URANUS.
Contrast between Uranus and the other great Planets--William
Herschel--His Birth and Parentage--Herschel's Arrival in
England--His Love of Learning--Commencement of his Astronomical
Studies--The Construction of Telescopes--Construction of
Mirrors--The Professor of Music becomes an Astronomer--The
Methodical Research--The 13th March, 1781--The Discovery of
Uranus--Delicacy of Observation--Was the Object a Comet?--The
Significance of this Discovery--The Fame of Herschel--George III.
and the Bath Musician--The King's Astronomer at Windsor--The Planet
Uranus--Numerical Data with reference thereto--The Four Satellites
of Uranus--Their Circular Orbits--Early Observations of
Uranus--Flamsteed's Observations--Lemonnier saw Uranus--Utility of
their Measurements--The Elliptic Path--The Great Problem thus
Suggested.
To the present writer it has always seemed that the history of Uranus,
and of the circumstances attending its discovery, forms one of the most
pleasing and interesting episodes in the whole history of science. We
here occupy an entirely new position in the study of the solar system.
All the other great planets were familiarly known from antiquity,
however erroneous might be the ideas entertained in connection with
them. They were conspicuous objects, and by their movements could hardly
fail to attract the attention of those whose pursuits led them to
observe the stars. But now we come to a great planet, the very existence
of which was utterly unknown to the ancients; and hence, in approaching
the subject, we have first to describe the actual discovery of this
object, and then to consider what we can learn as to its physical
nature.
We have, in preceding pages, had occasion to mention the revered name of
William Herschel in connection with various branches of astronomy; but
we have hitherto designedly postponed any more explicit reference to
this extraordinary man until we had arrived at the present stage of our
work. The story of Uranus, in its earlier stages at all events, is the
story of the early career of William Herschel. It would be alike
impossible and undesirable to attempt to separate them.
William Herschel, the illustrious astronomer, was born at Hanover in
1738. His father was an accomplished man, pursuing, in a somewhat humble
manner, the calling of a professor of music. He had a family of ten
children, of whom William was the fourth; and it may be noted that all
the members of the family of whom any record has been preserved
inherited their father's musical talents, and became accomplished
performers. Pleasing sketches have been given of this interesting
family, of the unusual aptitude of William, of the long discussions on
music and on philosophy, and of the little sister Caroline, destined in
later years for an illustrious career. William soon learned all that his
master could teach him in the ordinary branches of knowledge, and by the
age of fourteen he was already a competent performer on the oboe and the
viol. He was engaged in the Court orchestra at Hanover, and was also a
member of the band of the Hanoverian Guards. Troublous times were soon
to break up Herschel's family. The French invaded Hanover, the
Hanoverian Guards were overthrown in the battle of Hastenbeck, and young
William Herschel had some unpleasant experience of actual warfare. His
health was not very strong, and he decided that he would make a change
in his profession. His method of doing so is one which his biographers
can scarcely be expected to defend; for, to speak plainly, he deserted,
and succeeded in making his escape to England. It is stated on
unquestionable authority that on Herschel's first visit to King George
III., more than twenty years afterwards, his pardon was handed to him by
the King himself, written out in due form.
At the age of nineteen the young musician began to seek his fortunes in
England. He met at first with very considerable hardship, but industry
and skill conquered all difficulties, and by the time he was twenty-six
years of age he was thoroughly settled in England, and doing well in his
profession. In the year 1766 we find Herschel occupying a position of
some distinction in the musical world; he had become the organist of the
Octagon Chapel at Bath, and his time was fully employed in giving
lessons to his numerous pupils, and with his preparation for concerts
and oratorios.
Notwithstanding his busy professional life, Herschel still retained that
insatiable thirst for knowledge which he had when a boy. Every moment he
could snatch from his musical engagements was eagerly devoted to study.
In his desire to perfect his knowledge of the more abstruse parts of the
theory of music he had occasion to learn mathematics; from mathematics
the transition to optics was a natural one; and once he had commenced to
study optics, he was of course brought to a knowledge of the telescope,
and thence to astronomy itself.
