William F. Denning.

Telescopic Work for Starlight Evenings online

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has a manifest disadvantage because of the artificial illumination of
the atmosphere. But for general telescopic work the conditions do not
offer a serious impediment, especially if the observer is careful to
seize the many suitable occasions that must occur. The direction of the
wind relatively to his position and the central part of the city, will
occasion considerable differences to an observer who uses a telescope
in a suburban locality.

_Photography._—Upon this branch of practical astronomy not much will
be said in this volume, as it is rather beyond its scope, and possibly
also beyond the resources of ordinary amateurs, so far as really
valuable work is concerned. A reference must, however, be made to an
innovation which has deservedly assumed a very prominent place, and is
clearly destined to exert an accelerating influence on the progress of
_exact_ astronomy. At present it is impossible to foretell how far it
may be employed and extended, but judging from recent developments its
applications will be as manifold as they will be valuable. Photographic
records possess a great advantage over others, because they are more
accurate and therefore more reliable. They are pictures from Nature
taken by means free from the bias and error inseparable from mere
eye-estimations or hand-drawings. The latter are full of discordances
when compared one with another, and can seldom be implicitly trusted;
but in the photograph a different state of things prevails. Here we
have a faithful portrayal or reproduction of the object impressed by
itself upon the plate. Hence it can be depended upon, because there
has been no intermediate meddling either with its position or features
by what may be termed artistic misrepresentation. True, there may be
imperfections in the process; trifling flaws and obstructions will
invariably creep in wherever comparatively new and novel work is
attempted, but these will but little detract from the value of its
results. Photography is obviously a means of discovery as well as a
means of accurate record; for nebulæ and faint stars quite invisible to
the eye have been distinguished for the first time upon the negatives.
Those of our amateurs who intend working in this branch will find it a
productive one, and not decaying in interest; but the necessary outfit
will be expensive if thoroughly capable instruments are to be employed
in the service.

[Illustration:

LONDON STEREOSCOPIC CO. PHOTOMEZZOTYPE

STANMORE OBSERVATORY.

OUTSIDE VIEW]

_Publications._—The observer of to-day may esteem himself particularly
fortunate in regard to the number and quality of the astronomical
journals within his reach. Discoveries and current events receive
prompt notice in these, and readers are fully informed upon the
leading topics. Among the best of the periodicals alluded to are ‘The
Observatory’ (Taylor & Francis, London), ‘The Sidereal Messenger’
(Northfield, Minn., U.S.A.), and _L’Astronomie_ (Gautier-Villars,
Paris). The _Astronomische Nachrichten_ (Kiel, Germany) is a very old
and valued serial, and ‘The Astronomical Journal’ (Cambridge, Mass.,
U.S.A.) may also be favourably mentioned. The ‘Monthly Notices’ of the
Royal Astronomical Society and the ‘Journals’ of the Liverpool and
British Astronomical Societies contain many interesting materials.
‘Nature,’ ‘The English Mechanic,’ and ‘Knowledge’ are among the
English journals which devote part of their space to the science;
and the beautiful illustrations in the latter entitle it to special
recognition. It is evident, from this short summary, the amateur will
find that his literary appetite may be amply satisfied, and should he
desire a channel for recording his own work or ideas the publications
referred to offer him every facility and encouragement.

As to almanacks, the ‘Nautical’ which has been termed “The Astronomer’s
Bible,” includes a mass of tabular matter, some portion of which is of
utility to the amateur, but it does not give data which are to be found
in some other publications. I refer particularly to ephemerides of the
satellites of Mars, Saturn, Uranus, and Neptune, to the dates of max.
and min. of variable stars, to the times of rising and setting of the
Sun, Moon, and planets, to the epochs and positions of meteor-showers,
&c. The annual ‘Companion to the Observatory’ furnishes most of
these details, and ‘Whitaker’s Almanack’ and Brown & Sons’ ‘Nautical
Almanack’ each contain a large amount of serviceable information.
The latter, however, is chiefly devoted to topics connected with
Navigation, while ‘Whitaker’s Almanack’ is an extensive repertory of
general facts.

