_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
separated near the outer border of the penumbra, and sufficiently so to
allow sections of the umbral layer of the Sun to be observed through
the interstices. The lighter tint of the interior part of the penumbra
is stated to be due to contrast; but this is a mistake. The difference
is too definite and distinct to permit such an explanation. Mr. Maunder
says “that usually (not invariably) the penumbra darkens towards the
umbra, and that the phenomenon as ordinarily described is merely an
effect of contrast.” My own observations, however, appear to show that
there is an actual difference of detail in the outer and inner portions
of the penumbra, which gives a darker tone to the former.
In drawing the forms of sun-spots the observer must be expeditious,
because of the variations which are quickly and constantly affecting
them. In concluding a sketch I find it essential to make several
alterations in it, owing to the changes which have occurred in the
spots during the interval of a quarter of an hour or so since it was
commenced. The details must be filled in consecutively, each one
being the result of a careful scrutiny. When finished, the whole
sketch should be compared with the object itself and amended if found
necessary. The observer should also mark upon the sheet the measured
or estimated latitude and longitude of the spot, and make a finished
drawing from the basis of his sketch as soon as possible afterwards.
At Stonyhurst Observatory excellent delineations of solar phenomena
are made; and the late Father Perry, who lost his life in the cause
of science, thus described the method:—“On every fine day the image
of the Sun is projected on a thin board attached to the telescope,
and a drawing of the Sun is made, 10½ inches in diameter, showing the
position and outline of the spots visible. It is the first duty of the
assistant who makes the drawings to note the position of the spots,
and sketch their outlines. He then proceeds to shade in the penumbra
and to draw the finer details, comparing the drawing from time to time
by placing it alongside the projected image of the spots. The position
of the faculæ is then filled in with a red pencil, so that the eye can
at once recognize their grouping with respect to sun-spots, and the
other details drawn with a black pencil.” The same astronomer also
stated that, “as a general rule, careful drawings of the projected
image of the Sun give much more satisfactory pictures of the solar
surface than the photographs taken even at our best observatories. It
is quite true that occasionally an exquisite photograph on an enlarged
scale may be obtained, which exhibits features such as no pencil could
portray as accurately, but rarely indeed will the photograph furnish
all the details that a practised eye and hand, kept patiently at the
sketch-board, will detect and faithfully describe. And the reason
is not far to seek; for any experienced observer knows that, even
on the finest day, the definition is continually changing with the
sky, and that it is only at comparatively rare moments we can expect
those perfect conditions that enable the finest details to stand out
sharply, as Schiaparelli expresses it, like the faintest lines of a
steel engraving. A photograph may be accidentally taken during one
of these exceptionally favoured moments; but a patient draughtsman
is almost sure to secure several of these best opportunities at each
prolonged visit to his sketch-board. What would, therefore, be a great
acquisition at present is a series of careful solar drawings, taken at
short intervals of time, on days when characteristic spots are visible
upon the Sun; and this would be the surest way of adding much valuable
information to that already possessed concerning the changes that take
place in the solar photosphere.”
With regard to ascertaining the dimensions of sun-spots, very precise
results require accurate means of measurement and some mathematical
knowledge. For the general purposes of the amateur, who will only
want round numbers, simple methods may be adopted with success. I have
used, on a 4-inch refractor, a graduated piece of plane glass, mounted
suitably for insertion in the focus of the eyepiece, and marked with
divisions 1/200 of an inch apart. With power 65 I find the Sun’s disk
at max. distance covers 83 divisions of the graduated lens; so that one
division = 22″·8, the Sun’s min. diameter being 1892″. Each division,
therefore, is equal to 10,434 miles, the Sun’s real diameter being
866,000 miles.
[Illustration: Fig. 20.
Sun-spot of June 19, 1889, 2^h P.M.]
I viewed a large spot on June 19, 1889, and found its major axis
covered 2·6 divisions, = 59″·3[9]; so that its apparent length was
about 27,000 miles. For
1892″ : 866,000 miles :: 59″·3 : 27,143 miles.
The same method may be adopted if the image is thrown upon a screen.
