mutual perturbations to some extent unraveled. They have proved
exceedingly interesting to students of celestial mechanics. Titan
has, in this department, chiefly to be reckoned with. He exercises in
the Saturnian system a similar overpowering influence to that wielded
by Jupiter in the Solar System.
FOOTNOTES:
[28] A ninth satellite, Phœbe, was discovered in 1904. Its
existence had been suspected for many years, and it was discovered
at the Arequipa Observatory, Peru, on March 14, 1899, by means of
photography. Since that date, it has been several times lost and
rediscovered.—E. S.
URANUS AND NEPTUNE.—WILLIAM F. DENNING
While Sir W. Herschel was a musician at Bath he formed the design of
making a telescopic survey of the heavens. While engaged in this, he
accidentally effected a discovery of great importance, for on the
night of March 13, 1781, an object entered the field of his 6.3-inch
reflector which ultimately proved to be a new major planet of our
system.
The acute eye of Herschel, directly it alighted upon the strange
body, recognized it as one of unusual character, for it had a
perceptible disk, and could be neither fixed star nor nebula. He
afterward found the object to be in motion, and its appearance being
“hazy and ill-defined,” with very high powers, he was led to regard
it as a comet, and communicated his discovery to the Royal Society at
its meeting on April 26, 1781.
The supposed comet soon came under the observation of others,
including Maskelyne, the Astronomer Royal, and Messier, the “Comet
Ferret,” of Paris. The latter, in a letter to Herschel, said:
“Nothing was more difficult than to catch it, and I can not conceive
how you could have hit this star or comet several times, for it
was absolutely necessary for me to observe it for several days in
succession before I could perceive that it was in motion.”
As observations began to accumulate, it was seen that a parabolic
orbit failed to accommodate them. Ultimately the secret was revealed.
The only orbit to represent the motion of the new body was found
to be an approximately circular one situated far outside the path
of Saturn, and the inference became irresistible that the supposed
“comet” must in reality be a new primary planet revolving on the
outskirts of the Solar System. This conclusion was justified by
facts of a convincing nature, and its announcement created no small
excitement in the scientific world. Every telescope was directed to
that part of the firmament which contained the new orb, and its pale
blue disk, wrapped in tiny proportions, was viewed again and again
with all the delight that so great a novelty could inspire. From the
earliest period of ancient history, no discovery of the same kind had
been effected. The Chaldeans were acquainted with five major planets,
in addition to the earth, and the number had remained constant until
the vigilant eye of Herschel enlarged our knowledge, and Saturn was
relieved as the sentinel planet going his rounds on the distant
frontiers of our system.
When the elements of the new body had been computed, a search was
instituted among the records of previous observers, and it was found
that Herschel’s planet had been seen on many occasions, but it had
invariably been mistaken for a fixed star. Flamsteed observed it
on six occasions between 1690 and 1715, while Le Monnier saw it on
twelve nights in the years 1750 to 1771, and it seems to have been
pure carelessness on the part of the latter which prevented him from
anticipating Herschel in one of the greatest discoveries of modern
times.
The name Uranus was applied to the new planet, though the discoverer
himself called it Georgium Sidus, and there were others who termed
it Herschel in honor of the man through whose sagacity it had been
revealed.
Uranus revolves around the sun in 30,687 days, which very slightly
exceeds 84 terrestrial years. His mean distance from the sun is
1,782,000,000 miles, but the interval varies between 1,699 and 1,865
millions of miles. The apparent diameter of the planet undergoes
little variation; the mean is 3″.6, but observers differ. His real
diameter is approximately 31,000 miles, and the polar compression
about 1/13, though this value is not that found by all authorities.
The planet near opposition shines like a star of the sixth magnitude,
and is observable with the naked eye. He emits a bluish light.
While engaged in meteoric observations, I have sometimes followed
the planet with the naked eye during several months, and noted the
changes in his position relatively to the stars near. It is clear
from this that Uranus admitted of detection before the invention of
the telescope.
A luminous ring, similar to that of Saturn, was at first supposed
to surround Uranus, and Herschel suspected the existence of such a
feature on several occasions; but it scarcely survived his later
researches, and modern observations have finally disposed of it.
In May and June, 1883, Professor Young, having the advantage of the
fine 23-inch refractor at the Princeton Observatory, observed two
faint belts, one on each side of the equator, and much like the belts
of Saturn. On March 18, 1884, Messrs. Thollon and Perrotin, with the
14-inch equatorial at Nice, remarked dark spots similar to those on
Mars, toward the centre of the disk, and a white spot was seen on the
limb. Two different tints were perceived, the color of the Northwest
Hemisphere being dark and that of the Southeast a bluish-white color.
In April observations were continued, and the white spot was seen
“rather as a luminous band than a simple spot,” but it was most
conspicuous near the limb. The observers thought the appearances
indicated a rotation-period of about ten hours. The brothers Henry
at Paris, in 1884, invariably noticed two belts lying parallel to
each other, and including between them the brighter equatorial zone
of the planet. Their results apparently show that the angle between
the plane of the Uranian equator and that of the satellite-orbits is
about 41°.
M. Perrotin, with the great 30-inch equatorial at Nice, reobserved
the belts in May and June, 1889. He wrote that dark parallel bands
were noticed several times, and they were very similar to the belts
of Jupiter. M. Perrotin notes that the bands of Uranus do not always
present the same aspect. They vary in size and number in different
parts of their circumference.
For many years it was supposed that Uranus possessed six satellites,
all of which were discovered by Sir W. Herschel, but later
observations proved that four of these had no existence. They were
small stars near the planet. But two of Herschel’s satellites were
fully corroborated, and two new ones were discovered by Lassell and
Struve. The number of satellites attending Uranus is four, and it is
probable that many others exist, though they are too minute to be
distinguished in the most powerful instruments hitherto constructed.