His beginnings were made on a very modest scale. It was through a small
and imperfect telescope that the great astronomer obtained his first
view of the celestial glories. No doubt he had often before looked at
the heavens on a clear night, and admired the thousands of stars with
which they were adorned; but now, when he was able to increase his
powers of vision even to a slight extent, he obtained a view which
fascinated him. The stars he had seen before he now saw far more
distinctly; but, more than this, he found that myriads of others
previously invisible were now revealed to him. Glorious, indeed, is this
spectacle to anyone who possesses a spark of enthusiasm for natural
beauty. To Herschel this view immediately changed the whole current of
his life. His success as a professor of music, his oratorios, and his
pupils were speedily to be forgotten, and the rest of his life was to be
devoted to the absorbing pursuit of one of the noblest of the sciences.
Herschel could not remain contented with the small and imperfect
instrument which first interested him. Throughout his career he
determined to see everything for himself in the best manner which his
utmost powers could command. He at once decided to have a better
instrument, and he wrote to a celebrated optician in London with the
view of making a purchase. But the price which the optician demanded
seemed more than Herschel thought he could or ought to give. Instantly
his resolution was taken. A good telescope he must have, and as he could
not buy one he resolved to make one. It was alike fortunate, both for
Herschel and for science, that circumstances impelled him to this
determination. Yet, at first sight, how unpromising was the enterprise!
That a music teacher, busily employed day and night, should, without
previous training, expect to succeed in a task where the highest
mechanical and optical skill was required, seemed indeed unlikely. But
enthusiasm and genius know no insuperable difficulties. From conducting
a brilliant concert in Bath, when that city was at the height of its
fame, Herschel would rush home, and without even delaying to take off
his lace ruffles, he would plunge into his manual labours of grinding
specula and polishing lenses. No alchemist of old was ever more deeply
absorbed in a project for turning lead into gold than was Herschel in
his determination to have a telescope. He transformed his home into a
laboratory; of his drawing-room he made a carpenter's shop. Turning
lathes were the furniture of his best bedroom. A telescope he must have,
and as he progressed he determined, not only that he should have a good
telescope, but a very good one; and as success cheered his efforts he
ultimately succeeded in constructing the greatest telescope that the
world had up to that time ever seen. Though it is as an astronomer that
we are concerned with Herschel, yet we must observe even as a telescope
maker also great fame and no small degree of commercial success flowed
in upon him. When the world began to ring with his glorious discoveries,
and when it was known that he used no other telescopes than those which
were the work of his own hands, a demand sprang up for instruments of
his construction. It is stated that he made upwards of eighty large
telescopes, as well as many others of smaller size. Several of these
instruments were purchased by foreign princes and potentates.[29] We
have never heard that any of these illustrious personages became
celebrated astronomers, but, at all events, they seem to have paid
Herschel handsomely for his skill, so that by the sale of large
telescopes he was enabled to realise what may be regarded as a fortune
in the moderate horizon of the man of science.
Up to the middle of his life Herschel was unknown to the public except
as a laborious musician, with considerable renown in his profession, not
only in Bath, but throughout the West of England. His telescope-making
was merely the occupation of his spare moments, and was unheard of by
most of those who knew and respected his musical attainments. It was in
1774 that Herschel first enjoyed a view of the heavens through an
instrument built with his own hands. It was but a small one in
comparison with those which he afterwards fashioned, but at once he
experienced the advantage of being his own instrument maker. Night after
night he was able to add the improvements which experience suggested; at
one time he was enlarging the mirrors; at another he was reconstructing
the mounting, or trying to remedy defects in the eye-pieces. With
unwearying perseverance he aimed at the highest excellence, and with
each successive advance he found that he was able to pierce further into
the sky. His enthusiasm attracted a few friends who were, like himself,
ardently attached to science. The mode in which he first made the
acquaintance of Sir William Watson, who afterwards became his warmest
friend, was characteristic of both. Herschel was observing the mountains
in the moon, and as the hours passed on, he had occasion to bring his
telescope into the street in front of his house to enable him to
continue his work. Sir William Watson happened to pass by, and was
arrested by the unusual spectacle of an astronomer in the public street,
at the dead of night, using a large and quaint-looking instrument.
