THE
TROUVELOT
ASTRONOMICAL DRAWINGS
MANUAL
BY
E. L. TROUVELOT,
FORMERLY CONNECTED WITH THE OBSERVATORY OF HARVARD COLLEGE; FELLOW OF THE
AMERICAN ACADEMY OF ARTS AND SCIENCES, AND MEMBER OF THE SELENO-GRAPHICAL
SOCIETY OF GREAT BRITAIN; IN CHARGE OF A
GOVERNMENT EXPEDITION TO OBSERVE THE
TOTAL SOLAR ECLIPSE OF 1878.
NEW YORK
CHARLES SCRIBNER'S SONS
1882
INTRODUCTION
During a study of the heavens, which has now been continued for more
than fifteen years, I have made a large number of observations
pertaining to physical astronomy, together with many original drawings
representing the most interesting celestial objects and phenomena.
With a view to making these observations more generally useful, I was
led, some years ago, to prepare, from this collection of drawings, a
series of astronomical pictures, which were intended to represent the
celestial phenomena as they appear to a trained eye and to an
experienced draughtsman through the great modern telescopes, provided
with the most delicate instrumental appliances. Over two years were
spent in the preparation of this series, which consisted of a number of
large drawings executed in pastel. In 1876, these drawings were
displayed at the United States Centennial Exhibition at Philadelphia,
forming a part of the Massachusetts exhibit, in the Department of
Education and Science.
The drawings forming the present series comprise only a part of those
exhibited at Philadelphia; but, although fewer in number, they are quite
sufficient to illustrate the principal classes of celestial objects and
phenomena.
While my aim in this work has been to combine scrupulous fidelity and
accuracy in the details, I have also endeavored to preserve the natural
elegance and the delicate outlines peculiar to the objects depicted; but
in this, only a little more than a suggestion is possible, since no
human skill can reproduce upon paper the majestic beauty and radiance of
the celestial objects.
The plates were prepared under my supervision, from the original pastel
drawings, and great care has been taken to make the reproduction exact.
The instruments employed in the observations, and in the delineation of
the heavenly bodies represented in the series, have varied in aperture
from 6 to 26 inches, according to circumstances, and to the nature of
the object to be studied. The great Washington refractor, kindly placed
at my disposal by the late Admiral C. H. Davis, has contributed to this
work, as has also the 26 inch telescope of the University of Virginia,
while in the hands of its celebrated constructors, Alvan Clark & Sons.
The spectroscope used was made by Alvan Clark & Sons. Attached to it is
an excellent diffraction grating, by Mr. L. M. Rutherfurd, to whose
kindness I am indebted for it.
Those unacquainted with the use of optical instruments generally suppose
that all astronomical drawings are obtained by the photographic process,
and are, therefore, comparatively easy to procure; but this is not true.
Although photography renders valuable assistance to the astronomer in
the case of the Sun and Moon, as proved by the fine photographs of these
objects taken by M. Janssen and Mr. Rutherfurd; yet, for other subjects,
its products are in general so blurred and indistinct that no details of
any great value can be secured. A well-trained eye alone is capable of
seizing the delicate details of structure and of configuration of the
heavenly bodies, which are liable to be affected, and even rendered
invisible, by the slightest changes in our atmosphere.
The method employed to secure correctness in the proportions of the
original drawings is simple, but well adapted to the purpose in view. It
consists in placing a fine reticule, cut on glass, at the common focus
of the objective and the eye-piece, so that in viewing an object, its
telescopic image, appearing projected on the reticule, can be drawn very
accurately on a sheet of paper ruled with corresponding squares. For a
series of such reticules I am indebted to the kindness of Professor
William A. Rogers, of the Harvard College Observatory.
The drawings representing telescopic views are inverted, as they appear
in a refracting telescope--the South being upward, the North downward,
the East on the right, and the West on the left. The Comet, the
Milky-Way, the Eclipse of the Moon, the Aurora Borealis, the Zodiacal
Light and the Meteors are represented as seen directly in the sky with
the naked eye. The Comet was, however, drawn with the aid of the
telescope, without which the delicate structure shown in the drawing
would not have been visible.
The plate representing the November Meteors, or so-called "Leonids," may
be called an ideal view, since the shooting stars delineated, were not
observed at the same moment of time, but during the same night. Over
three thousand Meteors were observed between midnight and five o'clock
in the morning of the day on which this shower occurred; a dozen being
sometimes in sight at the same instant. The paths of the Meteors,
whether curved, wavy, or crooked, and also their delicate colors, are in
all cases depicted as they were actually observed.
In the Manual, I have endeavored to present a general outline of what is
known, or supposed, on the different subjects and phenomena illustrated
in the series. The statements made are derived either from the best
authorities on physical astronomy, or from my original observations,
which are, for the most part, yet unpublished.
The figures in the Manual relating to distance, size, volume, mass,
etc., are not intended to be strictly exact, being only round numbers,
which can, therefore, be more easily remembered.
