accompanied by faculæ resembling those seen near the ordinary spots.
They are frequently seen in the polar regions, but are there always of
small size and of short duration. The veiled spots are larger, and more
apt to arrange themselves into groups, in the regions occupied by the
ordinary spots, and it is not rare to observe such spots transform
themselves into ordinary spots, and vice versa. The veiled spots,
therefore, seem to be ordinary spots filled up, or covered over by the
granulations and semi-transparent gases composing the chromospheric
layer. That it is so, becomes more evident, from the fact that large
Sun-spots in process of diminution are sometimes gradually covered with
faint and scattered granules which descend in long, narrow filaments,
and become less and less distinguishable as they attain greater depth.
This phenomenon, associated with the fact that the luminous bridges seen
over the Sun-spots which are closing up are sometimes transformed into
branches which show the characteristic structure of the photosphere,
goes far to prove that the solar envelopes are mainly composed of an
innumerable quantity of radial filaments of varying height.
[PLATE I.--GROUP OF SUN-SPOTS AND VEILED SPOTS.
Observed on June 17th 1875 at 7 h. 30 m. A.M.]
The group of Sun-spots represented in Plate I., was observed and drawn
on June 17th, 1875, at 7h. 30m. A. M. The first traces of this group
were seen on June 15th, at noon, and consisted of three small black dots
disseminated among the granulations. At that time, no disturbance of the
surface was noticeable, and no faculæ were seen in the vicinity of these
spots. On June 16th, at 8 o'clock A. M., the three small spots had
become considerably enlarged, and, as usual, the group consisted of two
principal spots. Between these two spots all was in motion: the
granulations, stretched into long, wavy, parallel lines, had somewhat
the appearance of a liquid in rapid motion. At 1 o'clock, P. M., on the
same day, the group had considerably enlarged; the faculæ, the
granulations, and the penumbral filaments being interwoven in an
indescribable manner. On the morning of the 17th, these spots had
assumed the complicated form and development represented in the drawing;
while at the same time two conspicuous veiled spots were seen on the
left hand, at some distance above the group.
Some luminous bridges are visible upon the left hand spot, traversing
the penumbra and umbra of this spot in various directions. The umbra of
one of the spots is occupied, and partly filled with gray and rosy
veils, similar to those above described, and the granulations of the
solar surface form a background to the group of spots.
This group of spots was not so remarkable for its size as for its
complicated structure. The diameter of the group from east to west was
only 2½ minutes of arc, or about 67,000 miles. The upper part of the
umbra of the spot situated on the right hand side of the group was
nearly 7,000 miles in diameter, or less by 1,000 miles than the diameter
of the Earth. Some of the long filaments composing that part of the
penumbra, situated on the left hand side of the same spot, were 17,000
miles in length. One of these fiery elements would be sufficient to
encircle two-thirds of the circumference of the Earth.
SOLAR PROTUBERANCES
PLATE II
The chromosphere forming the outlying envelope of the Sun, is subject,
as has been shown above, to great disturbances in certain regions,
causing considerable upheavals of its surface and violent outbursts of
its gases. From these upheavals and outbursts of the chromosphere result
certain curious and very interesting forms, which are known under the
name of "_Solar Protuberances_," "_Prominences_," or "_Flames_."
These singular forms, which could, until recently, be observed only
during the short duration of the total eclipses of the Sun, can now be
seen on every clear day with the spectroscope, thanks to Messrs. Janssen
and Lockyer, to whose researches solar physics is so much indebted.
[PLATE II.--SOLAR PROTUBERANCES.
Observed on May 5, 1873 at 9h, 40m. A.M.]
The solar protuberances, the Sun-spots, and the faculæ to which they
are closely related, are confined within the same general regions of the
Sun, although the protuberances attain higher heliocentric latitudes.