Approximate values are to be obtained by means of fine cross wires
fixed in the eyepiece. Note the exact interval occupied by the Sun in
crossing the vertical wire, and also the interval occupied by the large
spot or group. If the Sun is 133 seconds in passing the wire, and the
group 6·5 seconds, then
133 seconds : 866,000 miles :: 6·5 seconds : 42,323 miles.
This plan is likely to be most successful when the Sun is near its
meridian passage; but it may be applied at any hour, if care is taken
to adjust the eyepiece so that the Sun’s motion is precisely at right
angles to the vertical wire. One other plan may be mentioned. Draw on
cardboard, with compasses, a circle about 10 or 12 inches diameter,
and divide this with 31 parallel lines. Subdivide each of the spaces
into 5, less prominently marked. Then, during observation, keep both
eyes open, and hold or fix the circular disk at a distance enabling it
to coincide with the telescopic image of the Sun. By carefully noting
how many divisions the group covers on the cardboard, its dimensions
may be readily found, because one division will be equal to about
5410 miles. Of course these methods[10] are simply approximate, and
only strictly applicable to objects not far removed from the central
regions of the Sun, because the spots are portions of a sphere, and not
angles subtended by a flat surface. When close to the E. or W. limbs,
foreshortening is considerable, though the polar diameter of a spot is
not affected by it then.
Presuming an observer to have his 3-or 4-inch telescope duly fitted
with a solar diagonal and tinted glass, he may naturally ask, after
his curiosity has been satisfied by the contemplation of his first
sun-spot, what he can do further: What special features is he to look
for? What changes ought to be recorded? What are the doubtful points
that require to be cleared up as regards the Sun’s physical appearance?
In what way are new and novel facts likely to be glimpsed? In a word,
he desires to know in what manner he may employ his eyes and instrument
usefully for science, while also gaining pleasure for himself.
Information like this is often needed by the young student, and
sometimes indeed by men who have already gained a little experience,
and who possess much larger instruments than we have intimated above.
In endeavouring to offer suggestions in response to such inquiries, I
would remark that the nature and direction of a research essentially
depend upon several conditions, viz. the observer’s inclination, his
instrumental equipment, his place of observation, and the amount of
time he can devote to the pursuit of his object. There are very few men
who, like Schwabe of Dessau, will confront the Sun on nearly every day
for more than forty years in order to learn something of its secrets.
Such extraordinary pertinacity is fortunately not required, except in
special cases. Amateurs may effect much valuable work in the short
intervals which many of them steal either from business or domestic
ties and offer at the shrine of astronomy.
There are quite a considerable number of attractive phenomena and
features on which the solar observer will find ample employment, and to
the principal of these it may be as well to make individual references.
_Eclipses of the Sun._—These phenomena deservedly rank amongst the most
important and impressive events displayed by the heavenly bodies, and
they are specially interesting to the possessors of small telescopes.
Solar eclipses have been so often made the subject of observation
and discussion, that our knowledge of the appearances presented may
be considered to be nearly complete. The various aspects of Nature
on such occasions have been so attentively studied in their manifold
bearings, that virtually nothing remains for the ordinary observer but
to reexamine and corroborate facts already well ascertained. He can
expect to glean few materials in a field where a plentiful harvest has
just been reaped. But the eclipsed Sun, if it has revealed most of
its secrets to previous investigators, has certainly not declined in
attractiveness; and the amateur will find the spectacle still capable
of exhibiting features which, though not full of the charms of novelty,
will be sufficiently striking and diversified to be remembered long
after the event has passed.
[Illustration: Fig. 21.
1891, June 6. 1899, June 8. 1900, May 28. 1905, Aug. 30.
1908, June 28. 1912, April 17. 1914, Aug. 21. 1916, Feb. 3.
At sunset.
1919, Nov. 22. 1920, Nov. 10. 1921, April 8. 1922, March 28.
At sunset. At sunset.
Solar Eclipses visible in England, 1891 to 1922.]
[Illustration: Fig. 22.
Total Solar Eclipse of August 19, 1887.]