Having a taste for astronomy, Sir William stopped, and when Herschel
took his eye from the telescope, asked if he might be allowed to have a
look at the moon. The request was readily granted. Probably Herschel
found but few in the gay city who cared for such matters; he was quickly
drawn to Sir W. Watson, who at once reciprocated the feeling, and thus
began a friendship which bore important fruit in Herschel's subsequent
career.
At length the year 1781 approached, which was to witness his great
achievement. Herschel had made good use of seven years' practical
experience in astronomy, and he had completed a telescope of exquisite
optical perfection, though greatly inferior in size to some of those
which he afterwards erected. With this reflector Herschel commenced a
methodical piece of observation. He formed the scheme of systematically
examining all the stars which were above a certain degree of brightness.
It does not quite appear what object Herschel proposed to himself when
he undertook this labour, but, in any case, he could hardly have
anticipated the extraordinary success with which the work was to be
crowned. In the course of this review the telescope was directed to a
star; that star was examined; then another was brought into the field of
view, and it too was examined. Every star under such circumstances
merely shows itself as a point of light; the point may be brilliant or
not, according as the star is bright or not; the point will also, of
course, show the colour of the star, but it cannot exhibit recognisable
size or shape. The greater, in fact, the perfection of the telescope,
the smaller is the telescopic image of a star.
How many stars Herschel inspected in this review we are not told; but at
all events, on the ever-memorable night of the 13th of March, 1781, he
was pursuing his self-allotted task among the hosts in the constellation
Gemini. Doubtless, one star after another was admitted to view, and was
allowed to pass away. At length, however, an object was placed in the
field which differed from every other star. It was not a mere point of
light; it had a minute, but still a perfectly recognisable, disc. We say
the disc was perfectly recognisable, but we should be careful to add
that it was so in the excellent telescope of Herschel alone. Other
astronomers had seen this object before. Its position had actually been
measured no fewer than nineteen times before the Bath musician, with his
home-made telescope, looked at it, but the previous observers had only
seen it in small meridian instruments with low magnifying powers. Even
after the discovery was made, and when well-trained observers with good
instruments looked again under the direction of Herschel, one after
another bore testimony to the extraordinary delicacy of the great
astronomer's perception, which enabled him almost at the first glance to
discriminate between it and a star.
If not a star, what, then, could it be? The first step to enable this
question to be answered was to observe the body for some time. This
Herschel did. He looked at it one night after another, and soon he
discovered another fundamental difference between this object and an
ordinary star. The stars are, of course, characterised by their fixity,
but this object was not fixed; night after night the place it occupied
changed with respect to the stars. No longer could there be any doubt
that this body was a member of the solar system, and that an interesting
discovery had been made; many months, however, elapsed before Herschel
knew the real merit of his achievement. He did not realise that he had
made the superb discovery of another mighty planet revolving outside
Saturn; he thought that it could only be a comet. No doubt this object
looked very different from a great comet, decorated with a tail. It was
not, however, so entirely different from some forms of telescopic comets
as to make the suggestion of its being a body of this kind unlikely; and
the discovery was at first announced in accordance with this view. Time
was necessary before the true character of the object could be
ascertained. It must be followed for a considerable distance along its
path, and measures of its position at different epochs must be effected,
before it is practicable for the mathematician to calculate the path
which the body pursues; once, however, attention was devoted to the
subject, many astronomers aided in making the necessary observations.
These were placed in the hands of mathematicians, and the result was
proclaimed that this body was not a comet, but that, like all the
planets, it revolved in nearly a circular path around the sun, and that
the path lay millions of miles outside the path of Saturn, which had so
long been regarded as the boundary of the solar system.
It is hardly possible to over-estimate the significance of this splendid
discovery. The five planets had been known from all antiquity; they were
all, at suitable seasons, brilliantly conspicuous to the unaided eye.
