while, at the same time, the darkness and gloom spread over nature
gradually disappear, and terrestrial objects begin to resume their
natural appearance. Finally the limb of the Moon separates from that of
the Sun at the instant of "fourth contact," and the eclipse is over.
The phenomena exhibited by the corona in different eclipses are very
complex, and, so far, they have not been sufficiently studied to enable
us to understand the true nature of the solar atmosphere. From the
spectral analysis of the corona, and the phenomena of polarization, it
has been learned, at least, that while the matter composing the upper
part of the solar atmosphere is chiefly composed of an unknown
substance, producing the green line 1474, its lower part is mainly
composed of hydrogen gas at different temperatures, a part of which is
self-luminous, while the other part only reflects the solar light. But
the proportion of the gaseous particles emitting light, to those simply
reflecting it, is subject to considerable variations in different
eclipses. At present it would seem that in years of great solar
disturbances, the particles emitting light are found in greater quantity
in the corona than those reflecting it; but further observations will be
required to confirm these views.
It is very difficult to understand how the corona, which in certain
eclipses extends only one diameter of the Sun, should, in other cases,
as in 1878, extend to the enormous distance of twelve times the same
diameter. Changes of such magnitude in the solar atmosphere, if due to
the operation of forces with which we are acquainted, cannot yet be
accounted for by what is known of such forces. Their causes are still as
mysterious as those concerned in the production of the monstrous tails
displayed by some comets on their approach to the Sun.
Plate 3, representing the total eclipse of the Sun of July 29th, 1878,
was drawn from my observations made at Creston, Wyoming Territory, for
the Naval Observatory. The eclipse is represented as seen in a
refracting telescope, having an aperture of 6⅓ inches, and as it
appeared a few seconds before totality was over, and when the
chromosphere was visible on the western limb of the Sun. The two long
wings seen on the east and west side of the Sun, appeared considerably
larger in the sky than they are represented in the picture.
THE AURORA BOREALIS
PLATE IV
The name of Polar Auroras is given to certain very remarkable luminous
meteoric phenomena which appear at intervals above the northern or the
southern horizons of both hemispheres of the Earth. When the phenomenon
is produced in our northern sky, it is called "Aurora Borealis," or
"Northern Lights;" and when it appears in the southern sky, it is called
"Aurora Australis," or southern aurora.
Marked differences appear in the various auroras observed from our
northern latitudes. While some simply consist in a pale, faint
luminosity, hardly distinguishable from twilight, others present the
most gorgeous and remarkable effects of brightness and colors.
A great aurora is usually indicated in the evening soon after twilight,
by a peculiar grayish appearance of the northern sky just above the
horizon. The grayish vapors giving that appearance, continuing to form
there, soon assume a dark and gloomy aspect, while they gradually take
the form of a segment of a circle resting on the horizon. At the same
time that this dark segment is forming, a soft pearly light, which seems
to issue from its border, spreads up in the sky, where it gradually
vanishes, being the brightest at its base. This arc of light, gradually
increasing in extent as well as in brightness, reaches sometimes as far
as the polar star. On some rare occasions, one or two, and even three,
concentric arches of bright light form one above the other over the dark
segment, where they appear as brilliant concentric rainbows. While the
aurora continues to develop and spread out its immense arc, the border
of the dark segment loses its regularity and appears indented at several
places by patches of light, which soon develop into long, narrow,
diverging rays and streamers of great beauty. For the most part the
auroral light is either whitish or of a pale, greenish tint; but in some
cases it exhibits the most beautiful colors, among which the red and
green predominate. In these cases the rays and streamers, which are
usually of different colors, produce the most magnificent effects by
their continual changes and transformations.
The brightness and extent of the auroral rays are likewise subject to
continual changes. An instant suffices for their development and
disappearance, which may be succeeded by the sudden appearance of others
elsewhere, as though the original streamers had been swiftly transported
to a new place while invisible. It frequently happens that all the
streamers seem to move sidewise, from west to east, along the arch,
continuing meanwhile to exhibit their various changes of form and color.
For a time, these appearances of motion continue to increase, a
succession of streamers alternately shooting forth and again fading,
when a sudden lull occurs, during which all motion seems to have ceased.
The stillness then prevailing is soon succeeded by slight pulsations of
light, which seem to originate on the border-of the dark segment, and
are propagated upwards along the streamers, which have now become more
numerous and active. Slow at first, these pulsations quicken by degrees,
and after a few minutes the whole northern sky seems to be in rapid
vibration. The lively upward and downward movement of these streamers
entitles them to the name of "merry dancers" given them in northern
countries where they are frequent.