It gives me pleasure to acknowledge that the experience acquired in
making the astronomical drawings published in Volume VIII. of the Annals
of the Harvard College Observatory, while I was connected with that
institution, has been of considerable assistance to me in preparing this
work; although no drawings made while I was so connected have been used
for this series.
E. L. TROUVELOT.
_Cambridge, March, 1882._
CONTENTS
INTRODUCTION
LIST OF PLATES
_THE SUN_
GENERAL REMARKS ON THE SUN
SUN-SPOTS AND VEILED SPOTS
SOLAR PROTUBERANCES
TOTAL ECLIPSE OF THE SUN
_THE AURORAL AND ZODIACAL LIGHTS_
THE AURORA BOREALIS
THE ZODIACAL LIGHT
_THE MOON_
THE MOON
ECLIPSES OF THE MOON
_THE PLANETS_
THE PLANETS
THE PLANET MARS
THE PLANET JUPITER
THE PLANET SATURN
_COMETS AND METEORS_
COMETS
SHOOTING-STARS AND METEORS
_THE STELLAR SYSTEMS_
THE MILKY-WAY OR GALAXY
THE STAR-CLUSTERS
THE NEBULÆ
APPENDIX
LIST OF PLATES[1]
PLATE
I. GROUP OF SUN-SPOTS AND VEILED SPOTS.
_Observed June 17, 1875, at 7 h. 30m. A. M._
II. SOLAR PROTUBERANCES.
_Observed May 5, 1873, at 9h. 40m. A. M._
III. TOTAL ECLIPSE OF THE SUN.
_Observed July 29, 1878, at Creston, Wyoming Territory._
IV. AURORA BOREALIS.
_As observed March 1, 1872, at 9h. 25m. P. M._
V. THE ZODIACAL LIGHT.
_Observed February 20, 1876._
VI. MARE HUMORUM.
_From a study made in 1875._
VII. PARTIAL ECLIPSE OF THE MOON.
_Observed October 24, 1874._
VIII. THE PLANET MARS.
_Observed September 3, 1877, at 11h. 55m. P. M._
IX. THE PLANET JUPITER.
_Observed November 1, 1880, at 9h. 30m. P. M._
X. THE PLANET SATURN.
_Observed November 30, 1874, at 5th. 50m. P. M._
XI. THE GREAT COMET OF 1881.
_Observed on the night of June 25-26, at 1h. 30m. A. M._
XII. THE NOVEMBER METEORS.
_As observed between midnight and 3 o'clock A. M., on the night
of November 13-14, 1868._
XIII. PART OF THE MILKY-WAY.
_From a study made during the years 1874, 1875 and 1876._
XIV. STAR-CLUSTER IN HERCULES.
_From a study made in June, 1877._
XV. THE GREAT NEBULA IN ORION.
_From a study made in the years 1873-76._
[Footnote 1: For Key to the Plates, see Appendix.]
_Reproduced from the Original Drawings, by Armstrong & Company,
Riverside Press, Cambridge, Mass._
GENERAL REMARKS ON THE SUN
The Sun, the centre of the system which bears its name, is a
self-luminous sphere, constantly radiating heat and light.
Its apparent diameter, as seen at its mean from the Earth, subtends an
angle of 32', or a little over half a degree. A dime, placed about six
feet from the eye, would appear of the same proportions, and cover the
Sun's disk, if projected upon it.
That the diameter of the Sun does not appear larger, is due to the great
distance which separates us from that body. Its distance from the Earth
is no less than 92,000,000 miles. To bridge this immense gap, would
require 11,623 globes like the Earth, placed side by side, like beads on
a string.
The Sun is an enormous sphere whose diameter is over 108 times the
diameter of our globe, or very nearly 860,000 miles. Its radius is
nearly double the distance from the Earth to the Moon. If we suppose,
for a moment, the Sun to be hollow, and our globe to be placed at the
centre of this immense spherical shell, not only could our satellite
revolve around us at its mean distance of 238,800 miles, as now, but
another satellite, placed 190,000 miles farther than the Moon, could
freely revolve likewise, without ever coming in contact with the solar
envelope.
The circumference of this immense sphere measures 2,800,000 miles. While
a steamer, going at the rate of 300 miles a day, would circumnavigate
the Earth in 83 days, it would take, at the same rate, nearly 25 years
to travel around the Sun.
The surface of the Sun is nearly 12,000 times the surface of the Earth,
and its volume is equal to 1,300,000 globes like our own. If all the
known planets and satellites were united in a single mass, 600 such
compound masses would be needed to equal the volume of our luminary.
Although the density of the Sun is only one-quarter that of the Earth,
yet the bulk of this body is so enormous that, to counterpoise it, no
less than 314,760 globes like our Earth would be required.
The Sun uniformly revolves around its axis in about 25½ days. Its
equator is inclined 7° 15' to the plane of the ecliptic, the axis of
rotation forming, therefore, an angle of 82° 45' with the same plane.
As the Earth revolves about the Sun in the same direction as that of the
Sun's rotation, the apparent time of this rotation, as seen by a
terrestrial spectator, is prolonged from 25½ days to about 27 days and
7 hours.