But it was now found that, far outside the outermost of these planets
revolved another splendid planet, larger than Mercury or Mars,
larger--far larger--than Venus and the earth, and only surpassed in bulk
by Jupiter and by Saturn. This superb new planet was plunged into space
to such a depth that, notwithstanding its noble proportions, it seemed
merely a tiny star, being only on rare occasions within reach of the
unaided eye. This great globe required a period of eighty-four years to
complete its majestic path, and the diameter of that path was
3,600,000,000 miles.
Although the history of astronomy is the record of brilliant
discoveries--of the labours of Copernicus, and of Kepler--of the
telescopic achievements of Galileo, and the splendid theory of
Newton--of the refined discovery of the aberration of light--of many
other imperishable triumphs of intellect--yet this achievement of the
organist at the Octagon Chapel occupies a totally different position
from any other. There never before had been any historic record of the
discovery of one of the bodies of the particular system to which the
earth belongs. The older planets were no doubt discovered by someone,
but we can say little more about these discoveries than we can about the
discovery of the sun or of the moon; all are alike prehistoric. Here was
the first recorded instance of the discovery of a planet which, like the
earth, revolves around the sun, and, like our earth, may conceivably be
an inhabited globe. So unique an achievement instantly arrested the
attention of the whole scientific world. The music-master at Bath,
hitherto unheard of as an astronomer, was speedily placed in the very
foremost rank of those entitled to the name. On all sides the greatest
interest was manifested about the unknown philosopher. The name of
Herschel, then unfamiliar to English ears, appeared in every journal,
and a curious list has been preserved of the number of blunders which
were made in spelling the name. The different scientific societies
hastened to convey their congratulations on an occasion so memorable.
Tidings of the discovery made by the Hanoverian musician reached the
ears of George III., and he sent for Herschel to come to the Court, that
the King might learn what his achievement actually was from the
discoverer's own lips. Herschel brought with him one of his telescopes,
and he provided himself with a chart of the solar system, with which to
explain precisely wherein the significance of the discovery lay. The
King was greatly interested in Herschel's narrative, and not less in
Herschel himself. The telescope was erected at Windsor, and, under the
astronomer's guidance, the King was shown Saturn and other celebrated
objects. It is also told how the ladies of the Court the next day asked
Herschel to show them the wonders which had so pleased the King. The
telescope was duly erected in a window of one of the Queen's apartments,
but when evening arrived the sky was found to be overcast with clouds,
and no stars could be seen. This was an experience with which Herschel,
like every other astronomer, was unhappily only too familiar. But it is
not every astronomer who would have shown the readiness of Herschel in
escaping gracefully from the position. He showed to his lady pupils the
construction of the telescope; he explained the mirror, and how he had
fashioned it and given the polish; and then, seeing the clouds were
inexorable, he proposed that, as he could not show them the real Saturn,
he should exhibit an artificial one as the best substitute. The
permission granted, Herschel turned the telescope away from the sky, and
pointed it towards the wall of a distant garden. On looking into the
telescope there was Saturn, his globe and his system of rings, so
faithfully shown that, says Herschel, even a skilful astronomer might
have been deceived. The fact was that during the course of the day
Herschel saw that the sky would probably be overcast in the evening, and
he had provided for the emergency by cutting a hole in a piece of
cardboard, the shape of Saturn, which was then placed against the
distant garden wall, and illuminated by a lamp at the back.
This visit to Windsor was productive of consequences momentous to
Herschel, momentous to science. He had made so favourable an impression,
that the King proposed to create for him the special appointment of
King's Astronomer at Windsor. The King was to provide the means for
erecting the great telescopes, and he allocated to Herschel a salary of
L200 a year, the figures being based, it must be admitted, on a somewhat
moderate estimate of the requirements of an astronomer's household.
