URANUS AND NEPTUNE
Hitherto we have been dealing with bodies which, from time immemorial,
have been known to man as planets. There must have been a period when
one by one the various members of our system known to the ancients
were discriminated from the fixed stars by unknown but patient and
skilful observers; but, from the dawn of historical astronomy, up to
the night of March 13, 1781, there had been no addition to the number
of those five primary planets the story of whose discovery is lost in
the mists of antiquity.
It may be questioned whether any one man, Kepler and Newton being
possible exceptions, has ever done so much for the science of
astronomy as was accomplished by Sir William Herschel. Certainly
no single observer has ever done so much, or, which is almost more
important than the actual amount of his achievement, has so completely
revolutionized methods and ideas in observing.
A Hanoverian by birth, and a member of the band of the Hanoverian
Guards, Herschel, after tasting the discomforts of war in the shape
of a night spent in a ditch on the field of Hastenbeck, where that
egregious general the Duke of Cumberland was beaten by the French,
concluded that he was not designed by Nature for martial distinction,
and abruptly solved the problem of his immediate destiny by recourse
to the simple and unheroic expedient of desertion. He came to England,
got employment after a time as organist of the Octagon Chapel at Bath,
and was rapidly rising into notice as a musician, when the force of
his genius, combined with a discovery which came certainly unsought,
but was grasped as only a great man can grasp the gifts of Fortune,
again changed the direction of his life, and gave him to the science
of astronomy.
He had for several years employed his spare time in assiduous
observation; and, finding that opticians' prices were higher than he
could well afford, had begun to make Newtonian reflectors for
himself, and had finally succeeded in constructing one of 6-1/2 inches
aperture, and of high optical quality. With this instrument, on the
night of March 13, 1781, he was engaged in the execution of a plan
which he had formed of searching the heavens for double stars, with a
view to measuring their distance from the earth by seeing whether the
apparent distance of the members of the double from one another varied
in any degree in the course of the earth's journey round the sun. He
was working through the stars in the constellation Gemini, when his
attention was fixed by one which presented a different appearance from
the others which had passed his scrutiny.
In a good telescope a fixed star shows only a very small disc, which
indeed should be but a point of light; and the finer the instrument
the smaller the disc. The disc of this object, however, was
unmistakably larger than those of the fixed stars in its
neighbourhood--unmistakably, that is, to an observer of such skill as
Herschel, though those who have seen Uranus under ordinary powers will
find their respect considerably increased for the skill which at
once discriminated the tiny greenish disc from that of a fixed star.
Subsequent observation revealed to Herschel that he was right in
supposing that this body was not a star, for it proved to be in motion
relatively to the stars among which it was seen. But, in spite of
poetic authority, astronomical discoveries do not happen quite so
dramatically as the sonnet 'On First looking into Chapman's Homer'
suggests.
'Then felt I like some watcher of the skies,
When a new planet swims into his ken'
is a noble simile, were it only true to the facts. But new planets
do not swim around promiscuously in this fashion; and in the case of
Uranus, which more nearly realizes the thought of Keats than any other
in the history of astronomy, the 'watcher of the skies' felt probably
more puzzlement than anything else. Herschel was far from realizing
that he had found a new planet. When unmistakable evidence was
forthcoming that the newly discovered body was not a fixed star, he
merely felt confirmed in the first conjecture which had been suggested
by the size of its disc--namely, that he had discovered a new comet;
and it was as a new comet that Uranus was first announced to the
astronomical world.
It quickly became evident, however, that the new discovery moved in no
cometary orbit, but in one which marked it out as a regular member
of the solar system. A search was then instituted for earlier
observations of the planet, and it was found to have been observed
and mistaken for a fixed star on twenty previous occasions! One
astronomer, Lemonnier, had actually observed it no fewer than twelve
times, several of them within a few weeks of one another, and, had he
but reduced and compared his observations, could scarcely have failed
to have anticipated Herschel's discovery. But perhaps an astronomer
who, like Lemonnier, noted some of his observations on a paper-bag
which had formerly contained hair-powder, and whose astronomical
papers have been described as 'the image of Chaos,' scarcely deserved
the honour of such a discovery!
When it became known that this new addition to our knowledge of the
solar system had been made by the self-taught astronomer at Bath,
Herschel was summoned to Court by George III., and enabled to devote
himself entirely to his favourite study by the bestowal of the not
very magnificent pension of £200 a year, probably the best investment
that has ever been made in the interests of astronomical science. In
gratitude to the penurious monarch who had bestowed on him this meagre
competence, Herschel wished to call his planet the Georgium Sidus--the
Georgian Star, and this title, shortened in some instances to the
Georgian, is still to be found in some ancient volumes on astronomy.
The astronomers of the Continent, however, did not feel in the least
inclined to elevate Farmer George to the skies before his due time,
and for awhile the name of Herschel was given to the new planet, which
still bears as its symbol the first letter of its discoverer's name
with a globe attached to the cross-bar [Uranus]. Finally, the name
Urᾰnus ('a' short) prevailed, and has for long been in universal
use.