The rotation of the Sun on its axis, like that of the Earth and the
other planets, is direct, or accomplished from West to East. To an
observer on the Earth, looking directly at the Sun, the rotation of this
body is from left to right, or from East to West.
The general appearance of the Sun is that of an intensely luminous disk,
whose limb, or border, is sharply defined on the heavens. When its
telescopic image is projected on a screen, or fixed on paper by
photography, it is noticed that its disk is not uniformly bright
throughout, but is notably more luminous in its central parts. This
phenomenon is not accidental, but permanent, and is due in reality to a
very rare but extensive atmosphere which surrounds the Sun, and absorbs
the light which that body radiates, proportionally to its thickness,
which, of course, increases towards the limb, to an observer on the
Earth.
THE ENVELOPING LAYERS OF THE SUN
The luminous surface of the Sun, or that part visible at all times, and
which forms its disk, is called the _Photosphere_, from the property it
is supposed to possess of generating light. The photosphere does not
extend to a great depth below the luminous surface, but forms a
comparatively thin shell, 3,000 or 4,000 miles thick, which is distinct
from the interior parts, above which it seems to be kept in suspense by
internal forces. From the observations of some astronomers it would
appear that the diameter of the photosphere is subject to slight
variations, and, therefore, that the solar diameter is not a constant
quantity. From the nature of this envelope, such a result does not seem
at all impossible, but rather probable.
Immediately above the photosphere lies a comparatively thin stratum,
less than a thousand miles in thickness, called the _Reversing Layer_.
This stratum is composed of metallic vapors, which, by absorbing the
light of particular refrangibilities emanating from the photosphere
below, produces the dark Fraunhofer lines of the solar spectrum.
Above the reversing layer, and resting immediately upon it, is a
shallow, semi-transparent gaseous layer, which has been called the
_Chromosphere_, from the fine tints which it exhibits during total
eclipses of the Sun, in contrast with the colorless white light radiated
by the photosphere below. Although visible to a certain extent on the
disk, the chromosphere is totally invisible on the limb, except with the
spectroscope, and during eclipses, on account of the nature of its
light, which is mainly monochromatic, and too feeble, compared with that
emitted by the photosphere, to be seen.
The chromospheric layer, which has a thickness of from 3,000 to 4,000
miles, is uneven, and is usually upheaved in certain regions, its matter
being transported to considerable elevations above its general surface,
apparently by some internal forces. The portions of the chromosphere
thus lifted up, form curious and complicated figures, which are known
under the names of _Solar Protuberances, or Solar Flames_.
Above the chromosphere, and rising to an immense but unknown height, is
the solar atmosphere proper, which is only visible during total eclipses
of the Sun, and which then surrounds the dark body of the Moon with the
beautiful rays and glorious nimbus, called the _Corona_.
These four envelopes: the photosphere, the reversing layer, the
chromosphere, and the corona, constitute the outer portions of our
luminary.
Below the photosphere little can be seen, although it is known, as will
appear below, that at certain depths cloud-like forms exist, and freely
float in an interior atmosphere of invisible gases. Beyond this all is
mystery, and belongs to the domain of hypothesis.
STRUCTURE OF THE PHOTOSPHERE AND CHROMOSPHERE
The apparent uniformity of the solar surface disappears when it is
examined with a telescope of sufficient aperture and magnifying powers.
Seen under good atmospheric conditions, the greater part of the solar
surface appears mottled with an infinite number of small, bright
granules, irregularly distributed, and separated from each other by a
gray-tinted background.
These objects are known under different names. The terms granules and
granulations answer very well for the purpose, as they do not imply
anything positive as to their form and true nature. They have also been
called _Luculœ_, _Rice Grains_, _Willow Leaves_, etc., by different
observers.
Although having different shapes, the granulations partake more or less
of the circular or slightly elongated form. Their diameter, which varies
considerably, has been estimated at from 0".5 to 3", or from 224 to
1,344 miles. The granulations which attain the largest size appear,
under good atmospheric conditions, to be composed of several granules,
closely united and forming an irregular mass, from which short
appendages protrude in various directions.
The number of granulations on the surface of the Sun varies considerably
under the action of unknown causes. Sometimes they are small and very
numerous, while at other times they are larger, less numerous, and more
widely separated. Other things being equal, the granulations are better
seen in the central regions of the Sun than they are near the limb.
Usually the granulations are very unstable; their relative position,
form, and size undergoing continual changes. Sometimes they are seen to
congregate or to disperse in an instant, as if acting under the
influence of attractive and repulsive forces; assembling in groups or
files, and oftentimes forming capricious figures which are very
remarkable, but usually of short duration. In an area of great solar
disturbances, the granulations are often stretched to great distances,
and form into parallel lines, either straight, wavy, or curved, and they
have then some resemblance to the flowing of viscous liquids.
The granulations are usually terminated either by rounded or sharply
pointed summits, but they do not all rise to the same height, as can be
ascertained with the spectroscope when they are seen sidewise on the
limb. In the regions where they are most abundant, they usually attain
greater elevations, and when observed on the limb with the spectroscope,
they appear as slender acute flames.