Eclipses recur in cycles of 18 years and 10 days (= 6585 days). This
period was determined by the ancients, and called the _saros_. By its
means the times and magnitudes of eclipses were roughly computed long
before astronomy became an exact science.
A solar eclipse is really an _occultation_ of the Sun by the Moon; for
the word _eclipse_, in its usual reference, denotes the obscuration of
one body by its immersion in the shadow of another. During any single
year there are never less than two eclipses, nor more than seven.
Whenever there are two only, both are solar.
Since the fine solar eclipse of December 22, 1870, no large eclipse
of the Sun has been visible in England. It is remarkable that during
the thirty years from 1870 to 1900 these phenomena are all of an
unimportant, minor character. Within the thirty years following 1891
there will be twelve solar eclipses, for which the Rev. S. J. Johnson
has given projections (as shown on p. 98) for the period of greatest
obscuration.
Total eclipses are extremely rare as regards their visibility at a
given station. Thus between 878 and 1715 not one was observed at
London, and during the next 500 years there will be a similar absence
of such a phenomenon. The observer of total eclipses must perforce
journey to those particular tracts of the earth’s surface over which
the band of totality passes. On such occasions photography plays an
important part; and the corona, the red flames, the shadow-bands, and
numerous other features become the subjects of necessarily hurried
observation and record, for totality endures for very few minutes[11].
As regards ordinary partial eclipses, amateurs usually find ample
entertainment in noting the serrated aspect of the Moon’s contour
projected on the bright Sun. It is also interesting to watch the
disappearance and reappearance of the solar spots visible at the time.
Rather a low magnifying power, with sufficiently expansive field to
include the entire disk, is commonly best for the purpose of these
observations.
_Periodicity of Spots._—This detail may be said to have been fully
investigated. Schwabe and Wolf have accomplished much in this
direction. A work of this kind must, by the nature of it, extend over
many years and entail many thousands of observations. It is therefore
more suited to the professional astronomer than to the amateur, whose
attention is more or less irregular owing to other calls. The sun-spot
cycle is one of about 11 years, during which there are alternately
few and many spots on the Sun. There appear to be some curious
fluctuations, disturbing the regular increase and decrease in the
number of spots; and these variations are worthy of more attention.
The following are the years of observed maxima and minima of sun-spot
frequency:—
Maxima. Minima.
1828. 1833.
1837. 1843.
1848. 1854.
1860. 1867.
1870. 1878.
1883-4. 1890 (?).
These phenomena have been rare during the past few years. The next
maximum may be expected in about 1894, when solar observers will
probably have an abundance of new materials to study.
_Crateriform Structure._—In 1769 Prof. Wilson, of Glasgow, while
watching a sun-spot with a Gregorian reflecting-telescope, remarked
that, as it approached near the limb, the penumbra became much
foreshortened on the interior side. He inferred from this that the
spots were cavities, and the idea has been generally accepted; so
that these objects are sometimes termed solar craters, and commonly
regarded as openings in the luminous atmosphere of the Sun. But the
conclusion appears to be based on data not uniformly supporting it. In
1886 the Rev. F. Howlett published some observations which “entirely
militate against the commonly received opinion that the spots are
to any extent sunk in the solar surface as to produce always those
effects of perspective foreshortening of the inner side of the penumbra
(when near the limb) which have been described in various works on
astronomy.” In a number of instances the penumbra is wider on the side
nearest the Sun’s centre, whereas the converse ought to be the case on
the cavity theory. The fine sun-spot of July 1889 offered an example
of this; for when it was near the W. limb the W. side of the penumbra
was obviously much narrower than the E. side, so that the appearance
would indicate the object as an elevation rather than a depression. The
observer should keep a register of the aspect of all pretty large spots
near the limb, and note the relative widths of the E. and W. sides of
the penumbra. An extensive table of such results would be interesting,
and certain to throw some light on the theory of spot-structure. It is
of course possible that occasionally the inner side of the penumbra is
broader than the outer, and thus appears wider even on the limb, though
really forming the side of a shallow depression.
