NEPTUNE.
Discovery of Neptune--A Mathematical Achievement--The Sun's
Attraction--All Bodies attract--Jupiter and Saturn--The Planetary
Perturbations--Three Bodies--Nature has simplified the
Problem--Approximate Solution--The Sources of Success--The Problem
Stated for the Earth--The Discoveries of Lagrange--The
Eccentricity--Necessity that all the Planets revolve in the same
Direction--Lagrange's Discoveries have not the Dramatic Interest of
the more Recent Achievements--The Irregularities of Uranus--The
Unknown Planet must revolve outside the Path of Uranus--The Data
for the Problem--Le Verrier and Adams both investigate the
Question--Adams indicates the Place of the Planet--How the Search
was to be conducted--Le Verrier also solves the Problem--The
Telescopic Discovery of the Planet--The Rival Claims--Early
Observation of Neptune--Difficulty of the Telescopic Study of
Neptune--Numerical Details of the Orbit--Is there any Outer
Planet?--Contrast between Mercury and Neptune.
We describe in this chapter a discovery so extraordinary that the whole
annals of science may be searched in vain for a parallel. We are not
here concerned with technicalities of practical astronomy. Neptune was
first revealed by profound mathematical research rather than by minute
telescopic investigation. We must develop the account of this striking
epoch in the history of science with the fulness of detail which is
commensurate with its importance; and it will accordingly be necessary,
at the outset of our narrative, to make an excursion into a difficult
but attractive department of astronomy, to which we have as yet made
little reference.
The supreme controlling power in the solar system is the attraction of
the sun. Each planet of the system experiences that attraction, and, in
virtue thereof, is constrained to revolve around the sun in an elliptic
path. The efficiency of a body as an attractive agent is directly
proportional to its mass, and as the mass of the sun is more than a
thousand times as great as that of Jupiter, which, itself, exceeds that
of all the other planets collectively, the attraction of the sun is
necessarily the chief determining force of the movements in our system.
The law of gravitation, however, does not merely say that the sun
attracts each planet. Gravitation is a doctrine much more general, for
it asserts that every body in the universe attracts every other body. In
obedience to this law, each planet must be attracted, not only by the
sun, but by innumerable bodies, and the movement of the planet must be
the joint effect of all such attractions. As for the influence of the
stars on our solar system, it may be at once set aside as inappreciable.
The stars are no doubt enormous bodies, in many cases possibly
transcending the sun in magnitude, but the law of gravitation tells us
that the intensity of the attraction decreases as the square of the
distance increases. Most of the stars are a million times as remote as
the sun, and consequently their attraction is so slight as to be
absolutely inappreciable in the discussion of this question. The only
attractions we need consider are those which arise from the action of
one body of the system upon another. Let us take, for instance, the two
largest planets of our system, Jupiter and Saturn. Each of these globes
revolves mainly in consequence of the sun's attraction, but every planet
also attracts every other, and the consequence is that each one is
slightly drawn away from the position it would have otherwise occupied.
In the language of astronomy, we would say that the path of Jupiter is
perturbed by the attraction of Saturn; and, conversely, that the path of
Saturn is perturbed by the attraction of Jupiter.
For many years these irregularities of the planetary motions presented
problems with which astronomers were not able to cope. Gradually,
however, one difficulty after another has been vanquished, and though
there are no doubt some small irregularities still outstanding which
have not been completely explained, yet all the larger and more
important phenomena of the kind are well understood. The subject is one
of the most difficult which the astronomer has to encounter in the
whole range of his science. He has here to calculate what effect one
planet is capable of producing on another planet. Such calculations
bristle with formidable difficulties, and can only be overcome by
consummate skill in the loftiest branches of mathematics. Let us state
what the problem really is.
When two bodies move in virtue of their mutual attraction, both of them
will revolve in a curve which admits of being exactly ascertained. Each
path is, in fact, an ellipse, and they must have a common focus at the
centre of gravity of the two bodies, considered as a single system. In
the case of a sun and a planet, in which the mass of the sun
preponderates enormously over the mass of the planet, the centre of
gravity of the two lies very near the centre of the sun; the path of the
great body is in such a case very small in comparison with the path of
the planet. All these matters admit of perfectly accurate calculation of
a somewhat elementary character. But now let us add a third body to the
system which attracts each of the others and is attracted by them. In
consequence of this attraction, the third body is displaced, and
accordingly its influence on the others is modified; they in turn act
upon it, and these actions and reactions introduce endless complexity
into the system. Such is the famous "problem of three bodies," which has
engaged the attention of almost every great mathematician since the time
of Newton. Stated in its mathematical aspect, and without having its
intricacy abated by any modifying circumstances, the problem is one that
defies solution. Mathematicians have not yet been able to deal with the
mutual attractions of three bodies moving freely in space. If the number
of bodies be greater than three, as is actually the case in the solar
system, the problem becomes still more hopeless.