The granulations terminated by sharply-pointed crests, although observed
in all latitudes, seem to be characteristic of certain regions. A daily
study of the chromosphere, extending over a period of ten years, has
shown me that the polar regions are rarely ever free from these objects,
which are less frequent in other parts of the Sun. In the polar regions
they are sometimes so abundant that they completely form the solar limb.
These forms of granulation are comparatively rare in the equatorial
zones, and when seen there, they never have the permanency which they
exhibit in the polar regions. When observed in the equatorial regions,
they usually appear in small groups, in the vicinity of sun spots, or
they are at least enclosed in areas of disturbances where such spots are
in process of formation. In these regions they often attain greater
elevations than those seen in high latitudes.
As we are certain that in the equatorial zones these slender flames (_i.
e._, granulations) are a sure sign of local disturbance, it may be
reasonably supposed that the same kind of energy producing them nearly
always prevails in the polar regions, although it is there much weaker,
and never reaches beyond certain narrow limits.
Studied with the spectroscope, the granulations are found to be composed
in the main of incandescent hydrogen gas, and of an unknown substance
provisionally called "helium." Among the most brilliant of them are
found traces of incandescent metallic vapors, belonging to various
substances found on our globe.
The chromosphere is not fixed, but varies considerably in thickness in
its different parts, from day to day. Its thickness is usually greater
in the polar regions, where it sometimes exceeds 6,700 miles. In the
equatorial regions the chromosphere very rarely attains this height, and
when it does, the rising is local and occupies only a small area. In
these regions it is sometimes so shallow that its depth is only a few
seconds, and is then quite difficult to measure. These numbers give, of
course, the extreme limits of the variations of the chromosphere; but,
nearly always, it is more shallow in the equatorial regions; and, as far
as my observations go, the difference in thickness between the polar and
equatorial zones is greater in years of calm than it is in years of
great solar activity. But ten years of observation are not sufficient to
warrant any definite conclusions on this subject.
There is undoubtedly some relation between the greater thickness of the
chromosphere in the polar regions, and the abundance and permanence of
the sharply-pointed granulations observed in the same regions. This
becomes more evident when we know that the appearance of
similarly-pointed flames in the equatorial zones is always accompanied
with a local thickening of the chromosphere. The thickening in the polar
regions may be only apparent, and not due to a greater accumulation of
chromospheric gases there; but may be caused by some kind of repulsive
action or polarity, which lifts up and extends the summit of the
granulations in a manner similar to the well-known mode of electric
repulsion and polarity.
As it seems very probable that the heat and light emanating from the Sun
are mainly generated at the base of the granulations, in the filamentary
elements composing the chromosphere and photosphere, it would follow
that, as the size and number of these objects constantly vary, the
amount of heat and light emitted by the Sun should also vary in the same
proportion.
The granulations of the solar surface are represented on Plate I., and
form the general background to the group of Sun-spots forming the
picture.
THE FACULÆ
Although the solar surface is mainly covered with the luminous
granulations and the grayish background above described, it is very rare
that its appearance is so simple and uniform as already represented. For
the most part, on the contrary, it is diversified by larger, brighter,
and more complicated forms, which are especially visible towards the
border of the Sun. Owing to their extraordinary brilliancy, these
objects have been called _Faculœ_ (torches).
Although the faculæ are very seldom seen well beyond 50 heliocentric
degrees from the limb, yet they exist, and are as numerous in the
central parts of the disk as they are towards the border; since they
form a part of the solar surface, and participate in its movement of
rotation. Their appearance near the limb has been attributed to the
effect of absorption produced by the solar atmosphere on the
light from the photosphere; but this explanation seems inadequate,
and does not solve the problem. The well-known fact that the solar
protuberances--which are in a great measure identical with the
faculæ--are much brighter at the base than they are at the summit,
perhaps gives a clue to the explanation of the phenomenon; especially
since we know that, in general, the summit of the protuberances is
considerably broader than their base. When these objects are observed in
the vicinity of the limb, they present their brightest parts to the
observer, since, in this position, they are seen more or less sidewise;
and, therefore, they appear bright and distinct. But as the faculæ
recede from the limb, their sides, being seen under a constantly
decreasing angle, appear more and more foreshortened; and, therefore,
these objects grow less bright and less distinct, until they finally
become invisible, when their bases are covered over by the broad, dusky
summit generally terminating the protuberances.
The faculæ appear as bright and luminous masses or streaks on the
granular surface of the Sun, but they differ considerably in form and
size. Two types at least are distinguishable. In their simplest form
they appear either as isolated white spots, or as groups of such spots
covering large surfaces, and somewhat resembling large flakes of snow.
In their most characteristic types they appear as intensely luminous,
heavy masses, from which, in most cases, issue intricate ramifications,
sometimes extending to great distances. Generally, the ramifications
issuing from the masses of faculæ have their largest branches directed
in the main towards the eastern limb of the Sun. Some of these branches
have gigantic proportions. Occasionally they extend over 60° and even
80° of the solar surface, and, therefore, attain a length of from
450,000 to 600,000 miles.
