_URANUS AND NEPTUNE._
Discovery of Uranus.—Mistaken for a Comet.—True character
revealed.—Period &c.—Observations.—Belts on Uranus.—Further
Observations required.—The Satellites.—Discovery of Neptune.—The
planet observed in 1795.—Period &c.—Observations.—Supposed
Ring.—Satellite.—A trans-Neptunian Planet.—Planetary Conjunctions.
_Discovery._—While Sir W. Herschel was a musician at Bath he formed
the design of making a telescopic survey of the heavens. When 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
afterwards 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. His paper begins as follows:—
“On Tuesday, March 13, 1781, between 10 and 11 in the evening, while I
was examining the small stars in the neighbourhood of H Geminorum, I
perceived one that appeared visibly larger than the rest. Being struck
with its uncommon magnitude, I compared it to H Geminorum and the small
star in the quartile between Auriga and Gemini, and finding it so much
larger than either of them suspected it to be a comet.... The power I
had on when I first saw the comet was 227.”
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 cannot 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.”
_True character revealed._—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 Chaldæans 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 amongst 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
12 nights in the years from 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 the _Georgium Sidus_, and there were others who
termed it “Herschel,” in honour of the man through whose sagacity it
had been revealed.
_Period &c._—Uranus revolves round 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.
_Observations._—The planet near opposition shines like a star of
the 6th 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.
Lassell, when working with his 2-foot speculum at Malta, thought he
saw a spot near the centre of the planet’s disk, but he considered
this might possibly be due to an optical illusion. In 1862, Jan. 29,
he said:—“I received an impression which I am unable to render certain
of an equatoreal dark belt.” In the early months of 1870, Mr. Buffham,
using a 9-inch “With” mirror, powers 212 and 320, saw bright spots and
zones on the planet, and inferred a rotation-period of about 12 hours.
On Jan. 16, 1873, when definition was very good, no traces of any
markings were visible in Lord Rosse’s 6-foot reflector. In May and June
1883 Prof. 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 equatoreal at
Nice, remarked dark spots similar to those on Mars, towards the centre
of the disk, and a white spot was seen on the limb. Two different
tints were perceived, the colour of the N.W. hemisphere being dark,
and that of the S.E. a bluish-white colour. 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
10 hours. The brothers Henry at Paris, in 1884, invariably noticed two
belts lying parallel to each other, and including between them the
brighter equatoreal 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°.
[Illustration: Fig. 43.
Uranus and his Belts. 1884.]
M. Perrotin, with the great 30-inch equatoreal at Nice, re-observed
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. On May 31 and June 1 and 7 the direction of the Uranian
belts was measured, and the mean result showed that the plane of the
equator of Uranus differs little (about 10°) from the common plane
of the orbits of the satellites. This deduction is not, it will be
observed, consistent with that of the Brothers Henry at Paris, who
found a difference of 41°. 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. This unequal distribution
raises the hope that by an attentive study of these bands it will be
possible to determine the duration of the planet’s rotation.
_Further Observations required._—In the case of an object so faint
and diminutive as Uranus, a powerful telescope is absolutely required
to deal with it effectively. A small instrument will readily show the
disk, and present the picture that caught the eye of Herschel more than
a century ago, but considerable light and power must be at command if
the observer would enter upon a study of the planet’s surface-markings.
With my 10-inch reflector I have suspected the existence of the belts,
but under high powers the image is too feeble to exhibit delicate forms
of this character. It is to be hoped that with the large telescopes now
available at various observatories, some attention will be given to
this planet, more especially with regard to the study of the belts and
determination of the rotation-period. Amateurs will have little trouble
in picking up Uranus; his position can be learnt from an ephemeris
and marked upon a star-map. A little careful sweeping with a low
power in the region indicated will soon reveal the object sought for,
and a higher power may then be applied to expand the disk and render
identification certain.
It may be mentioned as an interesting point that some fifty years after
the discovery of Uranus by Sir W. Herschel the planet was accidentally
rediscovered by his son Sir John Herschel, who mentioned the fact as
follows in a letter to Admiral Smyth, written on Aug. 8, 1830:—“I have
just completed two 20-foot reflectors, and have got some interesting
observations of the satellites of Uranus. The first sweep I made with
my new mirror I _rediscovered_ this planet by its _disk_, having
blundered upon it by the merest accident for 19 Capricorni.” Had the
father failed to detect this planet in 1781, the discovery might
therefore have been made by the son half a century later.
