William F. Denning.

Telescopic Work for Starlight Evenings online

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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.




CHAPTER VI.

_THE MOON._

Attractive aspect of the Moon.—Absence of air and water.—Only one
Hemisphere visible.—Earthshine.—Telescopic observations of the
lunar surface.—Eclipses.—Lunar changes.—Formations.—Plato and
other objects described.—Table of Moon’s age and formations near
terminator.—Occultations of stars.—Visibility of the new Moon.

“The western Sun withdraws: meanwhile the Moon,
Full orb’d, and breaking through the scatter’d clouds,
Shows her broad visage in the crimson’d east.”


Early in autumn, when the evenings are frequently clear, many persons
are led with more force than usual to evince an interest in our
satellite, and to desire information which may not be conveniently
obtained at the time. The aspect of the Moon at her rising, near the
time of the full, during the months of August, September, and October,
is more conspicuously noticeable than at any other season of the year,
on account of the position she then assumes on successive nights,
enabling her to rise at closely identical times for several evenings
together. The appearance of her large, ruddy globe at near the same
hour, and her increasing brilliancy as her horizontal rays give way
under a more vertical position, originated the title of “Harvest Moon,”
to commemorate the facility afforded by her light for the ingathering
of the corn preceding the time of the autumnal equinox.

It will be universally admitted that the Moon possesses special
attractions for us, as being situated nearer than any other celestial
body, and forming the inseparable companion or tributary world to the
Earth. The many important influences she exercises have led to her
becoming the object of close investigation; so that her motions and
physical appearances have been ascertained with a remarkable degree
of exactness and amplitude. Her movements regulate the tides; her
positions are of the utmost moment to the mariner; her light is the
welcome beacon of the wayfarer, and its picturesque serenity has ever
formed the theme of poets. To the practical astronomer she constitutes
an orb perfectly unique as regards extent and variety of detail; and
questions relating to the physical condition of her surface, now and in
past ages, supply a fund of endless speculation to the theorist.

The mean apparent diameter of the Moon is 31′ 5″, and it varies from
29′ 21″ at perigee to 33′ 31″ at apogee. Her real diameter is 2160
miles, and her mean distance slightly exceeds 237,000 miles. Her
revolution round the Earth (= sidereal period) is performed in 27^d 7^h
43^m 11^s·46, but the time from one new moon to another (= synodical
period) is 29^d 12^h 44^m 3^s. The Moon’s motion through the firmament
is at the rate of 13° 10′ 35″ per day and 32′ 56″ per hour. Thus she
travels over a space slightly exceeding her own diameter in one hour.
The linear value of 1″ at the distance of our satellite is 1·16 mile,
and of 1′ 69½ miles.

When we critically survey the face of the Moon with a good telescope,
we see at once that her surface is broken up into a series of craters
of various sizes, and that some very irregular formations are scattered
here and there, which present a similar appearance to elevated
mountain-ranges. The crateriform aspect of the Moon is perhaps the more
striking feature, from its greater extent; and we recognize in the
individual forms a _simile_ to the circular cavities formed in slag or
some other hard substances under the action of intense heat. In certain
regions of the Moon, especially those near the south pole, the disk
is one mass of abutting craters, and were it not for the obvious want
of symmetry in form and uniformity of size, the appearance would be
analogous to that of a gigantic honeycomb. These craters are commonly
surrounded by high walls or ramparts, and often include conical hills
rising from their centres to great heights. While the eye examines
these singular structures, and lingers amongst the mass of intricate
detail in which the whole surface abounds, we cannot but feel impressed
at the marvellous sharpness of definition with which the different
features are presented to our view. It matters not to what district we
direct our gaze, there is the same perfect serenity and clearness of
outline. Not the slightest indication can be discerned anywhere of mist
or other obscuring vapours hanging over the lunar landscape.

_Absence of Air and Water._—Now it is palpable from this that the Moon
has no atmosphere of sufficient density to render itself appreciable;
for such an appendage, if it existed in any visible form, would at once
obtrude upon the attention, and we should probably recognize some of
the characteristics common to the behaviour of our terrestrial clouds.
But nothing of the kind is apparent on the Moon: there is an unbroken
transparency spread over the whole extent of the Moon’s scenery; whence
we conclude that if any air exists on the surface it is of extremely
attenuated nature, and possibly confined to the bottom of the craters
and low-lying formations, which are arranged in such prolific manner on
our satellite.