With respect to handbooks much depends upon the direction of the
observer’s labours, for he will obviously require works dealing
expressly with his special subject. As a reliable companion to the
telescope, Webb’s ‘Celestial Objects for Common Telescopes’ (4th edit.,
1881) is indispensable; as a work of reference, and one forming an
exhaustive conspectus of astronomical facts, Chambers’s ‘Descriptive
Astronomy’ (4th edit., in 3 vols., 1889) may be recommended.
Ledger’s ‘The Sun, its planets and their satellites’ is another good
descriptive work. The beginner will find Noble’s ‘Hours with a 3-inch
Telescope’ full of very instructive and agreeable material; while the
more experienced astronomer, requiring a masterly exposition of the
principles of the science, must procure Sir J. Herschel’s ‘Outlines’
(11th edit., 1871). In departmental work books of more exclusive
character will be necessary. Thus, students of solar physics will
want Young’s volume on ‘The Sun;’ observers of our satellite will
need Neison’s ‘Moon.’ Those who find double stars interesting should
get Crossley, Gledhill, & Wilson’s ‘Handbook’ and Chambers’s revised
edition of Admiral Smyth’s ‘Cycle;’ others working on variable stars
will need the Catalogues of Chandler and Gore. Jovian phenomena are
well represented in Stanley Williams’s ‘Zenographic Fragments.’ Comets
have been fully treated of in works by Cooper, Hind, and Guillemin;
while to the observer of eclipses Johnson’s ‘Eclipses Past and
Future’ is a valuable guide. Everyone interested in nebulæ will of
course require Herschel-Dreyer’s ‘General Catalogue,’ containing 7840
objects and published by the Royal Astronomical Society in 1888. As
to planetary observations, the several works of Webb, Chambers (vol.
i.), and Ledger, first cited, supply a large amount of detail, almost
obviating the necessity for further books.

_Past and Future._—Observers and telescopes go on increasing day
by day, and the future of astronomy has a most brilliant outlook.
Photography has latterly effected a partial revolution in observation,
though it can never entirely supersede old methods. Spectrum analysis,
too, has formed a valuable acquisition during the last quarter of a
century. With the new and refined processes, and with the gigantic
instruments which have been erected, we may confidently anticipate
many additions to our knowledge, especially in regard to very small
and faint bodies which the inferior appliances of previous years
have failed to grasp. And it is certain that some of the presumed
discoveries of past times must be expunged, because not verified by the
more perfect and powerful researches of a later date. Let us place in
parallel columns (1) a few of the suspected objects thus to be erased,
and (2) some of those which the future will probably add to our store:-–

(1.) (2.)

Satellite of Venus. Satellites of Uranus and Neptune.
Vulcan. Ultra-Neptunian Planet.
Active Volcanoes on the Moon. Changes on the Moon.
Detached cusps of Venus and Mercury Rotation of Mercury, Venus,
indicating high mountains. Uranus, and Neptune.
Rings of Uranus and Neptune. Minor Planets.
Multiple companions to Polaris Periodical Comets.
and Vega. Nebulæ and Double Stars.

Whatever may be the direction of future enquiries or the departures
from old and tried methods, ordinary amateurs with small instruments,
though handicapped more heavily as regards the prospect of effecting
discoveries, may yet always be expected to accomplish useful work.
Even to him who simply makes the science a hobby and a source of
recreation in a leisure hour after the cares of business, the sky never
ceases to afford a means of agreeable entertainment. He may neither
achieve distinction nor seek it; but this he will assuredly do—gain
an instructive insight into the marvellous works of his Creator, and
acquire a knowledge which can only exercise an elevating tone to his
life. The observer who quietly, from his cottage window, surveys the
evening star or the new Moon through his little telescope often finds a
deeper pleasure than the proficient astronomer who, from his elevated
and richly appointed observatory, discovers new orbs with one of the
most powerful instruments ever made.