Nature, however, has in this matter dealt kindly with us. She has, it is
true, proposed a problem which cannot be accurately solved; but she has
introduced into the problem, as proposed in the solar system, certain
special features which materially reduce the difficulty. We are still
unable to make what a mathematician would describe as a rigorous
solution of the question; we cannot solve it with the completeness of a
sum in arithmetic; but we can do what is nearly if not quite as useful.
We can solve the problem approximately; we can find out what the effect
of one planet on the other is _very nearly_, and by additional labour we
can reduce the limits of uncertainty to as low a point as may be
desired. We thus obtain a practical solution of the problem adequate for
all the purposes of science. It avails us little to know the place of a
planet with absolute mathematical accuracy. If we can determine what we
want with so close an approximation to the true position that no
telescope could possibly disclose the difference, then every practical
end will have been attained. The reason why in this case we are enabled
to get round the difficulties which we cannot surmount lies in the
exceptional character of the problem of three bodies as exhibited in the
solar system. In the first place, the sun is of such pre-eminent mass
that many matters may be overlooked which would be of moment were he
rivalled in mass by any of the planets. Another source of our success
arises from the small inclinations of the planetary orbits to each
other; while the fact that the orbits are nearly circular also greatly
facilitates the work. The mathematicians who may reside in some of the
other parts of the universe are not equally favoured. Among the sidereal
systems we find not a few cases where the problem of three bodies, or
even of more than three, would have to be faced without any of the
alleviating circumstances which our system presents. In such groups as
the marvellous star Th Orionis, we have three or four bodies
comparable in size, which must produce movements of the utmost
complexity. Even if terrestrial mathematicians shall ever have the
hardihood to face such problems, there is no likelihood of their being
able to do so for ages to come; such researches must repose on accurate
observations as their foundation; and the observations of these distant
systems are at present utterly inadequate for the purpose.
The undisturbed revolution of a planet around the sun, in conformity
with Kepler's law, would assure for that planet permanent conditions of
climate. The earth, for instance, if guided solely by Kepler's laws,
would return each day of the year exactly to the same position which it
had on the same day of last year. From age to age the quantity of heat
received by the earth would remain constant if the sun continued
unaltered, and the present climate might thus be preserved indefinitely.
But since the existence of planetary perturbation has become recognised,
questions arise of the gravest importance with reference to the possible
effects which such perturbations may have. We now see that the path of
the earth is not absolutely fixed. That path is deranged by Venus and by
Mars; it is deranged, it must be deranged, by every planet in our
system. It is true that in a year, or even in a century, the amount of
alteration produced is not very great; the ellipse which represents the
path of our earth this year does not differ considerably from the
ellipse which represented the movement of the earth one hundred years
ago. But the important question arises as to whether the slight
difference which does exist may not be constantly increasing, and may
not ultimately assume such proportions as to modify our climates, or
even to render life utterly impossible. Indeed, if we look at the
subject without attentive calculation, nothing would seem more probable
than that such should be the fate of our system. This globe revolves in
a path inside that of the mighty Jupiter. It is, therefore, constantly
attracted by Jupiter, and when it overtakes the vast planet, and comes
between him and the sun, then the two bodies are comparatively close
together, and the earth is pulled outwards by Jupiter. It might be
supposed that the tendency of such disturbances would be to draw the
earth gradually away from the sun, and thus to cause our globe to
describe a path ever growing wider and wider. It is not, however,
possible to decide a dynamical question by merely superficial reasoning
of this character. The question has to be brought before the tribunal of
mathematical analysis, where every element in the case is duly taken
into account. Such an enquiry is by no means a simple one. It worthily
occupied the splendid talents of Lagrange and Laplace, whose
discoveries in the theory of planetary perturbation are some of the most
remarkable achievements in astronomy.
We cannot here attempt to describe the reasoning which these great
mathematicians employed. It can only be expressed by the formulae of the
mathematician, and would then be hardly intelligible without previous
years of mathematical study. It fortunately happens, however, that the
results to which Lagrange and Laplace were conducted, and which have
been abundantly confirmed by the labours of other mathematicians, admit
of being described in simple language.
