planet can affect another in virtue of their mutual attractions. Such
investigations demand the exercise of the highest mathematical
gifts. But not alone is intellectual ability necessary for success
in such inquiries. It must be united with a patient capacity for
calculations of an arduous type, protracted, as they frequently have
to be, through many years of labour. Le Verrier soon found in these
profound inquiries adequate scope for the exercise of his peculiar
gifts. His first important astronomical publication contained an
investigation of the changes which the orbits of several of the
planets, including the earth, have undergone in times past, and which
they will undergo in times to come.
As an illustration of these researches, we may take the case of the
planet in which we are, of course, especially interested, namely, the
earth, and we can investigate the changes which, in the lapse of
time, the earth's orbit has undergone, in consequence of the
disturbance to which it has been subjected by the other planets. In
a century, or even in a thousand years, there is but little
recognisable difference in the shape of the track pursued by the
earth. Vast periods of time are required for the development of the
large consequences of planetary perturbation. Le Verrier has,
however, given us the particulars of what the earth's journey through
space has been at intervals of 20,000 years back from the present
date. His furthest calculation throws our glance back to the state
of the earth's track 100,000 years ago, while, with a bound forward,
he shows us what the earth's orbit is to be in the future, at
successive intervals of 20,000 years, till a date is reached which is
100,000 years in advance of A.D. 1800.
The talent which these researches displayed brought Le Verrier into
notice. At that time the Paris Observatory was presided over by
Arago, a SAVANT who occupies a distinguished position in French
scientific annals. Arago at once perceived that Le Verrier was just
the man who possessed the qualifications suitable for undertaking a
problem of great importance and difficulty that had begun to force
itself on the attention of astronomers. What this great problem was,
and how astonishing was the solution it received, must now be
considered.
Ever since Herschel brought himself into fame by his superb discovery
of the great planet Uranus, the movements of this new addition to the
solar system were scrutinized with care and attention. The position
of Uranus was thus accurately determined from time to time. At
length, when sufficient observations of this remote planet had been
brought together, the route which the newly-discovered body pursued
through the heavens was ascertained by those calculations with which
astronomers are familiar. It happens, however, that Uranus possesses
a superficial resemblance to a star. Indeed the resemblance is so
often deceptive that long ere its detection as a planet by Herschel,
it had been observed time after time by skilful astronomers, who
little thought that the star-like point at which they looked was
anything but a star. From these early observations it was possible
to determine the track of Uranus, and it was found that the great
planet takes a period of no less than eighty-four years to accomplish
a circuit. Calculations were made of the shape of the orbit in which
it revolved before its discovery by Herschel, and these were compared
with the orbit which observations showed the same body to pursue in
those later years when its planetary character was known. It could
not, of course, be expected that the orbit should remain unaltered;
the fact that the great planets Jupiter and Saturn revolve in the
vicinity of Uranus must necessarily imply that the orbit of the
latter undergoes considerable changes. When, however, due allowance
has been made for whatever influence the attraction of Jupiter and
Saturn, and we may add of the earth and all the other Planets, could
possibly produce, the movements of Uranus were still inexplicable. It
was perfectly obvious that there must be some other influence at work
besides that which could be attributed to the planets already known.
Astronomers could only recognise one solution of such a difficulty.
It was impossible to doubt that there must be some other planet in
addition to the bodies at that time known, and that the perturbations
of Uranus hitherto unaccounted for, were due to the disturbances
caused by the action of this unknown planet. Arago urged Le Verrier
to undertake the great problem of searching for this body, whose
theoretical existence seemed demonstrated. But the conditions of the
search were such that it must needs be conducted on principles wholly
different from any search which had ever before been undertaken for a
celestial object. For this was not a case in which mere survey with
a telescope might be expected to lead to the discovery.
Certain facts might be immediately presumed with reference to the
unknown object. There could be no doubt that the unknown disturber
of Uranus must be a large body with a mass far exceeding that of the
earth. It was certain, however, that it must be so distant that it
could only appear from our point of view as a very small object.
Uranus itself lay beyond the range, or almost beyond the range, of
unassisted vision. It could be shown that the planet by which the
disturbance was produced revolved in an orbit which must lie outside
that of Uranus. It seemed thus certain that the planet could not be
a body visible to the unaided eye. Indeed, had it been at all
conspicuous its planetary character would doubtless have been
detected ages ago. The unknown body must therefore be a planet which
would have to be sought for by telescopic aid.
There is, of course, a profound physical difference between a planet
and a star, for the star is a luminous sun, and the planet is merely
a dark body, rendered visible by the sunlight which falls upon it.
