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’.