Let us suppose the case of the sun, and of two planets circulating
around him. These two planets are mutually disturbing each other, but
the amount of the disturbance is small in comparison with the effect of
the sun on each of them. Lagrange demonstrated that, though the ellipse
in which each planet moved was gradually altered in some respects by the
attraction of the other planet, yet there is one feature of the curve
which the perturbation is powerless to alter permanently: the longest
axis of the ellipse, and, therefore, the mean distance of the planet
from the sun, which is equal to one-half of it, must remain unchanged.
This is really a discovery as important as it was unexpected. It at once
removes all fear as to the effect which perturbations can produce on the
stability of the system. It shows that, notwithstanding the attractions
of Mars and of Venus, of Jupiter and of Saturn, our earth will for ever
continue to revolve at the same mean distance from the sun, and thus the
succession of the seasons and the length of the year, so far as this
element at least is concerned, will remain for ever unchanged.
But Lagrange went further into the enquiry. He saw that the mean
distance did not alter, but it remained to be seen whether the
eccentricity of the ellipse described by the earth might not be affected
by the perturbations. This is a matter of hardly less consequence than
that just referred to. Even though the earth preserved the same average
distance from the sun, yet the greatest and least distance might be
widely unequal: the earth might pass very close to the sun at one part
of its orbit, and then recede to a very great distance at the opposite
part. So far as the welfare of our globe and its inhabitants is
concerned, this is quite as important as the question of the mean
distance; too much heat in one half of the year would afford but
indifferent compensation for too little during the other half. Lagrange
submitted this question also to his analysis. Again he vanquished the
mathematical difficulties, and again he was able to give assurance of
the permanence of our system. It is true that he was not this time able
to say that the eccentricity of each path will remain constant; this is
not the case. What he does assert, and what he has abundantly proved, is
that the eccentricity of each orbit will always remain small. We learn
that the shape of the earth's orbit gradually swells and gradually
contracts; the greatest length of the ellipse is invariable, but
sometimes it approaches more to a circle, and sometimes becomes more
elliptical. These changes are comprised within narrow limits; so that,
though they may probably correspond with measurable climatic changes,
yet the safety of the system is not imperilled, as it would be if the
eccentricity could increase indefinitely. Once again Lagrange applied
the resources of his calculus to study the effect which perturbations
can have on the inclination of the path in which the planet moves. The
result in this case was similar to that obtained with respect to the
eccentricities. If we commence with the assumption that the mutual
inclinations of the planets are small, then mathematics assure us that
they must always remain small. We are thus led to the conclusion that
the planetary perturbations are unable to affect the stability of the
solar system.
We shall perhaps more fully appreciate the importance of these memorable
researches if we consider how easily matters might have been otherwise.
Let us suppose a system resembling ours in every respect save one. Let
that system have a sun, as ours has; a system of planets and of
satellites like ours. Let the masses of all the bodies in this
hypothetical system be identical with the masses in our system, and let
the distances and the periodic times be the same in the two cases. Let
all the planes of the orbits be similarly placed; and yet this
hypothetical system might contain seeds of decay from which ours is
free. There is one point in the imaginary scheme which we have not yet
specified. In our system all the planets revolve in the _same direction_
around the sun. Let us suppose this law violated in the hypothetical
system by reversing one planet on its path. That slight change alone
would expose the system to the risk of destruction by the planetary
perturbations. Here, then, we find the necessity of that remarkable
uniformity of the directions in which the planets revolve around the
sun. Had these directions not been uniform, our system must, in all
probability, have perished ages ago, and we should not be here to
discuss perturbations or any other subject.
Great as was the success of the eminent French mathematician who made
these beautiful discoveries, it was left for this century to witness the
crowning triumph of mathematical analysis applied to the law of
gravitation. The work of Lagrange lacks the dramatic interest of the
discovery made by Le Verrier and Adams, which gave still wider extent to
the solar system by the discovery of the planet Neptune revolving far
outside Uranus.
We have already alluded to the difficulties which were experienced when
it was sought to reconcile the early observations of Uranus with those
made since its discovery. We have shown that the path in which this
planet revolved experienced change, and that consequently Uranus must be
exposed to the action of some other force besides the sun's attraction.