Uranus revolves round the sun at a distance from him of about
1,780,000,000 miles, in an orbit which takes eighty-four of our years
to complete. Barnard gives his diameter at 34,900 miles, and if this
measure be correct, he is the third largest planet of the system.
Other measures give a somewhat smaller diameter, and place Neptune
above him in point of size.
Subsequent observers have been able to see but little more than
Herschel saw upon the diminutive disc to which even so large a body
is reduced at so vast a distance. When near opposition, Uranus can
readily be seen with the naked eye as a star of about the sixth
magnitude, and there is no difficulty in picking him up with the
finder of an ordinary telescope by means of an almanac and a good star
map, nor in raising a small disc by the application of a moderately
high power, say 200 and upwards. (Herschel was using 227 at the time
of his discovery.) But small telescopes do little more than give their
owners the satisfaction of seeing, pretty much as Herschel saw it,
the object on which his eye was the first to light. Nor have even the
largest instruments done very much more. Rings, similar to those of
Saturn, were once suspected, but have long since been disposed of,
and most of the observations of spots and belts have been gravely
questioned. The Lick observers in 1890 and 1891 describe the belts as
'the merest shades on the planet's surface.'
The spectrum of Uranus is marked by peculiarities which distinguish
it from that of the other planets. It is crossed by six dark
absorption-bands, which indicate at all events that the medium
through which the sunlight which it reflects to us has passed is of
a constitution markedly different from that of our own atmosphere. It
was at first thought that the spectrum gave evidence of the planet's
self-luminosity; but this has not proved to be the case, though
doubtless Uranus, like Jupiter and Saturn, is in the condition of
a semi-sun. Like the other members of the group of large exterior
planets, his density is small, being only 1/5 greater than that of
water.
Six years after his great discovery, Herschel, with the 40-foot
telescope of 4 feet in aperture which he had now built, discovered two
satellites, and believed himself to have discovered four more. Later
observations have shown that, in the case of the four, small stars
near the planet had been mistaken for satellites. Subsequently two
more were discovered, one by Lassell, and one by Otto Struve, making
the number of the Uranian retinue up to four, so far as our present
knowledge goes. These four satellites, known as Ariel, Umbriel,
Oberon, and Titania, are distinguished by the fact that their orbits
are almost perpendicular to the plane of the orbit of Uranus, and that
the motions of all of them are retrograde. Titania and Oberon, the two
discovered by Herschel, are the easiest objects; but although they are
said to have been seen with a 4·3-inch refractor, this is a feat which
no ordinary observer need hope to emulate. An 8-inch is a more likely
instrument for such a task, and a 12-inch more likely still;
the average observer will probably find the latter none too big.
Accordingly, they are quite beyond the range of such observation as we
are contemplating. The rotation period of Uranus is not known.
In a few years after the discovery of Uranus, it became apparent that
by no possible ingenuity could his places as determined by present
observation be satisfactorily combined with those determined by the
twenty observations available, as already mentioned, from the period
before he was recognised as a planet. Either the old observations were
bad, or else the new planet was wandering from the track which it had
formerly followed. It appeared to Bouvard, who was constructing the
tables for the motions of Uranus, the simplest course to reject the
old observations as probably erroneous, and to confine himself to the
modern ones. Accordingly this course was pursued, and his tables were
published in 1821, but only for it to be found that in a few years
they also began to prove unsatisfactory; discrepancies began to appear
and to increase, and it quickly became apparent that an attempt must
be made to discover the cause of them.
Bouvard himself appears to have believed in the existence of a planet
exterior to Uranus whose attraction was producing these disturbances,
but he died in 1843 before any progress had been made with the
solution of the enigma. In 1834 Hussey approached Airy, the Astronomer
Royal, with the suggestion that he might sweep for the supposed
exterior planet if some mathematician would help him as to the most
likely region to investigate. Airy, however, returned a sufficiently
discouraging answer, and Hussey apparently was deterred by it from
carrying out a search which might very possibly have been rewarded
by success. Bessel, the great German mathematician, had marked the
problem for his own, and would doubtless have succeeded in solving
it, but shortly after he had begun the gathering of material for his
researches, he was seized with the illness which ultimately proved
fatal to him.
The question was thus practically untouched when in 1841, John Couch
Adams, then an undergraduate of St. John's College, Cambridge, jotted
down a memorandum in which he indicated his resolve to attack it and
attempt the discovery of the perturbing planet, 'as soon as possible
after taking my degree.' The half-sheet of notepaper on which the
memorandum was made is still extant, and forms part of the volume
of manuscripts on the subject preserved in the library of St. John's
College.
