NEPTUNE.
We now come to the best known planet of the solar system, reckoning
outwards from the Sun, and though this planet itself, as an object to
look at, has no particular interest for the general public, yet the
history of its discovery is a matter of extreme interest. Moreover, it
is very closely mixed up with the history of the planet Uranus, which
has just been described. After Uranus had become fully recognised as a
regular member of the solar system, a French astronomer named Alexis
Bouvard set himself the task of exhaustively considering the movements
of Uranus with a view of determining its orbit with the utmost possible
exactness. His available materials ranged themselves in two groups:—the
modern observations between 1781 and 1820, and the early observations of
Flamsteed, Bradley, Mayer, and Le Monnier, extending from 1690 to 1771.
Bouvard found in substance that he could frame an orbit which would fit
in with each group of observations, but that he could not obtain an
orbit which would reconcile both sets of observations during the 130
years over which they jointly extended. He therefore rashly came to the
conclusion that the earlier observations, having been made when methods
and instruments were alike relatively imperfect, were probably
inaccurate or otherwise untrustworthy, and had better be rejected. This
seemed for awhile to solve the difficulty, and results which he
published in 1821 represented with all reasonable accuracy the then
movements of the planet. A very few years, however, sufficed to reveal
discordances between observation and theory, so marked and regular as to
make it perfectly clear that it was not Bouvard’s work which was faulty
but that Uranus itself had gone astray through the operation of definite
but as yet unknown causes. What these causes were could only be a matter
of surmise based upon the evident fact that there was some source of
disturbance which was evidently throwing Uranus out of its proper place
and regular course. First one and then another astronomer gave attention
and thought to the matter, and eventually the conclusion was arrived at
that there existed, more remote from the Sun than Uranus, an
undiscovered planet which was able to make its influence felt by
deranging the movements of Uranus in its ordinary journey round the Sun
every 84 years. This conclusion on the part of astronomers becoming
known, a young Cambridge student, then at St. John’s College, John Couch
Adams by name, resolved, in July 1841, to take up the subject, though it
was not until 1843 that he actually did so. The problem to be solved was
to suggest the precise place in the sky at a given time of an imaginary
planet massive enough to push, or pull, out of its normal place the
planet Uranus, which was evidently being pushed at one time and pulled
at another. It would also be part of the problem to predict the distance
from the Sun of the planet thus imagined to exist. Adams worked
patiently and silently at this very profound and difficult problem for
1¾ years when he found himself able to forward to Airy, who had become
Astronomer Royal after being a Cambridge Professor, some provisional
elements of an imaginary planet of a size, at a distance, and in a
position to meet the circumstances. It is greatly to be regretted, on
more grounds than one, that Airy did nothing but pigeon-hole Adams’s
papers. Had he done what might have been, and probably was, expected,
that is, had he made them public, or better still had he made telescopic
use of them, a long and unpleasant international controversy would have
been avoided, and Adams would not have been robbed in part of the
well-deserved fruits of his protracted labours.
We must now turn to consider something that was happening in France. In
the summer of 1845, just before Adams had finished his work, and one and
a half years after he commenced it a young Frenchman, who afterwards
rose to great eminence, U. J. J. Le Verrier, turned his attention to the
movements of Uranus with a view of ascertaining the cause of their
recognised irregularity. In November 1845 he made public the conclusion
that those irregularities did not exclusively depend upon Jupiter or
Saturn. He followed this up in June 1846 by a second memoir to prove
that an unknown exterior planet was the cause of all the trouble, and he
assigned evidence as to its position very much as Adams had done 8
months previously. Airy on receiving a copy of Le Verrier’s memoir seems
so far, at last, to have been roused that he took the trouble to compare
Le Verrier’s conclusions with those of Adams so long in his possession
neglected. Finding that a remarkably close accord existed between the
conclusions of the two men, he came to realise that both must be of
value, and he wrote a fortnight later to suggest to Professor Challis
the desirability of his instituting a search for the suspected planet.
Challis began within two days, but was handicapped by not having in his
possession any map of the stars in the neighbourhood suggested as the
_locale_ of the planet. He lost no time however in making such a map,
but, of course, the doing so caused an appreciable delay, and it was not
until September 29, 1846, that he found an object which excited his
suspicions and eventually proved to be the planet sought for. It was
subsequently ascertained that the planet had been recorded as a star on
August 4 and 12, and that the star of August 12 was missing from the
zone observed on July 30. The discovery of the planet was therefore just
missed on August 12 because the results of each evening’s work were not
adequately compared with what had gone before.