The following are the known satellites: 3d Ariel, discovered in 1847;
4th Umbriel, discovered in 1847; 1st Titania, discovered in 1787, and
2d Oberon, discovered in 1787.
Titania and Oberon are the two brightest satellites, but none of them
can be seen except in large instruments. From observations with large
modern instruments it appears highly probable that the four known
satellites must be considerably larger than any others which may be
revolving round the planet. A curious fact in connection with these
satellites is that their motions are retrograde.
The leading incidents in the narrative of the discovery of Uranus
and Neptune present a great dissimilarity—Uranus was discovered by
accident, Neptune by design. Telescopic power revealed the former,
while theory disclosed the latter. In one case optical appliance
afforded the direct means of success, while in the other the unerring
precision of mathematical analysis attained it. The telescope played
but a secondary part in the discovery of Neptune, for this instrument
was employed simply to realize or confirm what theory had proven.
Certain irregularities in the motion of Uranus could not be
explained but on the assumption of an undetected planet situated
outside the known boundaries of the system. Two able geometers
applied themselves to study the problem of these irregularities,
and to deduce from them the place of the disturbing body. This was
effected independently by Messrs. Le Verrier and Adams; and Dr. Galle
of Berlin, having received from Le Verrier the leading results of his
computations, and the intimation that the longitude of the suspected
planet was then 326°, found it with his telescope on the night of
September 23, 1846, in longitude 326° 52′. The calculated place by
Professor Adams was 329° 19′ for the same date.
The name given to the new planet was Neptune. When the elements were
computed it was found that they presented rather large differences
with those theoretically computed by Le Verrier and Adams. It was
also found that the planet had been previously observed by Lalande on
May 8 and 10, 1795, but its true character escaped detection. This
astronomer had observed a star of the eighth magnitude on May 8; but
on May 10, not finding the same star in the exact place noted on the
former evening, he rejected the first observation as inaccurate and
adopted the second, marking it doubtful. Lalande, like Le Monnier,
the unsuspecting discoverer of Uranus, let a valuable discovery slip
through his hands.
Neptune revolves round the sun in 60,126 days, which is equal to
rather more than 164½ of our years. His mean distance from the sun is
2,792,000,000 miles, and his usual diameter 2″.7. He exceeds Uranus
in dimensions, his real diameter being 37,000 miles.
Our knowledge of this distant orb is extremely limited, owing to his
apparently diminutive size and feebleness. No markings have ever been
sighted on his miniature disk, and we can expect to learn nothing
until one of the large telescopes is employed in the work. No doubt
this planet exhibits the same belted appearance as that of Uranus,
and there is every probability that he possesses numerous satellites.
Directly the new planet was discovered, Mr. Lassell turned his large
reflector upon it and sought to learn something of its appearance,
and possibly detect one or more of its satellites. On October 3 and
10, 1846, he was struck with the appearance of the disk, which was
obviously not spherical. He subsequently confirmed this impression,
and concluded that a ring, inclined about 70°, surrounded the planet.
Professor Challis supported this view, but later observations in
a purer sky led Mr. Lassell to abandon the idea. Thus the ring of
Neptune, like the ring of Uranus, though apparently obvious at first,
vanished in the light of more modern researches.
But if Mr. Lassell quite failed to demonstrate the existence of a
ring, he nevertheless succeeded in discovering a satellite belonging
to the planet. This was on October 10, 1846. The new satellite was
found to have a period of 5 days, 21 hours, and 3 minutes, and to be
situated about 220,000 miles distant from the planet.
END OF VOLUME ONE
TRANSCRIBER’S NOTE
Footnote [13] is referenced twice from page 102.
Obvious typographical errors and punctuation errors have been
corrected after careful comparison with other occurrences within
the text and consultation of external sources.
Some hyphens in words have been silently removed, some added,
when a predominant preference was found in the original book.
Except for those changes noted below, all misspellings in the text,
and inconsistent or archaic usage, have been retained.
Pg 3: ‘Caliph Al-Mamum’ replaced by ‘Caliph Al-Mamun’.
Pg 20: ‘Ninteenth Century’ replaced by ‘Nineteenth Century’.
Pg 21: ‘Map of the’ replaced by ‘Chart of the’.
Pg 21: ‘Hourglass Sea’ replaced by ‘Nine Views of the Hour-Glass Sea’.
Pg 74: ‘cose che redire’ replaced by ‘cose che ridire’.
Pg 74: ‘Nè sa, nè’ replaced by ‘Né sa, né’.
Pg 100: ‘Hesoid’ replaced by ‘Hesiod’.
Pg 122: ‘familar to most’ replaced by ‘familiar to most’.
Pg 150: ‘formed of myraids’ replaced by ‘formed of myriads’.
Pg 223: ‘may be interred’ replaced by ‘may be inferred’.
Pg 238: ‘Will some motral’ replaced by ‘Will some mortal’.
Pg 292: ‘its orbitual motion’ replaced by ‘its orbital motion’.
Pg 380: ‘the Mare Humorom’ replaced by ‘the Mare Humorum’.
Pg 390: ‘present themelves’ replaced by ‘present themselves’.
Pg 391: ‘Mr. Lowell remarks,*’ replaced by ‘Mr. Lowell remarks,’;
(the * anchor had no footnote and has been removed).
Pg 396: ‘permamently shut’ replaced by ‘permanently shut’.
Pg 418: ‘is a thining-out’ replaced by ‘is a thinning-out’.