The question arises as to the nature of these disturbing forces. From
what we have already learned of the mutual deranging influence between
any two planets, it seems natural to inquire whether the irregularities
of Uranus could not be accounted for by the attraction of the other
planets. Uranus revolves just outside Saturn. The mass of Saturn is much
larger than the mass of Uranus. Could it not be that Saturn draws Uranus
aside, and thus causes the changes? This is a question to be decided by
the mathematician. He can compute what Saturn is able to do, and he
finds, no doubt, that Saturn is capable of producing some displacement
of Uranus. In a similar manner Jupiter, with his mighty mass, acts on
Uranus, and produces a disturbance which the mathematician calculates.
When the figures had been worked out for all the known planets they were
applied to Uranus, and we might expect to find that they would fully
account for the observed irregularities of his path. This was, however,
not the case. After every known source of disturbance had been carefully
allowed for, Uranus was still shown to be influenced by some further
agent; and hence the conclusion was established that Uranus must be
affected by some unknown body. What could this unknown body be, and
where must it be situated? Analogy was here the guide of those who
speculated on this matter. We know no cause of disturbance of a planet's
motion except it be the attraction of another planet. Could it be that
Uranus was really attracted by some other planet at that time utterly
unknown? This suggestion was made by many astronomers, and it was
possible to determine some conditions which the unknown body should
fulfil. In the first place its orbit must lie outside the orbit of
Uranus. This was necessary, because the unknown planet must be a large
and massive one to produce the observed irregularities. If, therefore,
it were nearer than Uranus, it would be a conspicuous object, and must
have been discovered long ago. Other reasonings were also available to
show that if the disturbances of Uranus were caused by the attraction of
a planet, that body must revolve outside the globe discovered by
Herschel. The general analogies of the planetary system might also be
invoked in support of the hypothesis that the path of the unknown
planet, though necessarily elliptic, did not differ widely from a
circle, and that the plane in which it moved must also be nearly
coincident with the plane of the earth's orbit.
The measured deviations of Uranus at the different points of its orbit
were the sole data available for the discovery of the new planet. We
have to fit the orbit of the unknown globe, as well as the mass of the
planet itself, in such a way as to account for the various
perturbations. Let us, for instance, assume a certain distance for the
hypothetical body, and try if we can assign both an orbit and a mass for
the planet, at that distance, which shall account for the perturbations.
Our first assumption is perhaps too great. We try again with a lesser
distance. We can now represent the observations with greater accuracy. A
third attempt will give the result still more closely, until at length
the distance of the unknown planet is determined. In a similar way the
mass of the body can be also determined. We assume a certain value, and
calculate the perturbations. If the results seem greater than those
obtained by observations, then the assumed mass is too great. We amend
the assumption, and recompute with a lesser amount, and so on until at
length we determine a mass for the planet which harmonises with the
results of actual measurement. The other elements of the unknown
orbit--its eccentricity and the position of its axis--are all to be
ascertained in a similar manner. At length it appeared that the
perturbations of Uranus could be completely explained if the unknown
planet had a certain mass, and moved in an orbit which had a certain
position, while it was also manifest that no very different orbit or
greatly altered mass would explain the observed facts.
These remarkable computations were undertaken quite independently by two
astronomers--one in England and one in France. Each of them attacked,
and each of them succeeded in solving, the great problem. The scientific
men of England and the scientific men of France joined issue on the
question as to the claims of their respective champions to the great
discovery; but in the forty years which have elapsed since these
memorable researches the question has gradually become settled. It is
the impartial verdict of the scientific world outside England and
France, that the merits of this splendid triumph of science must be
divided equally between the late distinguished Professor J.C. Adams, of
Cambridge, and the late U.J.J. Le Verrier, the director of the Paris
Observatory.
Shortly after Mr. Adams had taken his degree at Cambridge, in 1843, when
he obtained the distinction of Senior Wrangler, he turned his attention
to the perturbations of Uranus, and, guided by these perturbations
alone, commenced his search for the unknown planet. Long and arduous was
the enquiry--demanding an enormous amount of numerical calculation, as
well as consummate mathematical resource; but gradually Mr. Adams
overcame the difficulties. As the subject unfolded itself, he saw how
the perturbations of Uranus could be fully explained by the existence of
an exterior planet, and at length he had ascertained, not alone the
orbit of this outer body, but he was even able to indicate the part of
the heavens in which the unknown globe must be sought. With his
researches in this advanced condition, Mr. Adams called on the
Astronomer-Royal, Sir George Airy, at Greenwich, in October, 1845, and
placed in his hands the computations which indicated with marvellous
accuracy the place of the yet unobserved planet. It thus appears that
seven months before anyone else had solved this problem Mr. Adams had
conquered its difficulties, and had actually located the planet in a
position but little more than a degree distant from the spot which it is
now known to have occupied. All that was wanted to complete the
discovery, and to gain for Professor Adams and for English science the
undivided glory of this achievement, was a strict telescopic search
through the heavens in the neighbourhood indicated.