Long waves of light, quickly succeeded by others, are propagated in an
instant from the horizon to the zenith; these, in their rapid passage,
cause bends and curves in the streamers, which then, losing their
original straightness, wave and undulate in graceful folds, resembling
those of a pennant in a gentle breeze. Although the coruscations add to
the grandeur of the spectacle, they tend to destroy the diverging
streamers, which, being disconnected from the dark segment, or torn in
various ways, are, as it were, bodily carried up towards the zenith.
In this new phase the aurora is transformed into a glorious crown of
light, called the "Corona." From this corona diverge in all directions
long streamers of different colors and forms, gracefully undulating in
numerous folds, like so many banners of light. Some of the largest of
these streamers appear like fringes composed of short transverse rays of
different intensity and colors, producing the most fantastic effects,
when traversed by the pulsations and coruscations which generally run
across these rays during the great auroral displays.
The aurora has now attained its full development and beauty. It may
continue in this form for half an hour, but usually the celestial fires
begin to fade at the end of fifteen or twenty minutes, reviving from
time to time, but gradually dying out. The northern sky usually appears
covered by gray and luminous streaks and patches after a great aurora,
these being occasionally rekindled, but more often they gradually
disappear, and the sky resumes its usual appearance.
The number of auroras which develop a corona near the zenith is
comparatively small in our latitudes; but many of them, although not
exhibited on so grand a scale, are nevertheless very interesting. On
some very rare occasions the auroral display has been confined almost
exclusively to the dark segment, which appeared then as if pierced along
its border by many square openings, like windows, through which appeared
the bright auroral light.
[PLATE IV.--AURORA BOREALIS.
As observed March 1, 1872, at 9h. 25m. P.M.]
Among the many auroras which I have had occasion to observe, none are
more interesting, excepting the type first described, than those which
form an immense arch of light spanning the heavens from East to West.
This form of aurora, which is quite rare, I last observed on September
12th, 1881. All the northern sky was covered with light vapors, when a
small auroral patch appeared in the East at about 20° above the
horizon. This patch of light, gradually increasing westward, soon
reached the zenith, and continued its onward progress until it arrived
at about 20° above the western horizon, where it stopped. The aurora
then appeared as a narrow, wavy band of light, crossed by numerous
parallel rays of different intensity and color. These rays seemed to
have a rapid motion from West to East along the delicately-fringed
streamer, which, on the whole, moved southward, while its extremities
remained undisturbed. Aside from the apparent displacement of the
fringes, a singular vibrating motion was observed in the auroral band,
which was traversed by pulsations and long waves of light. The phenomena
lasted for about twenty minutes, after which the arch was broken in many
places, and it slowly vanished.
The aurora usually appears in the early part of the evening, and attains
its full development between ten and eleven o'clock. Although the
auroral light may have apparently ceased, yet the phenomenon is not at
an end, as very often a solitary ray is visible from time to time; and
even towards morning these rays sometimes become quite numerous. On some
occasions the phenomenon even continues through the following day, and
is manifested by the radial direction of the cirrus-clouds in the
heights of our atmosphere. In 1872 I, myself, observed an aurora which
apparently continued for two or three consecutive days and nights. In
August, 1859, the northern lights remained visible in the United States
for a whole week.
The height attained by these meteors is considerable, and it is now
admitted that they are produced in the rarefied air of the upper regions
of our atmosphere. From the researches of Professor Elias Loomis on the
great auroras observed in August and September, 1859, it was ascertained
that the inferior part of the auroral rays had an altitude of 46 miles,
while that of their summits was 428 miles. These rays had, therefore, a
length of 382 miles. From the observation of thirty auroral displays, it
has been found that the mean height attained by the summit of these
streamers above the Earth's surface was 450 miles.
But if the auroral streamers are generally manifested at great heights
in our atmosphere, it would appear from the observations of persons
living in the regions where the auroras are most frequent, as also from
those who have been stationed in high northern and southern latitudes,
that the phenomenon sometimes descends very low. Both Sabine and Parry
saw the auroral rays projected on a distant mountain; Ross saw them
almost at sea-level projected on the polar ice; while Wrangel, Franklin,
and others observed similar phenomena. Dr. Hjaltalin, who has lived in
latitude 64° 46' north, and has made a particular study of the aurora,
on one occasion saw the aurora much below the summit of a hill 1,600
feet high, which was not very far off.
The same aurora is sometimes observed on the same night at places very
far distant from one another. The great aurora borealis of August 28th,
1859, for instance, was seen over a space occupying 150° in
longitude--from California to the Ural Mountains in Russia. It even
appears now very probable that the phenomenon is universal on our globe,
and that the northern lights observed in our hemisphere are simultaneous
with the aurora australis of the southern hemisphere. The aurora of
September 2d, 1859, was observed all through North and South America,
the Sandwich Islands, Australia, and Africa; the streamers and
pulsations of light of the north pole responding to the rays and
coruscations of the south pole. Of thirty-four auroras observed at
Hobart Town, in Tasmania, twenty-nine corresponded with aurora borealis
observed in our hemisphere.