Some spectroscopic observations of Uranus made in 1889 with Mr.
Common’s 5-foot reflector, appear to show that the planet “is to a
large extent self-luminous.” But Mr. Huggins on June 3 seems to have
obtained a different result (see ‘Monthly Notices,’ xlix. p. 404 _et
seq._).
_The Satellites._—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 known 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 periods, distances, &c., of the known satellites:—
+——————————————+————————————————————+———————+————————————————+
| | Mean distance. | | |
| Number and +——————————+————————-+ Max. | Date of |
| name. | Diameters| Miles. |Elonga-| Discovery. |
| |of Uranus.| | tion. | |
+——————————————+————————————————————+———————+————————————————+
| | | | ″ | |
| 3rd. Ariel | 4·03 | 125,000 | 12 | 1847, Sept. 14.|
| 4th. Umbriel | 5·61 | 174,000 | 15 | 1847, Oct. 8.|
| 1st. Titania | 9·19 | 285,000 | 33 | 1787, Jan. 11.|
| 2nd. Oberon | 12·32 | 382,000 | 44 | 1787, Jan. 11.|
+——————————————+————————————————————+———————+————————————————+
————————————-+——————————+
| |
Discoverer. | d h m |
| |
| |
————————————-+——————————+
| |
W. Lassell. | 2 12 29 |
O. Struve. | 4 3 27 |
W. Herschel.| 8 16 57 |
W. Herschel.| 13 11 7 |
————————————-+——————————+
Titania and Oberon are the two brightest satellites, but none of
them can be seen except in large instruments. The two outer ones are
said to have been glimpsed in a 4·3-inch refractor, but this feat is
phenomenal, and certainly no criterion of ordinary capacity. Sir J.
Herschel found them tolerably conspicuous in a reflector of 18 or 20
inches aperture, and mentioned a test-object by which observers might
determine whether their telescopes were adequate to reveal them.
This test is a minute double star lying between the stars β′ and β^2
Capricorni. The magnitudes are 15 and 16, and distance 3″. Relatively
to the satellites of Uranus this faint double is a “splendid object.”
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.
[Illustration: Fig. 44.
Apparent Orbits of the Satellites of Uranus, as seen in an Inverting
Telescope.
(The small circle in the above diagram represents the planet and is
on the same scale as the orbits. The arrows show the direction of the
motion of the satellites, and the figures indicate the number of days
from the time of the last North elongation.)]
_Discovery of Neptune._—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 appliances 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 Sept. 23, 1846, in
longitude 326° 52′. The calculated place by Prof. Adams was 329° 19′
for the same date and less accurate than the prediction of Le Verrier.
The former had priority both in attacking the problem and resolving
it, though unfortunately his efforts were not backed up in a practical
way. But for the supineness of certain officials, there is little doubt
that the planet would have been telescopically discovered in the autumn
of 1845, when it was within 1° 49′ of the place attributed to it by
Prof. Adams. Delays occurred owing to the doubts prevailing, and in the
meantime the planet was found elsewhere. This circumstance does not rob
Prof. Adams of his hard-earned laurels, though it shows how seriously
official negligence can mar the character of a discovery.
_Observations in 1795._—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 Messrs.
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 8th mag. 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.
Had Lalande exercised a little discretion, and confided in his work,
he would hardly have allowed the matter to rest here. A subsequent
observation would at once have exhibited the cause of the discrepancy,
and the mathematical triumph of Le Verrier and Adams, half a century
later, would have been forestalled. Lalande, like Le Monnier, the
unsuspecting observer of Uranus, let a valuable discovery slip through
his hands.
_Period &c._—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.
_Observations._—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 a numerous
retinue of satellites. In dealing with an object like this small
instruments are useless; they will display the disk, and enable us to
identify the object and determine its position if necessary, but beyond
this their powers are restricted by want of light.