There is certainly a very close relation between the faculæ and the
solar protuberances, since when a group of the faculæ traverses the
Sun's limb, protuberances are always seen at the same place. It seems
very probable that the faculæ and the protuberances are in the main
identical. The faculæ may be the brighter portion of the protuberances,
consisting of gases which are still undergoing a high temperature and
pressure; while the gases which have been relieved from this pressure
and have lost a considerable amount of their heat, may form that part of
the protuberances which is only visible on the Sun's limb.
A daily study of the solar protuberances, continued for ten years, has
shown me that these objects are distributed on two zones which are
equidistant from the solar equator, and parallel with it. The zone
arrangement of the protuberances is more easily recognized during the
years of minimum solar activity, as in these years the zones are very
narrow and widely separated. During these years the belt of
protuberances is situated between 40° and 45° of latitude, north and
south. In years of great solar activity the zones spread considerably on
either side of these limits, especially towards the equator, which they
nearly reach, only a narrow belt, usually free from protuberances,
remaining between them. Towards the poles the zones do not spread so
much, and there the space free from protuberances is considerably
greater than it is at the equator.
During years of maximum solar activity, the protuberances, like the
Sun-spots and the faculæ, are very numerous, very large, and very
complicated--sometimes occupying a great part of the whole solar limb.
As many as twenty distinct flames are sometimes observed at one time. In
years of minimum solar activity, on the contrary, the prominences are
very few in number, and they are of small size; but, as far as my
observations go, they are never totally absent.
In general, the solar flames undergo rapid changes, especially those
which are situated in the vicinity of Sun-spots, although they
occasionally remain unchanged in appearance and form for several hours
at a time. The protuberances situated in higher latitudes are less
liable to great and sudden changes, often retaining the same form for
several days. The changes observed in the protuberances of the
equatorial regions are due in part to the comparatively great changes in
their position with respect to the spectator, which are occasioned by
the rotation of the Sun. This rotation, of course, has a greater angular
velocity on the equator than in higher latitudes. In most cases,
however, the changes of the equatorial protuberances are too great and
too sudden to be thus explained. They are, in fact, due to the greater
solar activity developed in the equatorial zones, and wherever spots are
most numerous.
The solar protuberances appear under various shapes, and are often so
complicated in appearance that they defy description. Some resemble huge
clumsy masses having a few perforations on their sides; while others
form a succession of arches supported by pillars of different styles.
Others form vertical or inclined columns, often surmounted by cloud-like
masses, or by various appendages, which sometimes droop gracefully,
resembling gigantic palm leaves. Some resemble flames driven by the
wind; others, which are composed of a multitude of long and narrow
filaments, appear as immense fiery bundles, from which sometimes issue
long and delicate columns surmounted by torch-like objects of the most
fantastic pattern. Some others resemble trees, or animal forms, in a
very striking manner; while still others, apparently detached from the
solar limb, float above it, forming graceful streamers or clouds of
various shapes. Some of the protuberances are very massive, while others
are so thin and transparent as to form a mere veil, through which more
distant flames can easily be seen.
Notwithstanding this variety of form, two principal classes of solar
protuberances may be recognized: the cloud-like or quiescent, and the
eruptive or metallic protuberances.
The first class, which is the most common, comprises all the cloud-like
protuberances resting upon the chromosphere or floating about it. The
protuberances of this type often obtain enormous horizontal proportions,
and it is not rare to see some among them occupying 20° and 30° of the
solar limb. The height attained by protuberances of this class does not
correspond in general to their longitudinal extent; although some of
their branches attain considerable elevations. These prominences very
seldom have the brilliancy displayed by the other type, and are
sometimes so faint as to be seen with difficulty. Although it is
generally stated by observers that some of the protuberances belonging
to this class are detached from the solar surface, and kept in
suspension above the surface, like the clouds in our atmosphere, yet it
seems to me very doubtful whether protuberances are ever disconnected
from the chromosphere, since, in an experience of ten years, I have
never been able to satisfy myself that such a thing has occurred. Many
of them have appeared to me at first sight to be detached from the
surface, but with a little patience and attention I was always able to
detect faint traces of filamentary elements connecting them with the
chromosphere. Quite often I have seen bright protuberances gradually
lose their light and become invisible, while soon after they had
regained it, and were as clearly visible as before. Observations of this
kind seem to show that while the prominences are for the most part
luminous, there are also a few which are non-luminous and invisible to
the eye. These dark and invisible forms are most generally found in the
vicinity of Sun-spots in great activity. When observing such regions
with the spectroscope, it is not rare to encounter them in the form of
large dark spots projecting on the solar spectrum near the hydrogen
lines. On July 28th, 1872, I observed with the spectroscope a dark spot
of this kind issuing from the vicinity of a large Sun-spot, and
extending over one-fifth of the diameter of the Sun. This object had
been independently observed in France a little earlier by M. Chacornac
with the telescope, in which it appeared as a bluish streak.