Although the faculæ may be said to be seen everywhere on the surface of
the Sun, there is a vast difference in different regions, with regard to
their size, number, and brilliancy. They are largest, most abundant, and
brightest on two intermediate zones parallel to the solar equator, and
extending 35° or 40° to the north and to the south of this line. The
breadth of these zones varies considerably with the activity of the
solar forces. When they are most active, the faculæ spread on either
side, but especially towards the equator, where they sometimes nearly
meet those of the other zone. In years of little solar activity the
belts formed by the faculæ are very narrow--the elements composing them
being very few and small, although they never entirely disappear.
The faculæ are very unstable, and are constantly changing: those of the
small types sometimes form and vanish in a few minutes. When an area of
disturbance of the solar surface is observed for some time, all seems in
confusion; the movements of the granulations become unusually violent;
they congregate in all sorts of ways, and thus frequently form temporary
faculæ. Action of this kind is, for the most part, peculiar to the
polar regions of the Sun.
The larger faculæ have undoubtedly another origin, as they seem to be
mainly formed by the ejection of incandescent gases and metallic vapors
from the interior of the photosphere. In their process of development
some of the heavy masses of faculæ are swollen up to great heights,
being torn in all sorts of ways, showing large rents and fissures
through which the sight can penetrate.
Very few faculæ are represented in Plate I.; several streaks are shown
at the upper left-hand corner, some appearing as whitish ramifications
among the granulations representing the general solar surface.
SUN-SPOTS AND VEILED SPOTS
PLATE I
Besides the brilliant faculæ already described, much more conspicuous
markings, though of a totally different character, are very frequently
observed on the Sun. On account of their darkish appearance, which is in
strong contrast with the white envelope of our luminary, these markings
were called _Maculæ_, or Sun-spots, by their earlier observers.
The Sun-spots are not equally distributed on the solar surface; but like
the faculæ, to which they are closely related, they occupy two
zones--one on each side of the equator. These zones are comprised
between 10° and 35° of north latitude, and 10° and 35° of south
latitude. Between these two zones is a belt 20° in width, where the
Sun-spots are rarely seen.
Above the latitudes 35° north and south, the Sun-spots are rare, and it
is only occasionally, and during years of great solar activity, that
they appear in these regions; in only a few cases have spots of
considerable size been seen there. A few observers, however, have seen
spots as far as 40° and 50 from the equator; and La Hire even observed
one in 70° of north latitude; but these cases are exceedingly rare. It
is not uncommon, however, to see very small spots, or groups of such
spots, within 8° or 10° from the poles.
The activity of the Sun is subject to considerable fluctuation, and
accordingly the Sun-spots vary in size and number in different years.
During some years they are large, complicated, and very numerous; while
in others they are small and scarce, and are sometimes totally absent
for weeks and months together. The fluctuations in the frequency of
Sun-spots are supposed to be periodical in their character, although
their periods do not always appear to recur at exactly regular
intervals. Sometimes the period is found to be only nine years, while at
other times it extends to twelve years. The period generally adopted now
is 11⅒ years, nearly; but further investigations are needed to
understand the true nature of the phenomenon.
The number of Sun-spots does not symmetrically augment and diminish, but
the increase is more rapid than the diminution.
The period of increase is only about four years, while that of decrease
is over seven years; each period of Sun-spot maximum being nearer the
preceding period of Sun-spot minimum than it is to that next following.
The cause of these fluctuations in the solar energy is at present wholly
unknown. Some astronomers, however, have attributed it to the influence
of the planets Venus and Jupiter, the period of revolution of the latter
planet being not much longer than the Sun-spot period; but this
supposition lacks confirmation from direct observations, which, so far,
do not seem to be in favor of the hypothesis. At the present time the
solar activity is on the increase, and the Sun-spots will probably reach
their maximum in 1883. The last minimum occurred in 1879, when only
sixteen small groups of spots were observed during the whole year.
Sun-spots vary in size and appearance; but, unless they are very small,
in which case they appear as simple black dots, they generally consist
of two distinct and well-characterized parts, nearly always present.
There is first, a central part, much darker than the other, and sharply
divided from it, called the "_Umbra_;" second, a broad, irregular
radiated fringe of lighter shade, completely surrounding the first, and
called the "_Penumbra_."
Reduced to its simplest expression, a Sun-spot is a funnel-shaped
opening through the chromosphere and the photosphere. The inner end of
the funnel, or opening, gives the form to the umbra, while its sloping
sides form the penumbra.
The umbra of Sun-spots, whose outlines approximately follow the
irregularities of the penumbral fringe, has a diameter which generally
exceeds the width of the penumbral ring. Sometimes it appears uniformly
black throughout; but it is only so by contrast, as is proved when
either Mercury or Venus passes near a spot during a transit over the
Sun's disk. The umbra then appears grayish, when compared with the
jet-black disk of the planet.