“_Willow-Leaves._”—In 1861 the late Mr. Nasmyth announced that the
entire solar surface was composed of minute luminous filaments in
the shape of “willow-leaves,” which interlaced one another in every
possible variety of direction. This alleged discovery only met with
doubtful corroboration. The objects were stated by some authorities to
be simply identical with the “corrugations” and “bright nodules” of
Sir W. Herschel. Mr. Stone called them “rice-grains.” The eagle-eyed
Dawes thought “granulations” a more appropriate term, as it implied
no consistency of form and size. Secchi referred to them as oblong
filaments, and “rather like bits of cotton-wool of elongated form.” The
Rev. F. Howlett described the Sun as presenting a granulated, mottled
appearance in a 3-inch Dollond refractor, and mentioned that on the
morning of June 9, 1865, the aspect of its surface was like that of
new-fallen snow, the objects “being not rounded but sharply angular.”
The opinions of observers were thus singularly diverse, and the result
of several animated discussions at the Royal Astronomical Society was
that little unanimity was arrived at, except as to the fact that the
Sun’s surface was crowded with small luminous filaments of elongated
form, and either rounded or angular at the ends. There was no accord
as to their precise forms or distinctive manner of grouping. Some of
the observers averred that the “willow-leaves” or “rice-grains” had no
title whatever to be regarded as a new discovery, the same appearances
having been recognized long before. Gradually the contention ceased,
and though more than a quarter of a century has passed since the
discussion arose there has been little new light thrown on the subject.
Amateurs will therefore do well to probe deeper into this promising
branch of solar observation. As Mr. Nasmyth himself stated,
considerable telescopic power is required, combined with a good
atmosphere. But comparatively small instruments will also be useful,
because of their excellent definition and efficacy in displaying
details on a brilliant orb like the Sun. A power of 150 should be
employed in examining small regions of the general surface, and also
the edges of the umbra and penumbra of sun-spots. When definition is
unusually sharp, and the details very distinct, the magnifying power
should be increased if it can be done with advantage; and the observer
should utilize an occasion like this to the utmost extent. On a really
excellent day more may be sometimes detected than during several weeks
when the atmosphere is only moderately favourable. The observations,
being of a critical nature, should not be attempted in winter, when
the Sun is low. I have frequently secured fine views of the delicate
structure of the solar surface between about 8 and 9 A.M. in the summer
months; and this is often a convenient time for amateurs to snatch a
glimpse, before going to business.
With reference to the general question as to the existence of the
“willow-leaves,” my conception of the matter is that the features
described by Mr. Nasmyth are not new. His drawing of a spot in Sir J.
Herschel’s ‘Outlines’ and Chambers’s ‘Descriptive Astronomy’ exhibits
objects extremely uniform in shape and size, and this uniformity I
have never observed in the penumbra of spots. As to the engraving in
the ‘Outlines,’ showing the aspect of the interlaced “willow-leaves”
on the general surface, this is also not realized in observation. The
“corrugations” and “bright nodules” of Sir W. Herschel aptly represent
what is seen, and they are possibly identical with the “very small
bright and obscure points” and “lively and sombre streaks” of Scheiner,
though seen much better and in more profusion of detail through the
improved modern telescopes. The so-called “willow-leaves” are rounded
at the ends, and are consistent neither in size nor shape. They
encroach upon the umbra of the spots, and give a thatched appearance
to the edges. The penumbra also shows this in its outer limits, where
it is also fringed with lenticular particles. Drawings by Capocci and
Pastorff seventy-five years ago, and published in Arago’s ‘Popular
Astronomy,’ show the thatching at the edges of the umbra quite as
palpably as it is represented in recent drawings.
[Illustration: Fig. 23.
Belts of Sun-spots, visible October 29, 1868.]