Why, it may be said, was not such an enquiry instituted at once? No
doubt this would have been done, if the observatories had been generally
furnished forty years ago with those elaborate star-charts which they
now possess. In the absence of a chart (and none had yet been published
of the part of the sky where the unknown planet was) the search for the
planet was a most tedious undertaking. It had been suggested that the
new globe could be detected by its visible disc; but it must be
remembered that even Uranus, so much closer to us, had a disc so small
that it was observed nearly a score of times without particular notice,
though it did not escape the eagle glance of Herschel. There remained
then only one available method of finding Neptune. It was to construct a
chart of the heavens in the neighbourhood indicated, and then to compare
this chart night after night with the stars in the heavens. Before
recommending the commencement of a labour so onerous, the
Astronomer-Royal thought it right to submit Mr. Adams's researches to a
crucial preliminary test. Mr. Adams had shown how his theory rendered an
exact account of the perturbations of Uranus in longitude. The
Astronomer-Royal asked Mr. Adams whether he was able to give an equally
clear explanation of the notable variations in the distance of Uranus.
There can be no doubt that his theory would have rendered a satisfactory
account of these variations also; but, unfortunately, Mr. Adams seems
not to have thought the matter of sufficient importance to give the
Astronomer-Royal any speedy reply, and hence it happened that no less
than nine months elapsed between the time when Mr. Adams first
communicated his results to the Astronomer-Royal and the time when the
telescopic search for the planet was systematically commenced. Up to
this time no account of Mr. Adams's researches had been published. His
labours were known to but few besides the Astronomer-Royal and Professor
Challis of Cambridge, to whom the duty of making the search was
afterwards entrusted.
In the meantime the attention of Le Verrier, the great French
mathematician and astronomer, had been specially directed by Arago to
the problem of the perturbations of Uranus. With exhaustive analysis Le
Verrier investigated every possible known source of disturbance. The
influences of the older planets were estimated once more with every
precision, but only to confirm the conclusion already arrived at as to
their inadequacy to account for the perturbations. Le Verrier then
commenced the search for the unknown planet by the aid of mathematical
investigation, in complete ignorance of the labours of Adams. In
November, 1845, and again on the 1st of June, 1846, portions of the
French astronomer's results were announced. The Astronomer-Royal then
perceived that his calculations coincided practically with those of
Adams, insomuch that the places assigned to the unknown planet by the
two astronomers were not more than a degree apart! This was, indeed, a
remarkable result. Here was a planet unknown to human sight, yet felt,
as it were, by mathematical analysis with a certainty so great that two
astronomers, each in total ignorance of the other's labours, concurred
in locating the planet in almost the same spot of the heavens. The
existence of the new globe was thus raised nearly to a certainty, and it
became incumbent on practical astronomers to commence the search
forthwith. In June, 1846, the Astronomer-Royal announced to the visitors
of the Greenwich Observatory the close coincidence between the
calculations of Le Verrier and of Adams, and urged that a strict
scrutiny of the region indicated should be at once instituted. Professor
Challis, having the command of the great Northumberland equatorial
telescope at Cambridge, was induced to undertake the work, and on the
29th July, 1846, he began his labours.
The plan of search adopted by Professor Challis was an onerous one. He
first took the theoretical place of the planet, as given by Mr. Adams,
and after allowing a very large margin for the uncertainties of a
calculation so recondite, he marked out a certain region of the heavens,
near the ecliptic, in which it might be anticipated that the unknown
planet must be found. He then determined to observe all the stars in
this region and measure their relative positions. When this work was
once done it was to be repeated a second time. His scheme even
contemplated a third complete set of observations of the stars contained
within this selected region. There could be no doubt that this process
would determine the planet if it were bright enough to come within the
limits of stellar magnitude which Professor Challis adopted. The globe
would be detected by its motion relatively to the stars, when the three
series of measures came to be compared. The scheme was organised so
thoroughly that it must have led to the expected discovery--in fact, it
afterwards appeared that Professor Challis did actually observe the
planet more than once, and a subsequent comparison of its positions must
infallibly have led to the detection of the new globe.