The umbra of spots is rarely so simple as just described; but it is
frequently occupied, either partly or wholly, by grayish and rosy forms,
somewhat resembling loosely-entangled muscular fibres. These forms have
been called the _Gray and Rosy Veils_. Frequently these veils appear as
if perforated by roundish black holes, improperly called _Nuclei_, which
permit the sight to penetrate deeper into the interior. To all
appearance the gray and rosy veils are of the same nature as the
chromosphere and the faculæ, and are therefore mainly composed of
hydrogen gas.
Whatever can be known about the interior of the Sun, must be learned
from the observations of these openings, which are comparatively small.
But whatever this interior may be, we certainly know that it is not
homogeneous. Apparently, the Sun is a gigantic bubble, limited by a very
thin shell. Below this shell exists a large open space filled with
invisible gases, in which, through the openings constituting the
Sun-spots, the gray and rosy veils described above are occasionally seen
floating.
The fringe forming the penumbra of spots is much more complicated than
the umbra. In its simpler form, it is composed of a multitude of bright,
independent filaments of different forms and sizes, partly projecting
one above the other, on the sloping wall of the penumbra, from which
they seem to proceed. Seen from the Earth, these filaments have somewhat
the appearance of thatched straw, converging towards the centre of the
umbra. It is very rare, however, that the convergence of the penumbral
filaments is regular, and great confusion sometimes arises from the
entanglement of these filaments. Some of these elements appear straight,
others are curved or loop-shaped; while still others, much larger and
brighter than the rest, give a final touch to this chaos of filaments,
from which results the general thatched and radiating appearance of the
penumbra.
The extremities of the penumbral filaments, especially of those forming
the border of the umbra, are usually club-shaped and appear very
brilliant, as if these elements had been superheated by some forces
escaping through the opening of the spots.
Besides these characteristics, the Sun-spots have others, which,
although not always present, properly belong to them. Comparatively few
spots are so simple as the form just described. Very frequently a spot
is accompanied by brilliant faculæ, covering part of its umbra and
penumbra, and appearing to form a part of the spot itself.
When seen projected over Sun-spots, the faculæ appear intensely bright,
and from these peculiarities they have been called _Luminous Bridges_.
They are, in fact, bridges, but in most cases they are at considerable
heights above the spots, kept there by invisible forces. When such spots
with luminous bridges approach the Sun's limb, it is easy to see, by the
rapid apparent displacement which they undergo, that they are above the
general level.
When the spots are closing up, the inverse effect is sometimes observed.
On several occasions, I have seen huge masses of faculæ advance slowly
over the penumbra of a spot and fall into the depths of the umbra,
resembling gigantic cataracts. I have seen narrow branches of faculæ,
which, after having fallen to great depths in the umbra, floated across
it and disappeared under the photosphere on the opposite side. I have
also seen luminous bridges, resembling cables, tightly stretched across
the spots, slackening slowly, as if loosened at one end, and gently
curving into the umbra, where they formed immense loops, large enough to
receive our globe.
It is to be remarked that, in descending under the photospheric shell,
the bright faculæ and the luminous bridges gradually lose their
brilliancy. At first they appear grayish, but in descending farther they
assume more and more the pink color peculiar to the rosy veils. The
pinkish color acquired by the faculæ when they reach a certain depth
under the photosphere, is precisely the color of the chromosphere and of
the solar protuberances, as seen during total eclipses of the Sun--a
fact which furnishes another proof that the faculæ are of the same
nature as the protuberances.
I record here an observation which, at first sight, may appear
paradoxical; but which seems, however, to be of considerable importance,
as it shows unmistakably that the solar light is mainly, if not
entirely, generated on its surface, or at least very near to it. On May
26, 1878, I observed a large group of Sun-spots at a little distance
from the east limb of the Sun. The spot nearest to the limb was partly
covered over on its eastern and western sides by bright and massive
faculæ which concealed about two-thirds of the whole spot, only a
narrow opening, running from north to south, being left across the
middle of the spot. Owing to the rotundity of the Sun, the penumbra of
this spot, although partly covered by the faculæ, could, however, be
seen on its eastern side, since the sight of the observer could there
penetrate sidewise under the faculæ. Upon that part of the penumbra
appeared a strong shadow, representing perfectly the outline of the
facular mass situated above it. The phenomenon was so apparent that no
error of observation was possible, and a good drawing of it was secured.
If this faculæ had been as bright beneath as it was above, it is
evident that no shadow could have been produced; hence the light of
these faculæ must have been mainly generated on or very near their
exterior surfaces. This, with the well-proved fact that the bright
faculæ lose their light in falling into the interior of the Sun, seems
to suggest the idea that the bright light emitted by the faculæ, and
very probably all the solar light, can be generated only on its surface;
the presence of the coronal atmosphere being perhaps necessary to
produce it. Several times before this observation, I had suspected that
some faculæ were casting a shadow, but as this seemed so improbable, my
attention was not awakened until the phenomenon became so prominent that
it could not escape notice.
