telescopic stars down to the tenth magnitude, and if he had
compared his observations with this map as they were made, the process would
have been easy and the discovery quick. But he had no such map. Nevertheless
one was in existence. It had just been completed in that country of
enlightened method and industry - Germany. Doctor Bremiker had not indeed
completed his great work - a chart of the whole zodiac down to stars of the
tenth magnitude - but portions of it were completed, and the special region
where the new planet was expected to appear happened to be among the portions
finished. But in England this was not known.

Meanwhile Adams wrote to the astronomer-royal several additional
communications, making improvements in his theory, and giving what he
considered nearer and nearer approximations for the place of the planet. He
also now answered quite satisfactorily, but too late, the question about the
radius vector sent to him months before.

Leverrier was likewise engaged in improving this theory and in
considering how best the optical search could be conducted. Actuated probably
by the knowledge that in such matters as cataloguing and mapping Germany was
then, as now, far ahead of all the other nations, he wrote in September (the
same year that Sir John Herschel delivered his eloquent address at
Southampton) to Berlin. Leverrier wrote to Doctor Galle, head of the
observatory at Berlin, saying to him, clearly and decidedly, that the new
planet was now in or close to such and such a position, and that if he would
point his telescope to that part of the heavens he would see it; and moreover
that he would be able to tell it from a star by its having a sensible
magnitude, or disk, instead of being a mere point.

Galle got the letter on September 23, 1846. That same evening he pointed
his telescope to the place Leverrier told him, and saw the planet. He
recognized it first by its appearance. To his practised eye it did seem to
have a small disk, and not quite the same aspect as an ordinary star. He then
consulted Bremiker's great star-chart, the part just engraved and finished,
and, sure enough, no such star was there. Undoubtedly it was the planet.

The news flashed over Europe at the maximum speed with which news could
travel at that date (which was not very fast); and by October 1st Professor
Challis and Mr. Adams heard it at Cambridge, and realized that in so far as
there was competition in such a matter England was out of the race.

It was an unconscious race to all concerned, however. The French
scientists knew nothing of the search in England. Adams's papers had never
been published; and very annoyed the French were when a claim was set up in
his behalf to a share in this magnificent discovery. As for Adams himself, we
are told that by no word did he show resentment at the loss of the practical
consummation of his discovery. His part in any controversy that arose was
claim and dignified; but for a time his friends fought a public battle for his
fame. It so happened that the public took a keener interest than it usually
takes in scientific predictions; but the discussion has now settled down. All
the world honors the bright genius and mathematical skill of John Couch Adams,
and recognizes that he first solved the problem by calculation. All the
world, too, perceives clearly the no less eminent mathematical talents of M.
Leverrier, but it recognizes in him something more than the mere mathematician
- the man of energy, decision, and character.

 

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