The second class of solar protuberances, comprising the eruptive type,
is the most interesting, inasmuch as it conveys to us a conception of
the magnitude and violence of the solar forces. The protuberances of
this class, which are always intensely bright, appear for the most part
in the immediate vicinity of Sun-spots or faculæ. These protuberances,
which seem to be due to the outburst of the chromosphere, and to the
violent ejection of incandescent gases and metallic vapors from the
interior of the Sun, sometimes attain gigantic proportions and enormous
heights.
While the spectrum of the protuberances of the cloudy type is simple,
and usually composed of four hydrogen lines and the yellow line D{3}, that
of the eruptive class is very complicated, and, besides the hydrogen
lines and D{3}, it often exhibits the bright lines of sodium, magnesium,
barium, titanium, and iron, and occasionally, also, a number of other
bright lines.
The phenomena of a solar outburst are grand and imposing. Suddenly
immense and acute tongues and jets of flames of a dazzling brilliancy
rise up from the solar limb and extend in various directions. Some of
these fiery jets appear perfectly rigid, and remain apparently
motionless in the midst of the greatest disorder. Immense straps and
columns form and rise in an instant, bending and waving in all sorts of
ways and assuming innumerable shapes. Sometimes powerful jets resembling
molten metal spring up from the Sun, describing graceful parabolas,
while in their descent they form numerous fiery drops which acquire a
dazzling brilliancy when they approach the surface.
The upward motion of the protuberances in process of formation is
sometimes very rapid. Some protuberances have been observed to ascend in
the solar atmosphere at the rate of from 120 to 497 miles a second.
Great as this velocity may appear, it is nevertheless insignificant when
compared with that sometimes attained by protuberances moving in the
line of sight instead of directly upwards. Movements of this kind are
indicated by the displacement of the bright or dark lines in the
spectrum. A remarkable instance of this kind occurred on the 26th of
June, 1874. On that day I observed a displacement of the hydrogen C line
corresponding to a velocity of motion of 1,600 miles per second. The
mass of hydrogen gas in motion producing such a displacement was,
according to theory, moving towards the Earth at this incredible rate,
when it instantly vanished from sight as if it had been annihilated, and
was seen no more.
Until recently the protuberances had not been observed to rise more than
200,000 miles above the solar surface; but, on October 7th, 1880, a
flame, which had an elevation of 80,000 miles when I observed it at 8h.
55m. A. M., had attained the enormous altitude of 350,000 miles when it
was observed at noon by Professor C. A. Young. If we had such a
protuberance on the Earth, its summit would be at a height sufficient
not merely to reach, but to extend 100,000 miles beyond the Moon.
Although the solar protuberances represented in Plate II. have not the
enormous proportions attained by some of these objects, yet they are as
characteristic as any of the largest ones, and afford a good
illustration of the purely eruptive type of protuberances. The height of
the largest column in the group equals 4' 43", or a little over 126,000
miles. A large group of Sun-spots was in the vicinity of these
protuberances when they were observed and delineated.
TOTAL ECLIPSE OF THE SUN
PLATE III
A solar eclipse is due to the passage of the Moon directly between the
observer and the Sun. Such an eclipse can only occur at New Moon, since
it is only at that time that our satellite passes between us and the
Sun. The Moon's orbit does not lie precisely in the same plane as the
orbit of the Earth, but is inclined about five degrees to it, otherwise
an eclipse of the Sun would occur at every New Moon, and an eclipse of
the Moon at every Full Moon.