With due attention, some glimpses of the phenomenon can frequently be
observed through the openings of some of the faculæ projecting over the
penumbra of Sun-spots. It is very seldom that the structure of the
penumbra is seen through such openings, which usually appear as dark as
the umbra of the large spots, although they do not penetrate through the
photosphere like the latter. It is only when the rents in the faculæ
are numerous and quite large, that the penumbral structure is recognized
through them. Since these superficial rents in the faculæ do not extend
through the photosphere, and appear black, it seems evident that the
penumbra seen through them cannot be as bright as it is when no faculæ
are projected upon it, and therefore that the faculæ intercept light
from the exterior surface, which would otherwise reach the penumbra.
While the matter forming the faculæ sometimes falls into the interior
of the Sun, the same kind of matter is frequently ejected in enormous
quantities, and with great force, from the interior, through the visible
and invisible openings of the photosphere, and form the protuberances
described in the following section of this manual (Solar protuberances)
It is not only the incandescent hydrogen gas or the metallic vapors
which are thus ejected, but also cooler hydrogen gas, which sometimes
appears as dark clouds on the solar surface. On December 12, 1875, I
observed such a cloud of hydrogen issuing from the corner of a small
Sun-spot. It traveled several thousand miles on the solar surface, in a
north-easterly direction, before it became invisible.
Solar spots are formed in various ways; but, for the most part, the
apparition of a spot is announced beforehand, by a great commotion of
the solar surface at the place of its appearance, and by the formation
of large and bright masses of faculæ, which are usually swollen into
enormous bubbles by the pressure of the internal gases. These bubbles
become visible in the spectroscope while they are traversing the solar
limb, as they are then presented to us sidewise. Under the action of the
increasing pressure, the base of the faculæ is considerably stretched,
and, its weakest side finally giving way, the facular mass is torn in
many places from the solar surface, and is perforated by holes of
different sizes and forms. The holes thus made along the border of the
faculæ appear as small black spots, separated more or less by the
remaining portion of the lacerated faculæ, and they enlarge more and
more at the expense of the intervening portions, which thus become very
narrow. This perforated side of the faculæ, offering less resistance,
is gradually lifted up, as would be the cover of a box, for example,
while its opposite side remains attached to the surface. The facular
matter separating the small black holes is greatly stretched during this
action, and forms long columns and filaments. These appear as luminous
bridges upon the large and perfectly-formed spot, which is then seen
under the lifted facular masses. The spots thus made visible are soon
freed from the facular masses, which are gradually shifted towards the
opposite side.
In such cases the spots are undoubtedly formed under the faculæ before
they can be seen. This becomes evident when such spots, not yet cleared
from the faculæ covering them, are observed near the east limb; since
in this position the observer can see through the side-openings of the
faculæ, and sometimes recognize the spots under their cover.
It frequently occurs that the spots thus formed under the faculæ
continue to be partly covered by the facular clouds, the forces at work
in them being apparently too feeble to shift them aside. In such cases
these spots are visible when they are in the vicinity of the limb, where
they are seen sidewise; but when observed in the east, in being carried
forward by the solar rotation towards the centre of the disk, they
gradually diminish in size, and finally become invisible. The
disappearance of these spots, however, is only apparent, being due to
the fact that, as they advance towards the centre of the disk, our
lateral view of them is gradually lost, and they are finally hidden from
sight by the overhanging faculæ which then serve as a screen between
the observer and the spot. This class of spots may be called _Lateral
Spots_, from the fact that they can only be seen laterally, and near the
Sun's limb.
Solar spots are also formed in various other ways. Some, like those
represented in Plate I., appearing without being announced by any
apparent disturbance of the surface, or by the formation of any faculæ,
form and develop in a very short time. Others, appearing at first as
very small spots having an umbra and a penumbra, slowly and gradually
develop into very large spots. This mode of formation, which would seem
to be the most natural, is, however, quite rare. Spots of this class
have a duration and permanence not observed in those of any other type.
These spots of slow and regular development are never accompanied by
faculæ or luminous bridges, nor have they any gray or rosy veils in
their interior; a fact which may, perhaps, account for their permanent
character.
Another class of spots, which is also rare, appear as long and narrow
crevasses showing the penumbral structure of the ordinary spots; but
these rarely have any umbra. These long, and sometimes exceedingly
narrow fissures of the solar envelopes, with their radiated penumbral
structure, strongly suggest the idea that the photosphere is composed of
a multitude of filamentary elements having the granulations for summits.
Such a crevasse is represented on Plate I., and unites the two spots
which form the group.
The duration of Sun-spots varies greatly. Some last only for a few
hours; while others continue for weeks and even months at a time, but
not without undergoing changes.