Le Verrier was steadily maturing his no less elaborate investigations in
the same direction. He felt confident of the existence of the planet,
and he went so far as to predict not only the situation of the globe but
even its actual appearance. He thought the planet would be large enough
(though still of course only a telescopic object) to be distinguished
from the stars by the possession of a disc. These definite predictions
strengthened the belief that we were on the verge of another great
discovery in the solar system, so much so that when Sir John Herschel
addressed the British Association on the 10th of September, 1846, he
uttered the following words:--"The past year has given to us the new
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."
The time of the discovery was now rapidly approaching. On the 18th of
September, 1846, Le Verrier wrote to Dr. Galle of the Berlin
Observatory, describing the place of the planet indicated by his
calculations, and asking him to make its telescopic discovery. The
request thus preferred was similar to that made on behalf of Adams to
Professor Challis. Both at Berlin and at Cambridge the telescopic
research was to be made in the same region of the heavens. The Berlin
astronomers were, however, fortunate in possessing an invaluable aid to
the research which was not at the time in the hands of Professor
Challis. We have mentioned how the search for a telescopic planet can be
facilitated by the use of a carefully-executed chart of the stars. In
fact, a mere comparison of the chart with the sky is all that is
necessary. It happened that the preparation of a series of star charts
had been undertaken by the Berlin Academy of Sciences some years
previously. On these charts the place of every star, down even to the
tenth magnitude, had been faithfully engraved. This work was one of much
utility, but its originators could hardly have anticipated the brilliant
discovery which would arise from their years of tedious labour. It was
found convenient to publish such an extensive piece of surveying work by
instalments, and accordingly, as the chart was completed, it issued from
the press sheet by sheet. It happened that just before the news of Le
Verrier's labours reached Berlin the chart of that part of the heavens
had been engraved and printed.
It was on the 23rd of September that Le Verrier's letter reached Dr.
Galle at Berlin. The sky that night was clear, and we can imagine with
what anxiety Dr. Galle directed his telescope to the heavens. The
instrument was pointed in accordance with Le Verrier's instructions. The
field of view showed a multitude of stars, as does every part of the
heavens. One of these was really the planet. The new chart was unrolled,
and, star by star, the heavens were compared with it. As the
identification of the stars went on, one object after another was found
to lie in the heavens as it was engraved on the chart, and was of course
rejected. At length a star of the eighth magnitude--a brilliant
object--was brought into review. The chart was examined, but there was
no star there. This object could not have been in its present place when
the chart was formed. The object was therefore a wanderer--a planet. Yet
it was necessary to be cautious in such a matter. Many possibilities had
to be guarded against. It was, for instance, at least conceivable that
the object was really a star which, by some mischance, eluded the
careful eye of the astronomer who had constructed the map. It was even
possible that the star might be one of the large class of variables
which alternate in brightness, and it might have been too faint to have
been visible when the chart was made. Or it might be one of the minor
planets moving between Mars and Jupiter. Even if none of these
explanations would answer, it was still necessary to show that the
object was moving with that particular velocity and in that particular
direction which the theory of Le Verrier indicated. The lapse of a
single day was sufficient to dissipate all doubts. The next night the
object was again observed. It had moved, and when its motion was
measured it was found to accord precisely with what Le Verrier had
foretold. Indeed, as if no circumstance in the confirmation should be
wanting, the diameter of the planet, as measured by the micrometers at
Berlin, proved to be practically coincident with that anticipated by Le
Verrier.
The world speedily rang with the news of this splendid achievement.
Instantly the name of Le Verrier rose to a pinnacle hardly surpassed by
that of any astronomer of any age or country. The circumstances of the
discovery were highly dramatic. We picture the great astronomer buried
in profound meditation for many months; his eyes are bent, not on the
stars, but on his calculations. No telescope is in his hand; the human
intellect is the instrument he alone uses. With patient labour, guided
by consummate mathematical artifice, he manipulates his columns of
figures. He attempts one solution after another. In each he learns
something to avoid; by each he obtains some light to guide him in his
future labours. At length he begins to see harmony in those results
where before there was but discord. Gradually the clouds disperse, and
he discerns with a certainty little short of actual vision the planet
glittering in the far depths of space. He rises from his desk and
invokes the aid of a practical astronomer; and lo! there is the planet
in the indicated spot. The annals of science present no such spectacle
as this. It was the most triumphant proof of the law of universal
gravitation. The Newtonian theory had indeed long ere this attained an
impregnable position; but, as if to place its truth in the most
conspicuous light, this discovery of Neptune was accomplished.