Since the Moon's orbit is inclined to that of the Earth, it must
necessarily intersect this orbit at two opposite points. These points
are called the nodes of the Moon's orbit. When our satellite passes
through either of the nodes when the Moon is new, it appears interposed
to some extent between the Sun and the Earth, and so produces a solar
eclipse; while if it passes a node when the Moon is full, it is more or
less obscured by the Earth's shadow, which then produces an eclipse of
the Moon. But, on the other hand, when the New Moon and the Full Moon do
not coincide with the passage of our satellite through the nodes of its
orbit, no eclipse can occur, since the Moon is not then on a line with
the Sun and the Earth, but above or below that line.
Owing to the ellipticity of the Moon's orbit, the distance of our
satellite from the Earth varies considerably during each of its
revolutions around us, and its apparent diameter is necessarily subject
to corresponding changes. Sometimes it is greater, sometimes it is less,
than the apparent diameter of the Sun. If it is greater at the time of a
solar eclipse, the eclipse will be total to a terrestrial observer
stationed nearly on the line of the centres of the Sun and Moon, while
it will be only partial to another observer stationed further from this
line. But the Moon's distance from the Earth may be so great and its
apparent diameter consequently so small that even those observers
nearest the central line of the eclipse see the border of the Sun all
round the black disk of the Moon; the eclipse is then annular. Even
during the progress of one and the same eclipse the distance of the Moon
from the parts of the Earth towards which its shadow is directed may
vary so much that, while the eclipse is total to some observers, others
equally near the central line, but stationed at a different place, will
see it as annular.
The shadow cast by the Moon on the Earth during total eclipses, travels
along upon the surface of the Earth, in consequence of the daily
movement of rotation of our globe combined with the movements of the
Earth and Moon in their orbits. The track of the Moon's shadow over the
Earth's surface has a general eastward course, so that the more westerly
observers see it earlier than those east of them. An eclipse may
continue total at one place for nearly eight minutes, but in ordinary
cases the total phase is much shorter.
The nodes of the Moon's orbit do not invariably occupy the same
position, but move nearly uniformly, their position with regard to the
Sun, Earth, and Moon being at any time approximately what it formerly
was at a series of times separated by equal intervals from each other.
Each interval comprises 223 lunations, or 18 years, 11 days, and 7 or 8
hours. The eclipses which occur within this interval are almost exactly
repeated during the next similar interval. This period, called the
"Saros," was well known to the ancients, who were enabled by its means
to predict eclipses with some certainty.
[PLATE III.--TOTAL ECLIPSE OF THE SUN.
Observed July 29, 1878, at Creston, Wyoming Territory]
A total eclipse of the Sun is a most beautiful and imposing phenomenon.
At the predicted time the perfectly round disk of the Sun becomes
slightly indented at its western limb by the yet invisible Moon. This
phenomenon is known as the "first contact."
The slight indentation observed gradually increases with the advance of
the Moon from west to east, the irregularities of the surface of our
satellite being plainly visible on the border of the dark segment
advancing on the Sun's disk. With the advance of the Moon on the Sun,
the light gradually diminishes on the Earth. Every object puts on a dull
and gloomy appearance, as when night is approaching; while the bright
sky, losing its light, changes its pure azure for a livid grayish color.
Two or three minutes before totality begins, the solar crescent, reduced
to minute proportions, gives comparatively so little light that faint
traces of the Sun's atmosphere appear on the western side behind the
dark body of the Moon, whose limb then becomes visible outside of the
Sun. I observed this phenomenon at Creston during the eclipse of 1878.