_Attractions of Telescopic Work._—In concluding our comments we may
briefly refer to the importance and pleasure attached to telescopic
work, and the growing popularity of observation in the attractive and
diverse field of astronomy. A telescope may either be employed as an
instrument of scientific discovery and critical work, or it may be
made a source of recreation and instruction. By its means the powers
of the eye are so far assisted and expanded that we are enabled to form
a clearer conception of the wonderful works of the Creator than could
be obtained in any other way. Objects which appear to natural vision in
dim and uncertain characters are resolved, even in telescopes of the
smallest pretentions, into pictures of well-defined outlines containing
details of configuration far exceeding what are expected. And it is
entirely owing to the exact measurements obtained under telescopic
power that many of the most important problems of astronomy have been
satisfactorily solved. To this instrument we are indebted, not only
in a great measure for our knowledge of the physical features of many
celestial bodies, but also for the accurate information we have gained
as to their motions, distances, and magnitudes. Apart from this it is
capable of affording ample entertainment to all those who are desirous
of viewing for themselves some of the absorbing wonders of astronomy as
described in our handbooks. And a demonstration of this practical kind
is more effective than any amount of description in bringing home to
the comprehension of the uninitiated the unique and picturesque side of
astronomy.




CHAPTER V.

_THE SUN._

Solar Observations.—Early notices of Spots.—Difficulties of the
old observers.—Small instruments useful.—Tinted glass.—Solar
Diagonal.—Structure of a Spot.—Methods of Drawing.—Ascertaining
Dimensions.—Observer’s aims.—Eclipses of the Sun.—Periodicity of
Spots.—Crateriform structure.—“Willow-Leaves.”—Rotation of the
Sun.—Planetary bodies in transit.—Proper motion of Sun-spots.—Rise
and decay of Spots.—Black Nuclei in the umbræ.—Bright objects near
the Sun.—Cyclonic action.—Sudden outbursts of Faculæ.—Shadows
cast by Faculæ.—Veiled Spots.—Recurrent disturbances.—Recurrent
forms.—Exceptional position of Spots.—The Solar prominences.


“Along the skies the Sun obliquely rolls,
Forsakes, by turns, and visits both the poles;
Diff’rent his track, but constant his career,
Divides the times, and measures out the year.”

The Sun is not an object comprehended in the title of this volume.
But to have omitted reference to a body of such vast importance,
and one displaying so many interesting features to the telescopic
observer, would have been inexcusable. We may regard the Sun as the
dominant power, the controlling orb, and the great central luminary of
our system. The phenomena visibly displayed on his surface assume a
particular significance, as affecting a body occupying so high a place
in the celestial mechanism.

The mean apparent diameter of the Sun is 32′ 3″·6, and his real
diameter 866,000 miles. The apparent diameter varies from a minimum
of 31′ 32″ at the end of June to a maximum of 32′36″; at the end of
December; and the mean value is reached both at the end of March and
September. The Sun’s mean distance from the Earth is about 92,900,000
miles, computed from a solar parallax of 8″·8, which appears to agree
with the best of recent determinations. At this distance the linear
value of 1″ of arc is 447 miles.

The Sun’s apparent diameter is as follows on the first day of each
month:—

′ ″
Jan. 1 32 36·0
Feb. 1 32 31·8
Mar. 1 32 20·4
April 1 32 3·8
May 1 31 48·0
June 1 31 36·4
July 1 31 32·0
Aug. 1 31 35·8
Sept. 1 31 47·0
Oct. 1 32 2·6
Nov. 1 32 19·2
Dec. 1 32 31·6

Solar observations may be pursued with a facility greater than that
attending work in some other departments of practical astronomy. The
Moon, planets, and stars have to be observed at night, when cold air,
darkness, and other circumstances are the cause of inconvenience;
but the student of the Sun labours only in the light and warmth of
genial days, when all the incidentals to observation may be agreeably
performed. There are, however, some drawbacks even in this pleasant
sphere of work. The light of the Sun is so great that much persistent
observation is apt to have an injurious effect on the eye, and will
certainly deaden its sensitiveness on faint objects. In the summer
months the observer experiences discomfort during a lengthy observation
from remaining so long in the powerful rays of the Sun, some of which
must fall upon his face unless measures are adopted to shield it.
During the progress of solar work the student should always provide
for himself as much shelter as possible from the glare, which must
otherwise disturb that equanimity of feeling in the absence of which no
delicate research is likely to be successfully conducted.