Notwithstanding that a star is a sun thousands of times larger than
the planet and millions of times more remote, yet it is a singular
fact that telescopic planets possess an illusory resemblance to the
stars among which their course happens to lie. So far as actual
appearance goes, there is indeed only one criterion by which a planet
of this kind can be discriminated from a star. If the planet be
large enough the telescope will show that it possesses a disc, and
has a visible and measurable circular outline. This feature a star
does not exhibit. The stars are indeed so remote that no matter how
large they may be intrinsically, they only exhibit radiant points of
light, which the utmost powers of the telescope fail to magnify into
objects with an appreciable diameter. The older and well-known
planets, such as Jupiter and Mars, possess discs, which, though not
visible to the unaided eye, were clearly enough discernible with the
slightest telescopic power. But a very remote planet like Uranus,
though it possessed a disc large enough to be quickly appreciated by
the consummate observing skill of Herschel, was nevertheless so
stellar in its appearance, that it had been observed no fewer than
seventeen times by experienced astronomers prior to Herschel. In
each case the planetary nature of the object had been overlooked, and
it had been taken for granted that it was a star. It presented no
difference which was sufficient to arrest attention.
As the unknown body by which Uranus was disturbed was certainly much
more remote than Uranus, it seemed to be certain that though it might
show a disc perceptible to very close inspection, yet that the disc
must be so minute as not to be detected except with extreme care. In
other words, it seemed probable that the body which was to be sought
for could not readily be discriminated from a small star, to which
class of object it bore a superficial resemblance, though, as a
matter of fact, there was the profoundest difference between the two
bodies.
There are on the heavens many hundreds of thousands of stars, and the
problem of identifying the planet, if indeed it should lie among
these stars, seemed a very complex matter. Of course it is the
abundant presence of the stars which causes the difficulty. If the
stars could have been got rid of, a sweep over the heavens would at
once disclose all the planets which are bright enough to be visible
with the telescopic power employed. It is the fortuitous resemblance
of the planet to the stars which enables it to escape detection. To
discriminate the planet among stars everywhere in the sky would be
almost impossible. If, however, some method could be devised for
localizing that precise region in which the planet's existence might
be presumed, then the search could be undertaken with some prospect
of success.
To a certain extent the problem of localizing the region on the sky
in which the planet might be expected admitted of an immediate
limitation. It is known that all the planets, or perhaps I ought
rather to say, all the great planets, confine their movements to a
certain zone around the heavens. This zone extends some way on
either side of that line called the ecliptic in which the earth
pursues its journey around the sun. It was therefore to be inferred
that the new planet need not be sought for outside this zone. It is
obvious that this consideration at once reduces the area to be
scrutinized to a small fraction of the entire heavens. But even
within the zone thus defined there are many thousands of stars. It
would seem a hopeless task to detect the new planet unless some
further limitation to its position could be assigned.
It was accordingly suggested to Le Verrier that he should endeavour
to discover in what particular part of the strip of the celestial
sphere which we have indicated the search for the unknown planet
should be instituted. The materials available to the mathematician
for the solution of this problem were to be derived solely from the
discrepancies between the calculated places in which Uranus should be
found, taking into account the known causes of disturbance, and the
actual places in which observation had shown the planet to exist.
Here was indeed an unprecedented problem, and one of extraordinary
difficulty. Le Verrier, however, faced it, and, to the astonishment
of the world, succeeded in carrying it through to a brilliant
solution. We cannot here attempt to enter into any account of the
mathematical investigations that were necessary. All that we can do
is to give a general indication of the method which had to be
adopted.
Let us suppose that a planet is revolving outside Uranus, at a
distance which is suggested by the several distances at which the
other planets are dispersed around the sun. Let us assume that this
outer planet has started on its course, in a prescribed path, and
that it has a certain mass. It will, of course, disturb the motion
of Uranus, and in consequence of that disturbance Uranus will follow
a path the nature of which can be determined by calculation. It
will, however, generally be found that the path so ascertained does
not tally with the actual path which observations have indicated for
Uranus. This demonstrates that the assumed circumstances of the
unknown planet must be in some respects erroneous, and the astronomer
commences afresh with an amended orbit. At last after many trials,
Le Verrier ascertained that, by assuming a certain size, shape, and
position for the unknown Planet's orbit, and a certain value for the
mass of the hypothetical body, it would be possible to account for
the observed disturbances of Uranus. Gradually it became clear to
the perception of this consummate mathematician, not only that the
difficulties in the movements of Uranus could be thus explained, but
that no other explanation need be sought for. It accordingly
appeared that a planet possessing the mass which he had assigned, and
moving in the orbit which his calculations had indicated, must indeed
exist, though no eye had ever beheld any such body. Here was,
indeed, an astonishing result. The mathematician sitting at his
desk, by studying the observations which had been supplied to him of
one planet, is able to discover the existence of another planet, and
even to assign the very position which it must occupy, ere ever the
telescope is invoked for its discovery.
