William F. Denning.

Telescopic Work for Starlight Evenings online

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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



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 9 of 32)