For a moment it seemed as if the French were to enjoy the undivided
honour of this splendid triumph; nor would it, indeed, have been
unfitting that the nation which gave birth to Lagrange and to Laplace,
and which developed the great Newtonian theory by their immortal
labours, should have obtained this distinction. Up to the time of the
telescopic discovery of the planet by Dr. Galle at Berlin, no public
announcement had been made of the labours of Challis in searching for
the planet, nor even of the theoretical researches of Adams on which
those observations were based. But in the midst of the paeans of triumph
with which the enthusiastic French nation hailed the discovery of Le
Verrier, there appeared a letter from Sir John Herschel in the
_Athenaeum_ for 3rd October, 1846, in which he announced the researches
made by Adams, and claimed for him a participation in the glory of the
discovery. Subsequent enquiry has shown that this claim was a just one,
and it is now universally admitted by all independent authorities. Yet
it will easily be imagined that the French _savants_, jealous of the
fame of their countryman, could not at first be brought to recognise a
claim so put forward. They were asked to divide the unparalleled honour
between their own illustrious countryman and a young foreigner of whom
but few had ever heard, and who had not even published a line of his
work, nor had any claim been made on his part until after the work had
been completely finished by Le Verrier. The demand made on behalf of
Adams was accordingly refused any acknowledgment in France; and an
embittered controversy was the consequence. Point by point the English
astronomers succeeded in establishing the claim of their countryman. It
was true that Adams had not published his researches to the world, but
he had communicated them to the Astronomer-Royal, the official head of
the science in this country. They were also well known to Professor
Challis, the Professor of Astronomy at Cambridge. Then, too, the work of
Adams was published, and it was found to be quite as thorough and quite
as successful as that of Le Verrier. It was also found that the method
of search adopted by Professor Challis not only must have been
eventually successful, but that it actually was in a sense already
successful. When the telescopic discovery of the planet had been
achieved, Challis turned naturally to see whether he had observed the
new globe also. It was on the 1st October that he heard of the success
of Dr. Galle, and by that time Challis had accumulated observations in
connection with this research of no fewer than 3,150 stars. Among them
he speedily found that an object observed on the 12th of August was not
in the same place on the 30th of July. This was really the planet; and
its discovery would thus have been assured had Challis had time to
compare his measurements. In fact, if he had only discussed his
observations at once, there cannot be much doubt that the entire glory
of the discovery would have been awarded to Adams. He would then have
been first, no less in the theoretical calculations than in the optical
verification of the planet's existence. It may also be remarked that
Challis narrowly missed making the discovery of Neptune in another way.
Le Verrier had pointed out in his paper the possibility of detecting the
sought-for globe by its disc. Challis made the attempt, and before the
intelligence of the actual discovery at Berlin had reached him he had
made an examination of the region indicated by Le Verrier. About 300
stars passed through the field of view, and among them he selected one
on account of its disc; it afterwards appeared that this was indeed the
planet.
If the researches of Le Verrier and of Adams had never been undertaken
it is certain that the distant Neptune must have been some time
discovered; yet that might have been made in a manner which every true
lover of science would now deplore. We hear constantly that new minor
planets are observed, yet no one attaches to such achievements a
fraction of the consequence belonging to the discovery of Neptune. The
danger was, that Neptune should have been merely dropped upon by simple
survey work, just as Uranus was discovered, or just as the hosts of
minor planets are now found. In this case Theoretical Astronomy, the
great science founded by Newton, would have been deprived of its most
brilliant illustration.
Neptune had, in fact, a very narrow escape on at least one previous
occasion of being discovered in a very simple way. This was shown when
sufficient observations had been collected to enable the path of the
planet to be calculated. It was then possible to trace back the
movements of the planet among the stars and thus to institute a search
in the catalogues of earlier astronomers to see whether they contained
any record of Neptune, erroneously noted as a star. Several such
instances have been discovered. I shall, however, only refer to one,
which possesses a singular interest. It was found that the place of the
planet on May 10th, 1795, must have coincided with that of a so-called
star recorded on that day in the "Histoire Celeste" of Lalande. By
actual examination of the heavens it further appeared that there was no
star in the place indicated by Lalande, so the fact that here was
really an observation of Neptune was placed quite beyond doubt. When
reference was made to the original manuscripts of Lalande, a matter of
great interest was brought to light. It was there found that he had
observed the same star (for so he regarded it) both on May 8th and on
May 10th; on each day he had determined its position, and both
observations are duly recorded. But when he came to prepare his
catalogue and found that the places on the two occasions were different,
he discarded the earlier result, and merely printed the latter.