Meanwhile things had not been standing still in France. In August 1846,
Le Verrier published a third memoir intended to develope information
respecting the probable position of the planet in the heavens. In
September 23 a summary of this third memoir was received by Encke at
Berlin, accompanied by the request that he would cooperate
instrumentally in the search for it. Encke at once directed two of his
assistants named D’Arrest and Galle to do this, and they were
fortunately well circumstanced for the task. Unlike Challis, who, as we
have seen, could do nothing until he had made a map for himself, the
Berlin observers had one ready to hand, which by good chance had just
been published by the Berlin Academy for the part of the heavens which
both Adams and Le Verrier assigned as the probable locality in which the
anxiously desired planet would be found. Galle called out the visible
stars one by one whilst D’Arrest checked them by the map, and suddenly
he came upon an unmarked object which at the moment looked like an 8th
magnitude star. The following night showed that the suspicious object
was in motion, and it was soon ascertained to be the trans-Uranian
planet which was being searched for. The discovery when announced
excited the liveliest interest all over the world. It did more; it
created a bitter feeling of resentment on the part of French astronomers
that the laurels claimed by them should have been also claimed in an
equal share by a young and unknown Englishman, and accordingly the old
cry of “_perfide Albion_” arose on all sides. I have been particular in
stating the various dates which belong to this narrative, in order to
make as clear as possible the facts of the case. This is even now
necessary, because though the astronomers of England and Germany are
willing to give Adams and Le Verrier each their fair share of this great
discovery, the same impartial spirit is not to be found in France, for
nothing is more common, even in the present day, in looking at French
books of astronomy, than to find Adams’s name either glossed over or
absolutely suppressed altogether when the planet Neptune is under
discussion.
How remarkable a discovery this was, will perhaps be realized, when it
is stated that Adams was only 2½° out in assigning the position of the
new planet, whilst Le Verrier was even nearer, being barely 1° out.
We know practically nothing respecting the physical appearance of
Neptune, owing to its immense distance from us, and for the like reason
the Neptunian astronomers, if there are any, will know absolutely
nothing about the Earth; indeed, their knowledge of the Solar System
will be restricted to Uranus, Saturn, and the Sun. Even the Sun will
only have an apparent diameter of about 1′ of arc, and, therefore, will
only seem to be a very bright star, yielding light equal in amount,
according to Zöllner, to about 700 full moons. There is one satellite
belonging to Neptune, and as this has been calculated to exhibit a disc
10° in diameter, a certain amount of light will no doubt be afforded by
it especially if, as is not unlikely, Neptune is itself possessed of
some inherent luminosity independently of the Sun.
The fact that Neptune seems destitute of visible spots or belts, results
in our being ignorant of the period of its axial rotation, though it
should be stated that in 1883, Maxwell Hall in Jamaica, observed
periodical fluctuations in its light, which he thought implied that the
planet rotated on its axis in rather less than 8 hours. Several
observers thought 20 or 30 years ago, that they had noticed indications
of Neptune being surrounded by a ring like Saturn’s ring, but the
evidence as to this is very inconclusive. It is quite certain that none
but the very largest telescopes in the world would show any such
appendage, and this planet seems to have been neglected of late years,
by the possessors of such telescopes. Moreover, if a ring existed it
would only open out to its full extent once in every 82 years, being the
half of the period of the planet’s revolution round the Sun (just as
Saturn’s ring only opens out to the fullest extent every 14½ years), so
that, obviously, supposing suspicions of a ring dating back 30 or 40
years were well founded, it might well be that another 30 or 40 years
might need to elapse, before astronomers would be in a position to see
their suspicions revive.
Neptune revolves round the Sun in 164½ years, at a mean distance of 2791
millions of miles. Its apparent diameter scarcely varies from 2¾″. Its
true diameter is about 37,000 miles. No compression of the Poles is
perceptible. Its one satellite revolves round Neptune in 5¾ days, and in
a retrograde direction, at a mean distance of 223,000 miles, and shines
as a star of the 14th magnitude. This is a peculiarity which it only
shares with the satellites of Uranus, so far as it regards the planetary
members of the Solar System, though there are many retrograde Comets.
The question has often been mooted, whether there exists, and belonging
to the Solar System, a planet farther off than Neptune. There does seem
some evidence of this, as we shall better understand, when we come to
the subject of long-period Comets, though it cannot be said that much
progress has yet been made in arriving at a solution of the problem.
Unless there does exist a trans-Neptunian planet, a Neptunian astronomer
will know very little about planets, for Uranus and Saturn will alone be
visible to him. Both will of course be what we call “inferior planets,”
and under the best of circumstances will cut a poor figure in the
Neptunian sky.