Thus it was that the calculations of Le Verrier narrowed greatly the
area to be scrutinised in the telescopic search which was presently
to be instituted. It was already known, as we have just pointed out,
that the planet must lie somewhere on the ecliptic. The French
mathematician had now further indicated the spot on the ecliptic at
which, according to his calculations, the planet must actually be
found. And now for an episode in this history which will be
celebrated so long as science shall endure. It is nothing less than
the telescopic confirmation of the existence of this new planet,
which had previously been indicated only by mathematical
calculation. Le Verrier had not himself the instruments necessary
for studying the heavens, nor did he possess the skill of the
practical astronomer. He, therefore, wrote to Dr. Galle, of the
Observatory at Berlin, requesting him to undertake a telescopic
search for the new planet in the vicinity which the mathematical
calculation had indicated for the whereabouts of the planet at that
particular time. Le Verrier added that he thought the planet ought
to admit of being recognised by the possession of a disc sufficiently
definite to mark the distinction between it and the surrounding
stars.
It was the 23rd September, 1846, when the request from Le Verrier
reached the Berlin Observatory, and the night was clear, so that the
memorable search was made on the same evening. The investigation was
facilitated by the circumstance that a diligent observer had recently
compiled elaborate star maps for certain tracts of the heavens lying
in a sufficiently wide zone on both sides of the equator. These maps
were as yet only partially complete, but it happened that Hora. XXI.,
which included the very spot which Le Verrier's results referred to,
had been just issued. Dr. Galle had thus before his eyes a chart of
all the stars which were visible in that part of the heavens at the
time when the map was made. The advantage of such an assistance to
the search could hardly be over-estimated. It at once gave the
astronomer another method of recognising the planet besides that
afforded by its possible possession of a disc. For as the planet was
a moving body, it would not have been in the same place relatively to
the stars at the time when the map was constructed, as it occupied
some years later when the search was being made. If the body should
be situated in the spot which Le Verrier's calculations indicated in
the autumn of 1846, then it might be regarded as certain that it
would not be found in that same place on a map drawn some years
previously.
The search to be undertaken consisted in a comparison made point by
point between the bodies shown on the map, and those stars in the sky
which Dr. Galle's telescope revealed. In the course of this
comparison it presently appeared that a star-like object of the
eighth magnitude, which was quite a conspicuous body in the
telescope, was not represented in the map. This at once attracted
the earnest attention of the astronomer, and raised his hopes that
here was indeed the planet. Nor were these hopes destined to be
disappointed. It could not be supposed that a star of the eighth
magnitude would have been overlooked in the preparation of a chart
whereon stars of many lower degrees of brightness were set down. One
other supposition was of course conceivable. It might have been that
this suspicious object belonged to the class of variables, for there
are many such stars whose brightness fluctuates, and if it had
happened that the map was constructed at a time when the star in
question had but feeble brilliance, it might have escaped notice. It
is also well known that sometimes new stars suddenly develop, so that
the possibility that what Dr. Galle saw should have been a variable
star or should have been a totally new star had to be provided
against.
Fortunately a test was immediately available to decide whether the
new object was indeed the long sought for planet, or whether it was a
star of one of the two classes to which I have just referred. A star
remains fixed, but a planet is in motion. No doubt when a planet
lies at the distance at which this new planet was believed to be
situated, its apparent motion would be so slow that it would not be
easy to detect any change in the course of a single night's
observation. Dr. Galle, however, addressed himself with much skill
to the examination of the place of the new body. Even in the course
of the night he thought he detected slight movements, and he awaited
with much anxiety the renewal of his observations on the subsequent
evenings. His suspicions as to the movement of the body were then
amply confirmed, and the planetary nature of the new object was thus
unmistakably detected.
Great indeed was the admiration of the scientific world at this
superb triumph. Here was a mighty planet whose very existence was
revealed by the indications afforded by refined mathematical
calculation. At once the name of Le Verrier, already known to those
conversant with the more profound branches of astronomy, became
everywhere celebrated. It soon, however, appeared, that the fame
belonging to this great achievement had to be shared between Le
Verrier and another astronomer, J. C. Adams, of Cambridge. In our
chapter on this great English mathematician we shall describe the
manner in which he was independently led to the same discovery.
Directly the planetary nature of the newly-discovered body had been
established, the great observatories naturally included this
additional member of the solar system in their working lists, so that
day after day its place was carefully determined. When sufficient
time had elapsed the shape and position of the orbit of the body
became known. Of course, it need hardly be said that observations
applied to the planet itself must necessarily provide a far more
accurate method of determining the path which it follows, than would
be possible to Le Verrier, when all he had to base his calculations
upon was the influence of the planet reflected, so to speak, from
Uranus. It may be noted that the true elements of the planet, when
revealed by direct observation, showed that there was a considerable
discrepancy between the track of the planet which Le Verrier had
announced, and that which the planet was actually found to pursue.