_Rotation of the Sun._—By noting when the same individual spots return
to the same relative places on the disk, the approximate time of
rotation is easily deduced. This varies according to the latitude of
the spots[12]; whence it is evident the solar atmosphere is affected by
currents of different velocities, causing the spots to vary in their
longitudes with reference to each other. The Earth’s motion round
the Sun causes the spots to travel apparently more slowly than they
really do; for observations prove that a spot completes a rotation in
27 days 5 hours, whereas the actual time, after making allowance for
the earth’s orbital motion, is about 25 days 7-3/4 hours. The period of
rotation may be roughly found as follows, supposing a spot to return to
precisely the same part of the disk in 27 days 5 hours:—
365^d 5^h 49^m + 27^d 5^h = 392 10^h 49^m.
Then
392^d 10^h 49^m (= 565,129^m) : 365^d 5^h 49^m (= 525,949^m)
:: 27^d 5^h (= 39,180^m) : 25^d 7^h 44^m (= 36,464^m).
For exact results several circumstances have to be considered, such as
the direction of the spot-motions across the disk, as the chords vary
according to the season; thus in June and December the spots traverse
straight lines, while in March and September their paths are curved,
like a belt on Saturn when the planet is inclined. Some of the spots
display considerable proper motion; so that it is best to observe a
number of these objects, and reduce the times to a mean result. They
are not very durable, rarely lasting longer than a few weeks; but
some of the more extensive disturbances are sustained for several
months, during which many singular changes are effected. The period of
rotation, as determined by several observers, is as follows:—
d h m
1678. Cassini 25 13 55
1718. Bianchini 25 7 48
1775. Delambre 25 0 17
1841. Laugier 25 8 10
1846. Kysæus 25 2 10
1852. Böhm 25 12 29
1863. Carrington 25 9 7
1865. Schwabe 25 5 0
1868. Spörer 25 5 31
1888. Wilsing 25 5 47
The motion of rotation is similar in direction to that in which the
planets move found the Sun, namely from west to east. Hence the spots
come into view on the east limb of the Sun, and disappear at the west.
_Planetary Bodies in transit._—During observation the observer should
particularly watch any very dark, small spots that may be visible,
such as are isolated and pretty circular and definite in outline. If
an object of this character is seen it should be examined with a high
power, and its aspect critically noted. Should the observer entertain
any suspicion of its being of a planetary nature, he should carefully
determine its position on the disk, and, after a short interval,
re-observe it for traces of motion. If it remains stationary, its true
solar origin will be proved. If motion is shown, then the successive
positions of the object during its transit, and its place of egress,
with the time of each observation, should be recorded. In such a case
it would be a good plan to project the Sun’s image, and mark the place
of the suspicious object and chief sun-spots at short intervals. This
would be more accurate than mere eye-estimation. The observer who
scans the solar surface for intra-Mercurial planets must remember
that, if any such bodies exist, they will probably be very diminutive.
Venus, when on the Sun in December 1882, was a spot 63″ in diameter,
and easily perceptible to the naked eye. Mercury, at the transits of
1861, 1868, and 1881, was a little less than 10″, but in 1878 was
12″. If “Vulcan,” the suspected interior planet, has any existence
it may possibly be much smaller than Mercury, and will thus escape
observation, unless the observer exercises great care in the search.
The mobile, planetary spots asserted to have been seen on the Sun in
past years prove nothing definite, and appear to have been illusory.
_Proper Motion of Sun-spots._—This feature is one deserving more
investigation. The distances separating individual spots should either
be measured with a micrometer or determined by transits across a wire,
and the displacement recorded from hour to hour or from day to day.
Spots in different latitudes will almost certainly exhibit some change
of relative place; and objects in the same latitude must be watched,
for similar variations probably affect them. The physical peculiarities
of such spots should be remarked, and also the alterations of
appearance they undergo during the time they approach or recede from
each other.
_Rise and Decay of Spots._—Occasionally large spots are formed in an
incredibly short time, and the disappearance of others has been equally
sudden. Schwabe found, from many observations, that the western spots
of a group are obliterated first; but authorities differ. I have
usually observed that the smaller, outlying members of a group vanish
before the larger spot, which then contracts and is invaded by tongues
of faculæ; so that its effacement soon follows, and nothing remains to
indicate the disturbance but bright ridges of faculæ, which are very
conspicuous near the limb.