The modes of disappearance of Sun-spots are as various as those of their
apparition. The spots rarely close up by a gradual diminution or
contraction of their umbra and penumbra. This mode of disappearance
belongs exclusively to the spots deprived of faculæ and veils. One of
the most common modes of the disappearance of a spot is its invasion by
large facular masses, which slowly advance upon its penumbra and umbra
and finally cover it entirely. It is a process precisely the reverse of
that in which spots are formed by the shifting aside of the faculæ, as
above described. In other types, the spots close up by the gradual
enlargement of the luminous bridges traversing them, which are slowly
transformed into branches of the photosphere, all of the characteristics
of which they have acquired. In many cases, the spots covered over by
the faculæ continue to exist for some time, hidden under these masses,
as is often proved, either by the appearance of small spots on the
facular mass left at the place they occupied, or even by the
reappearance of the same spot.
Apart from the general movement of rotation of the solar surface, some
of the spots seem to be endowed with a proper motion of their own, which
becomes greater the nearer the spots are to the solar equator. According
to the observations of Mr. Carrington, the period of rotation of the
Sun, as deduced from the observations of the solar spots during a period
of seven years, is 25 days at the equator; while at 50° of heliocentric
latitude it is 27 days. But the period of rotation, as derived from the
observations of spots occupying the same latitude, is far from being
constant, as it varies at different times, with the frequency of the
spots and with the solar activity, so that at present the law of these
variations is not well known. From the character of the solar envelope,
it seems very natural that the rotation should differ in the different
zones and at different times, since this envelope is not rigid, but very
movable, and governed by forces which are themselves very variable.
Although it is a general law that the spots near the equator have a more
rapid motion than those situated in higher latitudes, yet, in many
cases, the proper motion of the spots is more apparent than real. For
the most part, the changes of form and the rapid displacements observed
in some spots are only apparent, and due to the fact that the large
masses of faculæ which are kept in suspense above them are very
unstable, and change position with the slightest change in the forces
holding them in suspension. Since in these cases we view the spots
through the openings of the faculæ situated above them, the slightest
motion of these objects produces an apparent motion in the spots,
although they have remained motionless. Accordingly, it has been
remarked that of all the spots, those which have the greater proper
motion are precisely those which have the most faculæ and luminous
bridges; while the other spots in the same regions, but not attended by
similar phenomena, are comparatively steady in their movement. These
last spots are undoubtedly better adapted than any others to exhibit the
rotation of the Sun; but it is probable that this period of rotation
will never be known with accuracy, simply because the solar surface is
unstable, and does not rotate uniformly.
The Sun-spots have a remarkable tendency to form into groups of various
sizes, but whatever may be the number of spots thus assembled, the group
is nearly always composed of two principal spots, to which the others
are only accessories. The tendency of the Sun-spots to assemble in pairs
is general, and is observed in all latitudes, even among the minute
temporary groups formed in the polar regions. Whenever several are
situated quite close together, those belonging to the same group can be
easily recognized by this character. Whatever may be the position of the
axis of the two principal spots of a group when it is first formed, this
axis has a decided tendency to place itself parallel to the solar
equator, no matter to what latitude the group belongs; and if it is
disturbed from this position, it soon returns to it when the disturbance
has ceased.
It is also remarkable that the spots observed at the same time remain in
nearly the same parallel of latitude for a greater or less period of
time; but they keep changing their position from year to year, their
latitude decreasing with the activity of the solar forces.
Among the Sun-spots, those associated with faculæ form the groups which
attain the largest proportions. When such groups acquire an apparent
diameter of 1' or more, they are plainly visible to the naked eye, since
for a spot to be visible to the naked eye on the Sun, it need only
subtend an angle of 50". I have sometimes seen such groups through a
smoky atmosphere, when the solar light was so much reduced that the disk
could be observed directly and without injury to the sight.
The largest spot which ever came under my observation was seen during
the period from the 13th to the 19th of November, 1870. This spot, which
was on the northern hemisphere of the Sun, was conspicuous among the
smaller spots constituting the group to which it belonged, and followed
them on the east. On November 16th, when it attained its largest size,
the diameter of its penumbra occupied fully one-fifth of the diameter of
the Sun; its real diameter being, therefore, not less than 172,000
miles, or nearly 22 times the diameter of the Earth. As the umbra of
this spot occupied a little more than one-third of its whole diameter,
seven globes like our own, placed side by side on a straight line, could
easily have passed through this immense gap. To fill the area of this
opening, about 45 such globes would have been needed. This spot was, of
course, very easily seen with the naked eye, its diameter being almost
eight times that required for a spot to be visible without a telescope.
Ancient historians often speak of obscurations of the Sun, and it has
been supposed by some astronomers that this phenomenon might have been
due in some cases to the apparition of large spots. A few spots on the
surface of the Sun, like that just described, would sensibly reduce its
light.
Besides the ordinary Sun-spots already described, others are at times
observed on the surface of the Sun, which show some of the same
characteristics, but never attain so large proportions. They always
appear as if seen through a fog, or veil, between the granulations of
the solar surface. On account of their vagueness and ill-defined
contours, I have proposed for these objects the term, "_Veiled Spots_."
Veiled spots have a shorter duration than the ordinary spots, the
smaller types sometimes forming and vanishing in a few minutes. Some of
the larger veiled spots, however, remain visible for several days in
succession, and show the characteristics of other spots in regard to the
arrangement of their parts.