On October 21, 1845, Adams, who had taken his degree (Senior Wrangler)
in 1843, communicated to Airy the results of his sixth and final
attempt at the solution of the problem, and furnished him with the
elements and mass of the perturbing planet, and an indication of its
approximate place in the heavens. Airy, whose record in the matter
reads very strangely, was little more inclined to give encouragement
to Adams than to Hussey. He replied by propounding to the young
investigator a question which he considered 'a question of vast
importance, an _experimentum crucis_,' which Adams seemingly
considered of so little moment, that strangely enough he never
troubled to answer it. Then the matter dropped out of sight, though,
had the planet been sought for when Adams's results were first
communicated to the Astronomer Royal, it would have been found within
3-1/2 lunar diameters of the place assigned to it.
Meanwhile, in France, another and better-known mathematician had taken
up the subject, and in three memoirs presented to the French Academy
of Sciences in 1846, Leverrier furnished data concerning the new
planet which agreed in very remarkable fashion with those furnished by
Adams to Airy. The coincidence shook Airy's scepticism, and he asked
Dr. Challis, director of the Cambridge Observatory, to begin a search
for the planet with the large Northumberland equatorial. Challis, who
had no complete charts of the region to be searched, began to make
observations for the construction of a chart which would enable him to
detect the planet by means of its motion. It is more than likely that
had he adopted Hussey's suggestion of simply sweeping in the vicinity
of the spot indicated, he would have been successful, for the
Northumberland telescope was of 11 inches aperture, and would have
borne powers sufficient to distinguish readily the disc of Neptune
from the fixed stars around it. However, Challis chose the more
thorough, but longer method of charting; and even to that he did
not devote undivided attention. 'Some wretched comet,' says Proctor,
'which he thought it his more important duty to watch, prevented
him from making the reductions which would have shown him that the
exterior planet had twice been recorded in his notes of observations.'
Indeed, a certain fatality seems to have hung over the attempts made
in Britain to realize Adams's discovery. In 1845, the Rev. W. R.
Dawes, one of the keenest and most skilful of amateur observers, was
so much impressed by some of Adams's letters to the Astronomer Royal
that he wrote to Lassell, asking him to search for the planet. When
Dawes's letter arrived, Lassell was suffering from a sprained ankle,
and laid the letter aside till he should be able to resume work. In
the meantime the letter was burned by an officious servant-maid, and
Lassell lost the opportunity of a discovery which would have crowned
the fine work which he accomplished as an amateur observer.
A very different fate had attended Leverrier's calculations. On
September 23, 1846, a letter from Leverrier was received at the Berlin
Observatory, asking that search should be made for the planet in the
position which his inquiries had pointed out. The same night Galle
made the search, and within a degree of the spot indicated an object
was found with a measurable disc of between two and three seconds
diameter. As it was not laid down on Bremiker's star-chart of the
region, it was clearly not a star, and by next night its planetary
nature was made manifest. The promptitude with which Leverrier's
results were acted upon by Encke and Galle is in strong contrast to
the sluggishness which characterized the British official astronomers,
who, indeed, can scarcely be said to have come out of the business
with much credit.
A somewhat undignified controversy ensued. The French astronomers,
very naturally, were eager to claim all the laurels for their
brilliant countryman, and were indignant when a claim was put in on
behalf of a young Englishman whose name had never previously been
heard of. Airy, however, displayed more vigour in this petty squabble
than in the search for Neptune, and presented such evidence in support
of his fellow-countryman's right to recognition that it was impossible
to deny him the honour which, but for official slackness, would have
fallen to him as the actual as well as the potential discoverer of the
new planet. Adams himself took no part in the strife; spoke, indeed,
no words on the matter, except to praise the abilities of Leverrier,
and gave no sign of the annoyance which most men in like circumstances
would have displayed.
Galle suggested that the new planet should be called Janus; but the
name of the two-faced god was felt to be rather too pointedly suitable
at the moment, and that of Neptune was finally preferred. Neptune
is about 32,900 miles in diameter, his distance from the sun is
2,792,000,000 miles, and he occupies 165 years in the circuit of his
gigantic orbit. The spectroscopic evidence, such as it is, seems to
point to a condition somewhat similar to that of Uranus.
Neptune had only been discovered seventeen days when Lassell found him
to be attended by one satellite. First seen on October 10, 1846, it
was not till the following July that the existence of this body
was verified by Lassell himself and also by Otto Struve and Bond
of Harvard. From the fact that it is visible at such an enormous
distance, it is evident that this satellite must be of considerable
size--probably at least equal to our own moon.
Small instruments can make nothing of Neptune beyond, perhaps,
distinguishing the fact that, whatever the tiny disc may be, it is
not that of a star. His satellite is an object reserved for the very
finest instruments alone.
Should Neptune have any inhabitants, their sky must be somewhat barren
of planets. Jupiter's greatest elongation from the sun would be about
10°, and he would be seen under somewhat less favourable conditions
than those under which we see Mercury; while the planets between
Jupiter and the sun would be perpetually invisible. Saturn and Uranus,
however, would be fairly conspicuous, the latter being better seen
than from the earth.
Suspicions have been entertained of the existence of another planet
beyond Neptune, and photographic searches have been made, but hitherto
without success. So far as our present knowledge goes, Neptune is the
utmost sentinel of the regular army of the solar system.