_Black Nuclei in the Umbræ._—Dawes was the first to announce that the
umbra sometimes included a much darker area or nucleus. This is present
in nearly all large spots. A part of the umbra seems covered or veiled
by a slightly luminous medium, and the portion unaffected looks black
by contrast. On October 1, 1881, with a 2½-inch refractor, I saw a
large sun-spot, the umbra of which was broken up into 7 fragments, and
the S. preceding part appeared very black while the others showed a
much lighter tint. In the fine spot of June 1889 a nucleus was also
distinctly apparent; and this feature is sometimes so obvious in large
spots that it may be observed with an instrument of only 2-inches
aperture. I have usually remarked the nucleus on one side of the
umbra, and abutting the penumbra. It may be formed by light patches of
transparent material floating over the umbra, and leaving a part free
where the Sun’s dark body is fully exposed. This light material is
possibly suspended far above the umbra and inconstant in its position;
so that the place and form of the nucleus should always be noted for
traces of change. It is necessary that such details should be closely
watched during an entire day, or several days; for the variations could
then be followed, and perhaps reduced to some law. This persistence
is very necessary, in order to solve many of the peculiarities of
sun-spots, which, though pretty well known in appearance, have not been
thoroughly studied in their various developments.
_Bright Objects near the Sun._—Small, rapidly moving bodies have been
occasionally reported as seen passing over the Sun. In several cases
these have been prematurely assigned a meteoric origin. They have been
described as luminous bodies of irregular shape, as moving in a common
direction, and as being very distinct when projected on the dark sky
just outlying the bright limb of the Sun. There is little doubt they
are either the pappus of different kinds of seed, or convolutions of
gossamer, which have been lifted to great heights in the air, and
are rendered bright by reflection from the bordering Sun. In this
connection I may mention some observations of my own with a 4-inch
refractor:—
“1889, MAY 20, 0^h 30^m P.M.—Bright points and little misty forms kept
passing from the Sun’s limb, at the average rate of 13 in a minute.
They moved in the same direction as the clouds and wind. Some of them
were followed by tails, which were far from straight. I saw them best
when I focused the telescope for an object much nearer than the Sun.
One of these forms would occasionally halt and pursue an irregular
flight. It was evident they were terrestrial objects, with motions
controlled by the wind.
“3^h P.M.—Many bright objects still passing from the Sun’s limb.”
“1889, MAY 22, 9^h A.M.—Observed vast numbers of luminous particles
floating about contiguous to the Sun’s margin. They were clearly
carried along by the wind; but this being very slight, their motions
were extremely slow, and now and then many of them became nearly
stationary. Their directions were far from uniform, though the general
tendency was obviously in a common line of flight. I watched them for
some time passing in a plentiful shower.”
These objects are always noticed in summer-time, and I believe they
would much more frequently attract remark but for the fact that they
require a longer focus than the Sun and cannot be recognized when on
the disk, to which the observer is usually giving the whole of his
attention. Those who are often employed in solar work will find it an
interesting diversion to look for these bodies. The instrument should
be focused as for a distant terrestrial object, and only a part of
the Sun’s limb should be retained in the field of view of an eyepiece
of moderately low power. Then, looking intently at the dark sky near
the limb, the bright objects will be sometimes seen sailing past in
considerable numbers.
_Cyclonic Action._—The appearance in detail of certain spots, coupled
with evidences of rotatory motion round their own centres, has induced
the belief that they are liable to action in some degree similar to the
cyclonic storms[13] which disturb and rend the terrestrial atmosphere.
Such indications should be looked for in fairly conspicuous spots,
and any peculiarities of the nature alluded to made the subject of
close investigation. A spot showing features having a spiral tendency
may not, however, have a gyratory movement about its centre. This can
only be determined by critically noting the details, and frequently
reobserving them for traces of motion. The penumbra always shows
radiations converging on the umbra as a centre; but this is merely a
form of structure, and proves nothing in evidence of a revolving storm.