“Spots on the Sun” were remarked long before the telescope came into
service. In the early Chinese annals many references are made to these
objects; thus, in A.D. 188, February 14, it is recorded—“The colour
of the Sun reddish-yellow; a fleckle in the Sun (bird-shaped).” Other
ancient notices compare the spots to a flying bird, an apple, or an
egg. Many spots were seen in later years, especially in 321, 807, 840,
1096, &c. In 807 a large black spot upon the Sun was watched during a
period of eight days. It reflects much credit upon observers of a past
age that they performed so many useful feats of observation, though
relying simply upon the powers with which Nature alone had endowed
them. They anticipated the telescope in some important discoveries.
Large sun-spots are not, it is true, difficult features to perceive
with the naked eye under certain circumstances; for whenever there
is a fog or haze sufficiently dense to veil the lustre of the Sun in
suitable degree, they can be readily seen, presuming, of course, that
such spots are in existence at the time. They are sometimes observed,
in a purely casual way, by people who may happen to glance at the Sun
when he is involved in fog and looks like a dull, red ball suspended in
the firmament. On one occasion, near sunset, in the autumn of 1870, I
saw four large spots on different parts of the Sun, and these phenomena
were very numerous at about this time. When spots attain a diameter of
50″ or more they may be detected by persons of good sight; but if the
Sun is high and clear, coloured glass must be used to defend the eye.

Doubt hangs over the question as to the first telescopic observer of
the spots. It is certain that Fabricius, Galilei, Harriot, and Scheiner
all remarked them in about the year 1611; and of these Fabricius
perhaps deserves the chief praise, as the first who published a memoir
on the subject. Galilei appears undoubtedly to have had priority in
recognizing the bright spots, or _faculæ_. Scheiner discovered that the
black spots, or _maculæ_, are composed of a dark umbra and a fainter
outlying shade, called the penumbra. Arago quotes him as having also
described the Sun as “covered over its whole surface with very small,
bright, and obscure points, or with lively and sombre streaks of
very slender dimensions, crossing each other in all directions.” He
announced, too, that the spots were confined to a narrow zone on the
north and south sides of the equator, and this he termed the “Royal
Zone.”

Some grave difficulties appear to have marked the attempts of the
earlier observers; for they did not all use coloured glasses, and
the dazzling light of the Sun, intensified by their lenses, often
overpowered the sight, and so we find them awaiting opportunities when
fog partly obscured the Sun near his rising or setting. Thus Harriot,
who seems to have noticed and figured three sun-spots as early as 1610,
Dec. 8, says:—“The altitude of the Sonne being 7 or 8 degrees, and it
being a frost and a mist, I saw the Sonne in this manner.” His drawing
followed. On another occasion he says:—“A notable mist: I observed
the Sonne at sundry times, when it was fit.” Fabricius advised other
observers to commence their observations by admitting only a small
portion of the Sun into the field, so that the eye might be prepared to
receive the light of the entire disk. Galilei was equally unaware of
the advantage of tinted glass, and adopted the expedient of scanning
the Sun when placed in the vicinity of the horizon. He remarks that
“the spot of 1612, April 5 appeared at sunset;” and his writings
contain other references of similar import. Scheiner, however, appears
to have been more alive to the requirements of the work, and employed a
plain green glass placed in front of the object-lens of his telescope.

Under the various circumstances we have been alluding to, the views
obtained of the solar surface must necessarily have been of a very
defective character, and the old observers at least deserve our
sympathy in their exertions. No such obstacles confront the observer
now. He has everything provided for him. Instrumental devices rob the
Sun of his noonday brilliancy, and the eye serenely scans the details
of his expansive image without the slightest pain or effort.