Had Lalande possessed a proper confidence in his own observations, an
immortal discovery lay in his grasp; had he manfully said, "I was right
on the 10th of May and I was right on the 8th of May; I made no mistake
on either occasion, and the object I saw on the 8th must have moved
between that and the 10th," then he must without fail have found
Neptune. But had he done so, how lamentable would have been the loss to
science! The discovery of Neptune would then merely have been an
accidental reward to a laborious worker, instead of being one of the
most glorious achievements in the loftiest department of human reason.
Besides this brief sketch of the discovery of Neptune, we have but
little to tell with regard to this distant planet. If we fail to see in
Uranus any of those features which make Mars or Venus, Jupiter or
Saturn, such attractive telescopic objects, what can we expect to find
in Neptune, which is half as far again as Uranus? With a good telescope
and a suitable magnifying power we can indeed see that Neptune has a
disc, but no features on that disc can be identified. We are
consequently not in a position to ascertain the period in which Neptune
rotates around its axis, though from the general analogy of the system
we must feel assured that it really does rotate. More successful have
been the attempts to measure the diameter of Neptune, which is found to
be about 35,000 miles, or more than four times the diameter of the
earth. It would also seem that, like Jupiter and like Saturn, the planet
must be enveloped with a vast cloud-laden atmosphere, for the mean
density of the globe is only about one-fifth that of the earth. This
great globe revolves around the sun at a mean distance of no less than
2,800 millions of miles, which is about thirty times as great as the
mean distance from the earth to the sun. The journey, though
accomplished at the rate of more than three miles a second, is yet so
long that Neptune requires almost 165 years to complete one revolution.
Since its discovery, some fifty years ago, Neptune has moved through
about one-third of its path, and even since the date when it was first
casually seen by Lalande, in 1795, it has only had time to traverse
three-fifths of its mighty circuit.
Neptune, like our earth, is attended by a single satellite; this
delicate object was discovered by Mr. Lassell with his two-foot
reflecting telescope shortly after the planet itself became known. The
motion of the satellite of Neptune is nearly circular. Its orbit is
inclined at an angle of about 35 deg. to the Ecliptic, and it is specially
noteworthy that, like the satellites of Uranus, the direction of the
motion runs counter to the planetary movements generally. The satellite
performs its journey around Neptune in a period of a little less than
six days. By observing the motions of this moon we are enabled to
determine the mass of the planet, and thus it appears that the weight of
Neptune is about one nineteen-thousandth part of that of the sun.
No planets beyond Neptune have been seen, nor is there at present any
good ground for believing in their existence as visual objects. In the
chapter on the minor planets I have entered into a discussion of the way
in which these objects are discovered. It is by minute and diligent
comparison of the heavens with elaborate star charts that these bodies
are brought to light. Such enquiries would be equally efficacious in
searching for an ultra-Neptunian planet; in fact, we could design no
better method to seek for such a body, if it existed, than that which is
at this moment in constant practice at many observatories. The labours
of those who search for small planets have been abundantly rewarded with
discoveries now counted by hundreds. Yet it is a noteworthy fact that
all these planets are limited to one region of the solar system. It has
sometimes been conjectured that time may disclose perturbations in the
orbit of Neptune, and that these perturbations may lead to the discovery
of a planet still more remote, even though that planet be so distant and
so faint that it eludes all telescopic research. At present, however,
such an enquiry can hardly come within the range of practical astronomy.
Its movements have no doubt been studied minutely, but it must describe
a larger part of its orbit before it would be feasible to conclude, from
the perturbations of its path, the existence of an unknown and still
more remote planet.
We have thus seen that the planetary system is bounded on one side by
Mercury and on the other by Neptune. The discovery of Mercury was an
achievement of prehistoric times. The early astronomer who accomplished
that feat, when devoid of instrumental assistance and unsupported by
accurate theoretical knowledge, merits our hearty admiration for his
untutored acuteness and penetration. On the other hand, the discovery of
the exterior boundary of the planetary system is worthy of special
attention from the fact that it was founded solely on profound
theoretical learning.
Though we here close our account of the planets and their satellites, we
have still two chapters to add before we shall have completed what is to
be said with regard to the solar system. A further and notable class of
bodies, neither planets nor satellites, own allegiance to the sun, and
revolve round him in conformity with the laws of universal gravitation.
These bodies are the comets, and their somewhat more humble associates,
the shooting stars. We find in the study of these objects many matters
of interest, which we shall discuss in the ensuing chapters.