_Sudden Outbursts of Faculæ._—In September 1859 Carrington and Hodgson
independently observed a striking outburst of faculæ in front
of a large group of spots which they were examining. It remained
visible about five minutes, during which interval several patches
of light travelled over a region nearly 34,000 miles in extent. An
extraordinary magnetic disturbance was simultaneously recorded at the
Kew Observatory, and sixteen hours afterwards there followed a magnetic
storm of unusual severity. On another occasion Dr. Peters observed
flashes of light cross and recross the umbra of a prominent spot with
electric velocity. Some other startling observations of solar phenomena
have been effected, and there is no question as to their having been
matters of fact. In the presence of effects so sudden, so obvious, and
so unexpected, no wonder the observers at first doubted the evidence
of their eyes and suspected the cause to lie in a fractured glass or
a fault of adjustment. But the corroboration afforded the clearest
proof as to the actuality of the events described. They will doubtless
occur again; but these phenomena cannot be definitely predicted as to
time, so that students of the solar surface should be prepared for a
repetition of them whenever they may occur.
[Illustration: Fig. 24.]
_Shadows cast by Faculæ._—M. Trouvelot, while examining a large
sun-spot on May 26, 1878, noticed that it was “completely surrounded
by very brilliant and massive faculæ.” “On one part of the penumbra
an extraordinary appearance was perceived, which resembled so closely
a shadow, as it would have been cast by the overhanging faculous mass,
that it seemed useless to seek, and it was impossible to admit, any
other explanation. This shadow, the outline of which was a little
diffused, had the same shape as, and reproduced with great exactness,
the outline of the faculous mass situated above it. It was not so
black as the opening in spots called the umbra, but of a very dark
tint.” A similar feature was seen by Kirk and Maclean on May 2, 1884,
and the ‘Observatory,’ vol. vii. pp. 146, 170, and 197, contains some
interesting particulars on this subject. Fig. 24 is a drawing by
Kirk, in which the shadow is represented by A, B; at C “it accurately
followed the outline of the intensely white margin of the spot.”
_Veiled Spots._—The late Father Perry described these objects at the
R.A.S. meeting on May 9, 1884, and said they are to be seen all over
the face of the Sun. They only exist for two or three minutes, and then
disappear. In one instance he observed a train of these veiled spots
stretching over “a tenth part of the Sun’s diameter, which was nearly
as obvious to the eye as the penumbra of an ordinary spot; it split
into two throughout its whole length, and disappeared in a minute.
The veiled spots seem to be of two classes: the one appear like small
greyish clouds, which disappear after a few minutes, as if they were
formed and rapidly evaporated by the Sun’s heat, and the others seem
to be connected with the umbra of ordinary spots; they appear about
them, and are more permanent than the ordinary veiled spots, lasting
sometimes two or three days, but never longer.” These markings appear
to have been first detected by Trouvelot in 1875, and he gives some
information as to this class of phenomena in the ‘Observatory,’ vol.
viii. pp. 228 _et seq._
_Recurrent Disturbances_[14].—It is supposed, and with good evidence
affirming the idea, that certain regions of the Sun’s surface are
subject to frequent outbursts of spots, which are possibly due to
forces acting from below the Sun’s bright atmosphere. After the
disappearance of large groups or isolated spots it is therefore
advisable to watch the same region for some time afterwards, to find
whether it remains perfectly quiescent, or whether it soon again
becomes a seat of activity and change.
_Recurrent Forms._—Certain spots observed at different times have
exhibited appearances so nearly resembling each other that it has
been considered the likeness may be due to something more than mere
accident. Whenever such suggestive coincidences are recognized the
observer should note them particularly, and secure drawings. It should
be his aim to determine the exact intervals elapsing between the
presentation of spots or groups of this character, and also whether
they occupy the same latitude and longitude on the Sun’s disk.
_Exceptional Position of Spots._—The ordinary spots are rarely seen
more than 35° distant from the solar equator or within 8° of it. They
usually appear in the zones from 8° to 20° N. and S. of the equator.