Nor is there any perceptible intimation of water upon the Moon. It is
true that several dark grey patches have been given names, leading
one directly to the inference that lakes and seas comprise part of
the surface phenomena. Thus there is the _Mare Serenitatis_ (“the sea
of serenity”) and many other designations of similar import, which
we cannot but insist are wrongly applied and calculated to lead to
misapprehension. Before the invention of the telescope furnished us
with the means of accurately determining the character of the lunar
features, such apellations may have been considered eligible; but now
that the non-existence of water in any extensive form is admitted,
the titles are rendered obsolete. Still their retention is in some
respects advisable, for any sweeping change in a recognized system of
nomenclature must cause confusion, and the names alluded to serve a
useful end in facilitating reference; so that, under the circumstances,
it would perhaps be unwise to attempt reform, or to introduce an
innovation which must occasion many difficulties.

_Only one Hemisphere visible._—In discussing the nature and appearances
of the lunar formations, it must be distinctly understood that our
remarks apply to those visible on the side invariably turned towards
the Earth. For, in point of fact, there is a considerable expanse
of the lunar disk never perceptible from the Earth at all. This is
occasioned by the circumstance that the Moon rotates upon her axis
in precisely the same time as she revolves around the Earth, and is
therefore enabled to present the same side towards us on all occasions.
A slight tilting (called libration) takes place, so that we are
allowed a glimpse of fragments of the side normally invisible, and its
analogous aspect leads us to suppose that there is no great distinction
between the features of the Moon’s visible and invisible hemispheres.
From exact computations it appears that we are enabled to see a
proportion of 59/100 of the surface, and that the remaining 41/100 are
permanently beyond our reach.

_Earthshine._—A few mornings before new moon, and on a few evenings
after it, the whole outline of the dark portion of the lunar globe
may be distinctly perceived. A feeble illumination like twilight
pervades the opaque part, and this is really earthlight thrown upon
our satellite, for near the times of new moon the Earth appears at
her brightest (her disk being fully illuminated) as seen from the
Moon. The French term for this light is _la lumière cendrée_, or “the
ashy light.” The appearance is often popularly referred to in our own
country as “the old Moon in the new Moon’s arms.” Some of the old
observers remarked that the waning Moon showed this earthlight more
strongly than the new Moon.

_Telescopic Observations of the Lunar Surface._—Our telescopes give
by far the most pleasing view of the Moon when she is in a crescent
shape. At such a period the craters and mountains, with their dark
shadows, are splendidly displayed. A good view is also obtainable
with the Moon at first or last quarter, or when the disk is gibbous.
But the full Moon is decidedly less attractive; for the shadows have
all disappeared, and the various formations have quite lost their
distinctive character. The disk is enveloped in a flood of light,
causing glare, and though there is a large amount of detail, including
systems of bright rays, many differences of tint, and bright spots,
yet the effect is altogether less satisfactory than at the time of a
crescent phase.

The nature of the work undertaken by the amateur must largely depend
upon his opportunities and the capacity of his appliances. It is
evident that in the investigation of lunar details it is essential to
be very particular in recording observations; for unless the conditions
of illumination are nearly the same, lunar objects will present little
resemblance. He should therefore examine the formations at intervals of
59^d 1^h 28^m, when the terminator is resting on nearly identical parts
of the surface. In periods of 442^d 23^h (= 15 lunations) there is
another repetition of similar phase; also in periods of 502^d 0^h 28^m
(= 17 lunations).

The observer, in entering results into his note-book, should state
the Moon’s age to the nearest minute, and give aperture and power of
telescope and state of sky. Those objects which he has recorded at one
lunation should be re-observed after an intervening lunation, or at
intervals of 59^d 1^h 28^m. He will then find his notes and drawings
are comparable. By the persistent scrutiny of special structures he
will discern more and more of their details; in other words, he will
find his eye soon acquires power with experience and familiarity with
the object. Comparisons of his own work with the charts and records
of previous observers will be sure to interest him greatly, and the
differences which he will almost certainly detect may exert a useful
influence in inciting him to ascertain the source of them. He must not
be premature in attributing such discordances to actual changes on the
Moon; for he must remember that perfect harmony is rarely to be found
in the experiences of different observers. But whenever his own results
are inconsistent with those of others, the fact should be carefully
noted and the observations repeated and rediscussed with a view to
reconcile them. The charts and descriptions of former selenographers



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 10 of 32)