Small telescopes are peculiarly well adapted for solar observations. A
good 3-inch refractor or 4-inch reflector will reveal an astonishing
diversity of structure in the spots, and show something of the
complicated _minutiæ_ of the general surface. If the aperture of
either instrument is 2 inches more than that stated, so much the
better; but further than this it is rarely advisable to go. When the
objective or mirror exceeds a diameter of 5 or 6 inches a stop often
improves the images, and even smaller instruments will perform better
when a little contracted. Definition is here the point to be desired;
of light we have a superabundance. But if the observer meditates a
critical analysis of the detail, either of a single spot, of a group of
spots, or of a small area of the luminous surface, then a fair amount
of aperture should be used, because greater aperture means greater
separating power, and the latter will be useful in resolving the
network of fibrous materials of which apparently the whole surface is
composed. But for the common requirements of the observer an instrument
of 3 or 4 inches will be found very effective, and it can either be
used on a short tripod stand, placed on a steady table near a window
having a south aspect, or it may be mounted on a tall garden stand and,
according to the owner’s pleasure, either fixed at his window or in
his garden. Two powers will be really necessary—one of about 60 and a
field of quite 33″ to contain the entire disk and give a good general
view, and another of 150 to which the observer will have recourse when
examining details. Additional eyepieces will be sometimes useful,
especially one of about 100; but the power of 60 previously recommended
will, if a Huygenian, answer the same purpose, for if the field-lens
is removed it will be increased to about 90. And should the observer
think that anything is to be gained by a higher magnifier than 150, let
him use the eye-lens only of that power. I have obtained many exquisite
views of sun-spots with a single lens, and, instead of purchasing
new eyepieces, a real advantage will be derived in adopting the plan
suggested. There will be a smaller field and more colour about the
image, but the improvement in definition is considerable, and more than
balances these disadvantages.

Tinted glass must always be employed, unless a dense fog prevails, in
which case the example of the old observers may be emulated. Several
coloured glasses, of various depths, are needed for use according as
the occasion requires. With a high Sun on a bright June day a darker
tint will be necessary than in the winter, when the Sun’s rays are
but feebly transmitted through the horizontal vapours. Red glass is
unsatisfactory, as there is much heat and glare with it; but when used
in combination with green the effect is excellent. Green alone is often
used, and answers well; but it is not always thick and dense enough for
the purpose. The plan of Sir W. Herschel, to interpose a glass trough
of diluted ink, has never become popular, though he found it to succeed
admirably. Smoked glass is also adapted for solar work, and recommends
itself as being always obtainable at a minute’s notice. Some observers
use a Barlow lens, with a thin film of silver deposited on the
surfaces. It is then sufficiently transparent to give a neutral tint
when held before a light, and sharp definition is said to be obtained
without additional protection. Mr. Thornthwaite has also employed a
coloured Barlow lens with effect.

A solar diagonal is a very necessary appliance if the observer would
ensure perfect safety; for any refractor exceeding 2-inches aperture
may, when turned on the Sun, focus enough heat to fracture the tinted
sun-glass. The diagonal, by preserving a part only of the solar rays
which are transmitted by the object-glass, enables observations to be
made in security. This little instrument is comparatively cheap, and no
telescope is complete without one. Dawes’s solar eyepiece serves the
same purpose in a different manner, but it is an expensive luxury. In
the latter construction there is a perforated diaphragm fixed near the
eyepiece and so arranged that the quantity of admitted light may be
modified consistently with the observer’s wishes.

In reflecting-telescopes with glass mirrors, effective views of the Sun
are obtainable by employing unsilvered mirrors; for sufficient light is
reflected by the glass surfaces to form good images of solar detail.

What, perhaps, interferes more than any other circumstance with
successful observation of the Sun, is the fact that the rays, falling
upon the telescope and objects near, induce a good deal of radiation,
the direct tendency of which is to impair the definition and give a
rippling effect to the disk. This is sometimes present in such force
that the spots are subject to an incessant commotion, which serves
to obliterate their more delicate features. A shady place is best,
therefore, for such work; and if the observer leaves his telescope
for a short time, intending to resume observations, it should never
be placed broadside to the Sun, or the tube wall get hot, and heated
currents must be generated in the interior, to the ruin of subsequent
views.

A large sun-spot consists of an apparently black nucleus, a brown
umbra, divided possibly by veins of bright matter or by encroachments
of the penumbra which surrounds it. The latter is of much lighter
tone than the umbra, though often similar in its general form. The
outer edges of the umbra are serrated or scalloped by rice-grain
protuberances. The inner region of the penumbra is much brighter than
the outer, and the latter often exhibits quite a dusky fringe, induced
by lines of dark material intervening with the brighter particles.
The filaments forming the penumbra—often grouped in a radial manner
with reference to the centre of a spot—would appear to be more widely



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 8 of 32)