_Supposed Ring._—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 perfectly spherical. He subsequently confirmed
this impression, and concluded that a ring, inclined about 70°,
surrounded the planet. Prof. 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.
_The Satellite._—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 Oct. 10, 1846. The
new satellite was found to have a period of 5^d 21^h 3^m, and to be
situated about 220,000 miles distant from the planet. Its apparent star
mag. is 14, and at max. elongation it extends its excursions to 18″ on
either side of its primary. Compared with the other satellites of our
system the one attending Neptune must be excessive in regard to size,
or it could not be discerned at the vast distance separating it from
the Earth.
[Illustration: Fig. 45.
Apparent Orbit of the Satellite of Neptune, as seen in an Inverting
Telescope.
(The small circle in the above diagram represents the planet, the
arrows show the direction of motion, and the figures indicate the
interval from the time of last North-east elongation.)]
_A trans-Neptunian Planet._—Is there a planet beyond Neptune? Prof.
Forbes wrote a memoir in 1880 tending to prove that two such planets
exist. From the influences exerted by these bodies on certain comets of
long period, he approximately deduced the positions of the former, and
they were searched for with the great Washington refractor, but without
success. Flammarion and Todd have also arrived at conclusions affirming
the existence of a planet outside Neptune; but the idea has not yet
been realized by its telescopic discovery.
_Planetary Conjunctions._—Before concluding this chapter, an allusion
should be made to a noteworthy class of events, viz., planetary
conjunctions. These include some of the most attractive aspects
displayed by the heavenly bodies, and they are sometimes witnessed
by ordinary persons with the same amount of gratification as by the
astronomical amateur. In almanacks the times of such conjunctions are
given, so that intending observers may always be prepared for these
events. In a strict sense a conjunction occurs at the instant when
two or more bodies have the same right ascension, but the term is
here intended to have a more general reference, _i. e._, to denote
the assembling together of two or more planets in the same region
of the firmament. Historical records furnish us with a considerable
number of planetary conjunctions, and some of them were attentively
observed long before the telescope came into use. Thus in 2012 B.C.,
Feb. 26, the Moon, Mercury, Venus, Jupiter, and Saturn were in the same
constellation, and within 14° of one another. In 1186 A.D., Sep. 14,
the Sun, Moon, and all the known planets are said to have been situated
in Libra. In 1524 Venus, Mars, Jupiter, and Saturn were near together.
Many similar instances might be quoted, but this is unnecessary.
Occasionally the conjunctions were so close that one planet appeared
to occult another. Kepler refers to an occultation of Jupiter by Mars
which he saw on January 9, 1591; but this would really be a transit of
Mars across the disk of Jupiter, if contact actually occurred, for the
apparent diameter of Jupiter always exceeds that of Mars. Mœstlin seems
to have witnessed an occultation of Mars by Venus on Oct. 3, 1590. It
is probable, however, that these were near approaches only. A genuine
occultation of Mercury by Venus was telescopically observed on May 17,
1737.
On the evening of March 3, 1881, the new Moon, Venus, Jupiter, and
Saturn formed a brilliant quartet in Pisces. On the morning of July
21, 1881, I saw the Moon, Venus, Mars, Jupiter, Saturn, and Aldebaran
in the same region above the eastern horizon. There was a very close
conjunction of Mars and Saturn on the morning of Sept. 20, 1889. Mr.
Marth computed that the nearest approach would occur at 8^h 7^m A.M.,
when the distance between the centres would be 54″·8 and less than
that (74″) observed at the time of the close conjunction of the same
planets on June 30, 1879.
The interest centred in the conjunction of Sept. 20, 1889, was enhanced
by the fact that Regulus was only 47′ distant, while Venus was also in
the same region. I observed this phenomenon in my 10-inch reflector,
and with the help of a comet-eyepiece made the above sketch of the
positions of the objects as they were presented in the field.
[Illustration: Fig. 46.
Mars, Saturn, and Regulus in same field, Sept. 20 1889, 4^h 45^m A.M.]
Perhaps there is not much scientific importance attached to the
observation of these conjunctions, though comparisons of colour and
surface-brilliancy are feasible at such epochs, and are not wholly
without value. As spectacles merely, they possess a high degree of
interest to everyone who “considers the heavens.”