From 15 to 20 seconds before totality, the narrow arc of the Sun's disk
not yet obscured by the Moon seems to break and separate towards the
extremities of its cusps, which, thus divided, form independent points
of light, which are called "_Baily's beads_." A moment after, the whole
solar crescent breaks into numerous beads of light, separated by dark
intervals, and, suddenly, they all vanish with the last ray of Sunlight,
and totality has begun with the "second contact." This phenomenon of
Baily's beads is undoubtedly caused by the irregularities of the Moon's
border, which, on reaching the solar limb, divide the thin solar
crescent into as many beads of light and dark intervals as there are
peaks and ravines seen sidewise on that part of the Moon's limb.
With the disappearance of the last ray of light, the planets and the
stars of the first and second magnitude seem to light up and become
visible in the sky. The darkness, which had been gradually creeping in
with the progress of the eclipse, is then at its maximum. Although
subject to great variations in different eclipses, the darkness is never
so great as might be expected from the complete obscuration of our
luminary, as the part of our atmosphere which is still exposed to the
direct rays of the Sun, reflects to us some of that light, which thus
diminishes the darkness resulting from the disappearance of the Sun.
Usually the darkness is sufficient to prevent the reading of common
print, and to deceive animals, causing them to act as if night was
really approaching. During totality the temperature decreases, while the
humidity of the atmosphere augments.
Simultaneously with the disappearance of Baily's beads, a pale, soft,
silvery light bursts forth from behind the Moon, as if the Sun, in
disappearing, had been vaporized and expanded in all directions into
soft phosphorescent rays and streamers. This pale light is emitted by
gases constituting the solar atmosphere surrounding the bright nucleus
now obscured by the dark body of our satellite. This solar atmosphere is
called _Corona_, from its distant resemblance to the aureola, or glory,
represented by ancient painters around the heads of saints.
With the bursting forth of the corona, a very thin arc of bright white
light is seen along the Moon's limb, where the solar crescent has just
disappeared. This thin arc of light is the reversing layer, which, when
observed with the spectroscope at that moment, exhibits bright lines
answering to the dark lines of the ordinary solar spectrum. Immediately
above this reversing layer, and concentric with it, appears the
pink-colored chromospheric layer, with its curiously shaped flames and
protuberances. During totality, the chromosphere and protuberances are
seen without the aid of the spectroscope, and appear of their natural
color, which, although somewhat varying in their different parts, is, on
the whole, pinkish, and similar to that of peach-blossoms; yet it is
mixed here and there with delicate prismatic hues, among which the pink
and straw colors predominate.
The color of the corona seems to vary in every eclipse, but as its tints
are very delicate, it may depend, in a great measure, upon the vision of
the observer; although there seems to be no doubt that there are real
variations. At Creston, in 1878, it appeared to both Professor W.
Harkness and myself of a decided pale greenish hue.
The corona appears under different forms, and has never been observed
twice alike. Its dimensions are also subject to considerable variations.
Sometimes it appears regular and very little extended, its distribution
around the Sun being almost uniform; although in general it spreads a
little more in the direction of the ecliptic, or of the solar equator.
At other times it appears much larger and more complicated, and forms
various wings and appendages, which in some cases, as in 1878, extend to
immense distances; while delicate rays radiate in straight or curved
lines from the spaces left in the polar regions between the wings. The
corona has sometimes appeared as if divided by immense dark gaps,
apparently free from luminous matter, and strongly resembling the dark
rifts seen in the tails of comets. This was observed in Spain and Sicily
during the total eclipse of the Sun in 1870. Different structures,
forming wisps and streamers of great length, and interlaced in various
ways, are sometimes present in the corona, while faint but more
complicated forms, distantly resembling enormous solar protuberances
with bright nuclei, have also been observed.
As the Moon continues its eastward progress, it gradually covers the
chromosphere and the solar protuberances on the eastern side of the Sun;
while, at the same time, the protuberances and the chromosphere on the
opposite limb gradually appear from under the retreating Moon. Then, the
thin arc of the reversing layer is visible for an instant, and is
instantly followed by the appearance of a point of dazzling white light,
succeeded immediately by the apparition of Daily's beads on each side,
and totality is over, with this third contact. The corona continues to
be visible on the eastern side of the Sun for several minutes longer,
and then rapidly vanishes.