Herschel mentioned these particulars to no one save to his constant and
generous friend, Sir W. Watson, who exclaimed, "Never bought monarch
honour so cheap." To other enquirers, Herschel merely said that the King
had provided for him. In accepting this post, the great astronomer took
no doubt a serious step. He at once sacrificed entirely his musical
career, now, from many sources, a lucrative one; but his determination
was speedily taken. The splendid earnest that he had already given of
his devotion to astronomy was, he knew, only the commencement of a
series of memorable labours. He had indeed long been feeling that it was
his bounden duty to follow that path in life which his genius indicated.
He was no longer a young man. He had attained middle age, and the years
had become especially precious to one who knew that he had still a
life-work to accomplish. He at one stroke freed himself from all
distractions; his pupils and concerts, his whole connection at Bath,
were immediately renounced; he accepted the King's offer with alacrity,
and after one or two changes settled permanently at Slough, near
Windsor.
It has, indeed, been well remarked that the most important event in
connection with the discovery of Uranus was the discovery of Herschel's
unrivalled powers of observation. Uranus must, sooner or later, have
been found. Had Herschel not lived, we would still, no doubt, have known
Uranus long ere this. The really important point for science was that
Herschel's genius should be given full scope, by setting him free from
the engrossing details of an ordinary professional calling. The
discovery of Uranus secured all this, and accordingly obtained for
astronomy all Herschel's future labours.[30]
Uranus is so remote that even the best of our modern telescopes cannot
make of it a striking picture. We can see, as Herschel did, that it has
a measurable disc, and from measurements of that disc we conclude that
the diameter of the planet is about 31,700 miles. This is about four
times as great as the diameter of the earth, and we accordingly see that
the volume of Uranus must be about sixty-four times as great as that of
the earth. We also find that, like the other giant planets, Uranus seems
to be composed of materials much lighter, on the whole, than those we
find here; so that, though sixty-four times as large as the earth,
Uranus is only fifteen times as heavy. If we may trust to the analogies
of what we see everywhere else in our system, we can feel but little
doubt that Uranus must rotate about an axis. The ordinary means of
demonstrating this rotation can be hardly available in a body whose
surface appears so small and so faint. The period of rotation is
accordingly unknown. The spectroscope tells us that a remarkable
atmosphere, containing apparently some gases foreign to our own, deeply
envelops Uranus.
There is, however, one feature about Uranus which presents many points
of interest to those astronomers who are possessed of telescopes of
unusual size and perfection. Uranus is accompanied by a system of
satellites, some of which are so faint as to require the closest
scrutiny for their detection. The discovery of these satellites was one
of the subsequent achievements of Herschel. It is, however, remarkable
that even his penetration and care did not preserve him from errors
with regard to these very delicate objects. Some of the points which he
thought to be satellites must, it would now seem, have been merely stars
enormously more distant, which happened to lie in the field of view. It
has been since ascertained that the known satellites of Uranus are four
in number, and their movements have been made the subject of prolonged
and interesting telescopic research. The four satellites bear the names
of Ariel, Umbriel, Titania, and Oberon. Arranged in order of their
distance from the central body, Ariel, the nearest, accomplishes its
journey in 2 days and 12 hours. Oberon, the most distant, completes its
journey in 13 days and 11 hours.
The law of Kepler declares that the path of a satellite around its
primary, no less than of the primary around the sun, must be an ellipse.
It leaves, however, boundless latitude in the actual eccentricity of the
curve. The ellipse may be nearly a circle, it may be absolutely a
circle, or it may be something quite different from a circle. The paths
pursued by the planets are, generally speaking, nearly circles; but we
meet with no exact circle among planetary orbits. So far as we at
present know, the closest approach made to a perfectly circular movement
is that by which the satellites of Uranus revolve around their primary.
We are not prepared to say that these paths are absolutely circular. All
that can be said is that our telescopes fail to show any measurable
departure therefrom. It is also to be noted as an interesting
circumstance that the orbits of the satellites of Uranus all lie in the
same plane. This is not true of the orbits of the planets around the
sun, nor is it true of the orbits of any other system of satellites
around their primary. The most singular circumstance attending the
Uranian system is, however, found in the position which this plane
occupies. This is indeed almost as great an anomaly in our system as are
the rings of Saturn themselves. We have already had occasion to notice
that the plane in which the earth revolves around the sun is very nearly
coincident with the planes in which all the other great planets revolve.