The name of the newly-discovered body had next to be considered. As
the older members of the system were already known by the same names
as great heathen divinities, it was obvious that some similar source
should be invoked for a suggestion as to a name for the most recent
planet. The fact that this body was so remote in the depths of
space, not unnaturally suggested the name "Neptune." Such is
accordingly the accepted designation of that mighty globe which
revolves in the track that at present seems to trace out the
frontiers of our system.
Le Verrier attained so much fame by this discovery, that when, in
1854, Arago's place had to be filled at the head of the great Paris
Observatory, it was universally felt that the discoverer of Neptune
was the suitable man to assume the office which corresponds in France
to that of the Astronomer Royal in England. It was true that the
work of the astronomical mathematician had hitherto been of an
abstract character. His discoveries had been made at his desk and
not in the observatory, and he had no practical acquaintance with the
use of astronomical instruments. However, he threw himself into the
technical duties of the observatory with vigour and determination. He
endeavoured to inspire the officers of the establishment with
enthusiasm for that systematic work which is so necessary for the
accomplishment of useful astronomical research. It must, however, be
admitted that Le Verrier was not gifted with those natural qualities
which would make him adapted for the successful administration of
such an establishment. Unfortunately disputes arose between the
Director and his staff. At last the difficulties of the situation
became so great that the only possible solution was to supersede Le
Verrier, and he was accordingly obliged to retire. He was succeeded
in his high office by another eminent mathematician, M. Delaunay,
only less distinguished than Le Verrier himself.
Relieved of his official duties, Le Verrier returned to the
mathematics he loved. In his non-official capacity he continued to
work with the greatest ardour at his researches on the movements of
the planets. After the death of M. Delaunay, who was accidentally
drowned in 1873, Le Verrier was restored to the directorship of the
observatory, and he continued to hold the office until his death.
The nature of the researches to which the life of Le Verrier was
subsequently devoted are not such as admit of description in a
general sketch like this, where the language, and still less the
symbols, of mathematics could not be suitably introduced. It may,
however, be said in general that he was particularly engaged with the
study of the effects produced on the movements of the planets by
their mutual attractions. The importance of this work to astronomy
consists, to a considerable extent, in the fact that by such
calculations we are enabled to prepare tables by which the places of
the different heavenly bodies can be predicted for our almanacs. To
this task Le Verrier devoted himself, and the amount of work he has
accomplished would perhaps have been deemed impossible had it not
been actually done.
The superb success which had attended Le Verrier's efforts to explain
the cause of the perturbations of Uranus, naturally led this
wonderful computer to look for a similar explanation of certain other
irregularities in planetary movements. To a large extent he
succeeded in showing how the movements of each of the great planets
could be satisfactorily accounted for by the influence of the
attractions of the other bodies of the same class. One circumstance
in connection with these investigations is sufficiently noteworthy to
require a few words here. Just as at the opening of his career, Le
Verrier had discovered that Uranus, the outermost planet of the then
known system, exhibited the influence of an unknown external body, so
now it appeared to him that Mercury, the innermost body of our
system, was also subjected to some disturbances, which could not be
satisfactorily accounted for as consequences of any known agents of
attraction. The ellipse in which Mercury revolved was animated by a
slow movement, which caused it to revolve in its plane. It appeared
to Le Verrier that this displacement was incapable of explanation by
the action of any of the known bodies of our system. He was,
therefore, induced to try whether he could not determine from the
disturbances of Mercury the existence of some other planet, at
present unknown, which revolved inside the orbit of the known
planet. Theory seemed to indicate that the observed alteration in
the track of the planet could be thus accounted for. He naturally
desired to obtain telescopic confirmation which might verify the
existence of such a body in the same way as Dr. Galle verified the
existence of Neptune. If there were, indeed, an intramercurial
planet, then it must occasionally cross between the earth and the
sun, and might now and then be expected to be witnessed in the actual
act of transit. So confident did Le Verrier feel in the existence of
such a body that an observation of a dark object in transit, by
Lescarbault on 26th March, 1859, was believed by the mathematician to
be the object which his theory indicated. Le Verrier also thought it
likely that another transit of the same object would be seen in
March, 1877. Nothing of the kind was, however, witnessed,
notwithstanding that an assiduous watch was kept, and the explanation
of the change in Mercury's orbit must, therefore, be regarded as
still to be sought for.
Le Verrier naturally received every honour that could be bestowed
upon a man of science. The latter part of his life was passed during
the most troubled period of modern French history. He was a
supporter of the Imperial Dynasty, and during the Commune he
experienced much anxiety; indeed, at one time grave fears were
entertained for his personal safety.