A few exceptions may be mentioned[15]. Mechain saw a spot in July
1780 having a latitude of 40-1/3°; in April 1826 Capocci recorded one
having 49° of S. latitude; Schwabe and Peters observed spots 50° from
the equator. Lahire, in the last century, described a spot as visible
in a latitude of 70°; but the accuracy of this observation has been
questioned. Whenever a spot is seen near the equator, or very far
removed from it, measures should be taken of its exact place; for
it is desirable to learn something more of those disturbances which
occasionally affect the more barren regions of the solar envelope.
_The Solar Prominences._—Those amateurs who have included a
spectroscope in their instrumental outfit will find the study of
the chromosphere and prominences a most productive one. Huggins and
Zöllner were the first to apply the “open-slit” method; and the study
of the shape of the hydrogen prominences commenced in 1869. Tupman
details (‘Monthly Notices R.A.S.,’ vol. xxxiii. p. 106) a series of
observations which he secured in 1872 with a refractor of 3-inches
aperture and a direct-vision spectroscope of five prisms. He mentions
the cost of the entire apparatus as only £18, and says he entertains
“no doubt that an equally effective instrument could be made for much
less.” The prominences appear to be of different kinds, and are known
as “cloud”- and “flame”-prominences. Both are liable to rapid changes.
Trouvelot, in June 1874, noticed “a gigantic comma-shaped prominence,
82,000 miles high, which vanished from before his eyes by a withdrawal
of light as sudden as the passage of a flash of lightning.” Since
the study of these remarkable forms was rendered feasible by using a
greater dispersion to open the slit of the spectroscope wide enough to
see them, they have been made the subject of daily study and record.
The results, so far as they have been investigated, show that the
region of the Sun’s limb in which the prominences are most frequent
reaches to some 40° on either side of the equator, which is somewhat
greater than the area of sun-spot frequency. The chromosphere itself
is probably of much the same character as the erupted prominences,
and formed of little flames arranged thickly together like “blades of
grass.”
In observing the Sun with a telescope the amateur will soon notice
that the surface is far more brilliant in the central parts than round
the margin of the disk. Vögel has estimated that immediately inside
the edges the brightness does not amount to one seventh that of the
centre. The difference is entirely due to the solar atmosphere, which
is probably very shallow relatively to the great diameter of the Sun.
FOOTNOTES:
[9] The Rev. F. Howlett measured this spot on the following day,
June 20, and found it 63″ in its largest diameter. He used a small
refractor, and projected the Sun’s image on to a screen sufficiently
distant for it to have a diameter of 3 feet.
[10] On May 13, 1890, at 3^h, I tested the three methods alluded
to on a scattered train of small spots, and derived the following
measurements of length:—
By glass micrometer 76,570 miles.
" cross wires 76,610 "
" cardboard disk 75,770 "
In this comparison I used an excellent 4-inch Cooke refractor,
belonging to a friend.
[11] The maximum duration of totality, under every favouring
circumstance, appears to be about 8 minutes. The great eclipse which
occurred on August 18, 1868, maintained the total phase for nearly 6
minutes 50 seconds in the Gulf of Siam. In reference to this eclipse,
Dr. Weiss says:—“In the records of ancient eclipses there are to be
found only two which may be compared in size with that of August 18,
1868, but none in which the totality lasted so long. The first of these
is the eclipse of Thales (28 May, 585 B.C.), which is said to have
been the first predicted, and to have terminated a bloody war between
the Lydians and the Medes. The second was visible on June 17, 1433, in
Scotland, and the time of its occurrence was long remembered by the
people of that country as ‘the black hour.’”
[12] Carrington found that spots near the equator gave a shorter
rotation-period than those far removed from it. This offers an analogy
to the spots on Jupiter, which move with greater celerity near the
equator, though the rule is not absolute.
[13] In 1852 Dawes observed and measured a rotatory motion affecting a
spot at the rate of about 17° per day.
[14] Lalande, in 1778, asserted that “there are spots of very
considerable magnitude, which, reappear in the same physical points of
the solar disk.”
[15] A spot was visible on June 30, 1889, in 40° South latitude.
Its recorded duration was 2 days. This object was observed at the
Stonyhurst Observatory and at a station in North America.