The same is true, to a large extent, of the orbits of the minor planets;
though here, no doubt, we meet with a few cases in which the plane of
the orbit is inclined at no inconsiderable angle to the plane in which
the earth moves. The plane in which the moon revolves also approximates
to this system of planetary planes. So, too, do the orbits of the
satellites of Saturn and of Jupiter, while even the more recently
discovered satellites of Mars form no exception to the rule. The whole
solar system--at least so far as the great planets are concerned--would
require comparatively little alteration if the orbits were to be
entirely flattened down into one plane. There are, however, some notable
exceptions to this rule. The satellites of Uranus revolve in a plane
which is far from coinciding with the plane to which all other orbits
approximate. In fact, the paths of the satellites of Uranus lie in a
plane nearly at right angles to the orbit of Uranus. We are not in a
position to give any satisfactory explanation of this circumstance. It
is, however, evident that in the genesis of the Uranian system there
must have been some influence of a quite exceptional and local
character.
Soon after the discovery of the planet Uranus, in 1781, sufficient
observations were accumulated to enable the orbit it follows to be
determined. When the path was known, it was then a mere matter of
mathematical calculation to ascertain where the planet was situated at
any past time, and where it would be situated at any future time. An
interesting enquiry was thus originated as to how far it might be
possible to find any observations of the planet made previously to its
discovery by Herschel. Uranus looks like a star of the sixth magnitude.
Not many astronomers were provided with telescopes of the perfection
attained by Herschel, and the personal delicacy of perception
characteristic of Herschel was a still more rare possession. It was,
therefore, to be expected that, if such previous observations existed,
they would merely record Uranus as a star visible, and indeed bright, in
a moderate telescope, but still not claiming any exceptional attention
over thousands of apparently similar stars. Many of the early
astronomers had devoted themselves to the useful and laborious work of
forming catalogues of stars. In the preparation of a star catalogue, the
telescope was directed to the heavens, the stars were observed, their
places were carefully measured, the brightness of the star was also
estimated, and thus the catalogue was gradually compiled in which each
star had its place faithfully recorded, so that at any future time it
could be identified. The stars were thus registered, by hundreds and by
thousands, at various dates from the birth of accurate astronomy till
the present time. The suggestion was then made that, as Uranus looked so
like a star, and as it was quite bright enough to have engaged the
attention of astronomers possessed of even very moderate instrumental
powers, there was a possibility that it had already been observed, and
thus actually lay recorded as a star in some of the older catalogues.
This was indeed an idea worthy of every attention, and pregnant with the
most important consequences in connection with the immortal discovery to
be discussed in our next chapter. But how was such an examination of the
catalogues to be conducted? Uranus is constantly moving about; does it
not seem that there is every element of uncertainty in such an
investigation? Let us consider a notable example.
