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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.
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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
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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
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