William F. Denning.

Telescopic Work for Starlight Evenings online

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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.”




CHAPTER XIV.

_COMETS AND COMET-SEEKING._

Ideas concerning Comets.—Appearance.—Large number
visible.—Nature of Apparition.—Tenuity of Comets.—Differences
of Orbit.—Discoveries of Comets.—Large Comets.—Periodical
Comets.—The Comets of Halley, Encke, Biela, Brorsen, Faye,
D’Arrest, Pons-Winnecke, and Tuttle.—Grouping.—Further Observations
required.—Nomenclature of Comets.—Curiosities of Comets.—Naked-eye
Comets.—Comet-seeking.—English weather.—Aperture and Power
required.—Annual rate of Discovery.—Telescopic Comets and
Nebulæ.—Ascertaining Positions.—Dr. Doberck’s hints.—Prizes.


Superstitious ideas with regard to comets as the harbingers of disaster
have long since been discarded for more rational opinions. They are no
longer looked upon as ill-omened presages of evil, or as

“From Saturnius sent,
To fright the nations with a dire portent.”

Many references are to be found among old writings to the supposed evil
influence of these bodies, and to the dread which their appearance
formerly incited in the popular mind. Shakespeare makes an allusion to
the common belief:—

“Hung be the heavens with black, yield day to night!
Comets, importing change of time and states,
Brandish your crystal tresses in the sky;”

and in relation to the habit of connecting historical events with their
apparition, he further says:—

“When beggars die, there are no comets seen;
The heavens themselves blaze forth the death of princes.”

But, happily, the notions prevalent in former times have been
superseded by the more enlightened views naturally resulting from
the acquirement and diffusion of knowledge; so that comets, though
still surrounded by a good deal of mystery, are now regarded with
considerable interest, and welcomed, not only as objects devoid of
malevolent character, but as furnishing many useful materials for
study. Mere superstition has been put aside as an impediment to real
progress, and a more intelligent age has recognized the necessity of
dealing only with _facts_ and explaining them according to the laws of
nature; for it is on facts, and their just interpretation, that all
true searchers after knowledge must rely. Comets are properly regarded
as bodies which, though far from being thoroughly understood in all the
details of their physical structure and behaviour, have yet a wonderful
history, and one which, could it be clearly elucidated, would unfold
some new and marvellous facts. Under these circumstances we need evince
no surprise that these visitors are invariably hailed with enthusiasm,
not only by scientific men, who make them the special subjects of close
observation, but by everyone who regards celestial “sights and signs”
with occasional attention.

_Appearance._—From whatever point of view a large comet is considered,
it deserves all the interest manifested in it and all the labour
expended in its investigation. Whilst its grand appearance in the
firmament arrests the notice of all classes alike, and is the subject
of much curious speculation amongst the uninformed, it merits, apart
from other considerations, the most assiduous observation on account
of the singular features it displays and the striking variations they
undergo. Indeed, the visible deportment of a comet during its rapid
career near perihelion is so extraordinary as to form a problem, the
solution of which continues to defy the most ingenious theories. The
remarkable changes in progress, the quickness and apparent irregularity
of their development, are the immediate result of a combination of
forces, the operations of which can neither be defined nor foreseen.
Jets of flame and wreaths of vapour start from the brilliant nucleus;
while, streaming away from the latter, in a direction opposite to the
Sun, is the fan-shaped tail, often traceable over a large span of the
heavens and commingling its extreme fainter limits with the star-dust
in the background.

_Large number visible._—The orbits of 400 comets have now been
computed, and more than 500 others have been observed; so that these
bodies are extremely plentiful. Kepler described them to be as numerous
as the fishes in the sea, and no doubt the allegory is justly applied.
Their vagaries of form, size, and place are equally noteworthy; and
those who enter upon the discussion of facts relating to these objects
will find an endless store of interesting materials, opening up a wide
field for conjecture.

_Nature of Apparition._—The apparition of a comet may be either gradual
or sudden. Usually the telescope gives us the earliest intimation
that one of these bodies is approaching us[41]. It is first seen as
a small round nebulosity, with probably a central condensation or
stellar nucleus of the 10th or 11th mag. The whole object brightens and
expands as its distance grows less, and it assumes an elongated form
preparatory to the formation of a tail. The latter varies greatly in
different instances: it may either be a narrow ray, as shown in the
southern comet of January 1887, or a fan-shaped extension like that of
the great comet of 1744. Barnard’s Comet of December 1886 exhibited
a duple tail. Occasionally a fine comet bursts upon us suddenly,
like that of 1843 or 1861. The former was sufficiently bright to be
discovered when only 4° from the Sun, and the latter presented itself
quite unexpectedly as a magnificent object even in the strong twilight
of a June sky.

_Tenuity of Comets._—Comets are noteworthy for the extreme thinness of
their material. The smallest stars may be discerned through the denser
portions of the head, without suffering any apparent diminution of
light. Yet such stars would be quite obscured by the interposition of
a minute speck of cloud or by a little fog or any vapour of trifling
density. Comets are visible in the form of transparent nebulosities;
and their mass must be inconceivably small relatively to the enormous
space over which they frequently extend. Sir J. Herschel has described
the “all but spiritual texture” of comets; and other authorities have
referred to them as feeble wreaths of vapour, which, though obeying
the laws of gravitation and suffering much perturbation, are yet
themselves incapable of exercising any disturbing influence upon the
other bodies near which they pass. It has been asserted that comets
would show phases were they rendered luminous by reflected sunlight,
and that, such features being absent, these bodies must possess a
phosphorescence of their own sufficient to cause the glow observed.
This idea, however, is hardly consistent with our present knowledge.
Comets are not compact and coherent masses of matter; they more
likely represent vast groups of planetary atoms, more or less loosely
dispersed and sometimes forming streams. The effect of sunshine
upon such assemblages will be that the whole mass becomes illumined
according to density, and that no phase will be apparent, inasmuch as
the light is enabled to penetrate through its entirety.

_Differences of Orbit._—When three trustworthy observations of a
comet’s place have been made, its orbit may be computed. This may be
either an ellipse, a parabola, or hyperbola. If an _ellipse_ the comet
is periodical, and the period depends upon the degree of eccentricity.
If a _parabola_ the comet will not be seen again, because this form of
orbit does not reunite; it consists of branches equally divergent and
uniting at perihelion, but extending outwards indefinitely in nearly
parallel lines and without convergence. If a _hyperbola_, the comet is
also not returnable; the branches of the orbit are widely divergent,
and show no tendency to parallelism. These several forms of orbit are
somewhat different as applied to various comets, but they are the same
in effect. Thus Tempel’s Comet of 1867 revolves in an ellipse having
an eccentricity of about 0·4630, while that of Halley’s Comet is
0·9674. No doubt some of the parabolic orbits applied to comets really
represent very eccentric ellipses; but the parabola is a convenient
form of orbit for computation, and unless ellipticity is very decided
it indicates the path with sufficient accuracy.

_Discoveries of Comets._—In the latter part of the last century
Messier, Mechain, and Miss Herschel shared nearly all the cometary
discoveries between them. Then Pons entered the field, and he may be
said to have monopolized this branch during the period from 1802 to
1827, for he was the first to announce thirty comets. Pons died in



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 19 of 32)