The great national observatory at Greenwich was founded in 1675, and the
first Astronomer-Royal was the illustrious Flamsteed, who in 1676
commenced that series of observations of the heavenly bodies which has
been continued to the present day with such incalculable benefits to
science. At first the instruments were of a rather primitive
description, but in the course of some years Flamsteed succeeded in
procuring instruments adequate to the production of a catalogue of
stars, and he devoted himself with extraordinary zeal to the
undertaking. It is in this memorable work, the "Historia Coelestis" of
Flamsteed, that the earliest observation of Uranus is recorded. In the
first place it was known that the orbit of this body, like the orbit of
every other great planet, was inclined at a very small angle to the
ecliptic. It hence follows that Uranus is at all times only to be met
with along the ecliptic, and it is possible to calculate where the
planet has been in each year. It was thus seen that in 1690 the planet
was situated in that part of the ecliptic where Flamsteed was at the
same date making his observations. It was natural to search the
observations of Flamsteed, and see whether any of the so-called stars
could have been Uranus. An object was found in the "Historia
Coelestis" which occupied a position identical with that which Uranus
must have filled on the same date. Could this be Uranus? A decisive test
was at once available. The telescope was directed to the spot in the
heavens where Flamsteed saw a sixth-magnitude star. If that were really
a star, then would it still be visible. The trial was made: no such star
could be found, and hence the presumption that this was really Uranus
could hardly be for a moment doubted. Speedily other confirmation flowed
in. It was shown that Uranus had been observed by Bradley and by Tobias
Mayer, and it also became apparent that Flamsteed had observed Uranus
not only once, but that he had actually measured its place four times in
the years 1712 and 1715. Yet Flamsteed was never conscious of the
discovery that lay so nearly in his grasp. He was, of course, under the
impression that all these observations related to different stars. A
still more remarkable case is that of Lemonnier, who had actually
observed Uranus twelve times, and even recorded it on four consecutive
days in January, 1769. If Lemonnier had only carefully looked over his
own work; if he had perceived, as he might have done, how the star he
observed yesterday was gone to-day, while the star visible to-day had
moved away by to-morrow, there is no doubt that Uranus would have been
discovered, and William Herschel would have been anticipated. Would
Lemonnier have made as good use of his fame as Herschel did? This seems
a question which can never be decided, but those who estimate Herschel
as the present writer thinks he ought to be estimated, will probably
agree in thinking that it was most fortunate for science that Lemonnier
did _not_ compare his observations.[31]
These early accidental observations of Uranus are not merely to be
regarded as matters of historical interest or curiosity. That they are
of the deepest importance with regard to the science itself a few words
will enable us to show. It is to be remembered that Uranus requires no
less than eighty-four years to accomplish his mighty revolution around
the sun. The planet has completed one entire revolution since its
discovery, and up to the present time (1900) has accomplished more than
one-third of another. For the careful study of the nature of the orbit,
it was desirable to have as many measurements as possible, and extending
over the widest possible interval. This was in a great measure secured
by the identification of the early observations of Uranus. An
approximate knowledge of the orbit was quite capable of giving the
places of the planet with sufficient accuracy to identify it when met
with in the catalogues. But when by their aid the actual observations
have been discovered, they tell us precisely the place of Uranus; and
hence, instead of our knowledge of the planet being limited to but
little more than one revolution, we have at the present time information
with regard to it extending over considerably more than two revolutions.
From the observations of the planet the ellipse in which it moves can be
ascertained. We can compute this ellipse from the observations made
during the time since the discovery. We can also compute the ellipse
from the early observations made before the discovery. If Kepler's laws
were rigorously verified, then, of course, the ellipse performed in the
present revolution must differ in no respect from the ellipse performed
in the preceding, or indeed in any other revolution. We can test this
point in an interesting manner by comparing the ellipse derived from the
ancient observations with that deduced from the modern ones. These
ellipses closely resemble each other; they are nearly the same; but it
is most important to observe that they are not _exactly_ the same, even
when allowance has been made for every known source of disturbance in
accordance with the principles explained in the next chapter. The law of
Kepler seems thus not absolutely true in the case of Uranus. Here is,
indeed, a matter demanding our most earnest and careful attention. Have
we not repeatedly laid down the universality of the laws of Kepler in
controlling the planetary motions? How then can we reconcile this law
with the irregularities proved beyond a doubt to exist in the motions of
Uranus?
Let us look a little more closely into the matter. We know that the laws
of Kepler are a consequence of the laws of gravitation. We know that the
planet moves in an elliptic path around the sun, in virtue of the sun's
attraction, and we know that the ellipse will be preserved without the
minutest alteration if the sun and the planet be left to their mutual
attractions, and if no other force intervene. We can also calculate the
influence of each of the known planets on the form and position of the
orbit. But when allowance is made for all such perturbing influences it
is found that the observed and computed orbits do not agree. The
conclusion is irresistible. Uranus does not move solely in consequence
of the sun's attraction and that of the planets of our system interior
to Uranus; there must therefore be some further influence acting upon
Uranus besides those already known. To the development of this subject
the next chapter will be devoted.