The auroral phenomena, although sometimes visible within the tropics,
are, however, quite rare in these regions. For the most part they are
confined within certain zones situated in high latitudes north and
south. The zone where they are most frequent in our hemisphere forms an
ellipse, which has the north pole at one of its foci; while the other is
situated somewhere in North America, in the vicinity of the magnetic
pole. The central line of the zone upon which the auroras seem to be
most frequent passes from the northern coast of Alaska through Hudson's
Bay and Labrador to Iceland, and then follows the northern coast of
Europe and Asia. The number of auroras diminishes as the observer
recedes from this zone, and it is only in exceptional cases that they
are seen near the equator. Near the pole the phenomenon is less frequent
than it is in the region described. In North America we occupy a
favorable position for the observation of auroras, as we are nearer the
magnetic poles than are the Europeans and Asiatics, and we consequently
have a greater number of auroras in corresponding latitudes.
The position of the dark auroral segment varies with the place occupied
by the observer, and its centre always corresponds with the magnetic
meridian. In our Eastern States the auroral segment appears a little to
the west of the north point; but as the observer proceeds westward it
gradually approaches this point, and is due north when seen from the
vicinity of Lake Winnipeg. At Point Barrow, in the extreme north-west of
the United States, the aurora is observed in the east. In Melville
Islands, Parry saw it in the south; while in Greenland it is directly in
the west.
It is stated that auroras are more numerous about the equinoxes than
they are at any other seasons; and also, when the earth is in perigee,
than when it is in apogee. An examination which I have made of a
catalogue by Professor Loomis, comprising 4,137 auroras observed in the
temperate zone of our hemisphere from 1776 to 1873, sustains this
statement. During this period, one hundred more auroras were recorded
during each of the months comprising the equinoxes, than during any
other months of the year; while eighty more auroras were observed when
the earth was in perigee, than when it was in apogee. But to establish
the truth of this assertion on a solid basis, more observations in both
hemispheres will be required.
The aurora is not simply a terrestrial phenomenon, but is associated in
some mysterious way with the conditions of the Sun's surface. It is a
well-known fact that terrestrial magnetism is influenced directly by the
Sun, which creates the diurnal oscillations of the magnetic needle.
Between sunrise and two o'clock, the north pole of the needle moves
towards the west in our northern hemisphere, and in the afternoon and
evening it moves the other way. These daily oscillations of the needle
are not uniform in extent; they have a period of regular increase and
decrease. At a given place the daily oscillations of the magnetic needle
increase and decrease with regularity during a period which is equal to
10⅓ years. As this period closely coincides with the Sun-spot period,
the connection between the variation of the needle and these solar
disturbances has been recognized.
Auroral phenomena generally accompany the extraordinary perturbations in
the oscillations of the magnetic needle, which are commonly called
"magnetic storms," and the greater the auroral displays, the greater are
the magnetic perturbations. Not only is the needle subject to unusual
displacements during an aurora, but its movements seem to be
simultaneous with the pulsations and waving motions of the delicate
auroral streamers in the sky. When the aurora sends forth a coruscation,
or a streamer in the sky, the magnetic needle responds to it by a
vibration. The inference that the auroral phenomena are connected with
terrestrial magnetism is further supported by the fact that the centre
of the corona is always situated exactly in the direction of that point
in the heavens to which the dipping needle is directed.
It has been found that the aurora is a periodical phenomenon, and that
its period corresponds very closely with those of the magnetic needle
and Sun-spots. The years which have the most Sun-spots and magnetic
disturbances have also the most auroras. There is an almost perfect
similarity between the courses of the three sets of phenomena, from
which it is concluded that the aurora is connected in some mysterious
way with the action of the Sun, as well as with the magnetic condition
of the earth.
A very curious observation, which has been supposed to have some
connection with this subject, was made on Sept. 1st, 1859, by Mr.
Carrington and Mr. Hodgson, in England. While these observers, who were
situated many miles from one another, were both engaged at the same time
in observing the same Sun-spot, they suddenly saw two luminous spots of
dazzling brilliancy bursting into sight from the edge of the Sun-spot.
These objects moved eastward for about five minutes, after which they
disappeared, having then traveled nearly 34,000 miles. Simultaneously
with these appearances, a magnetic disturbance was registered at Kew by
the self-registering magnetic instruments. The very night that followed
these observations, great magnetic perturbations, accompanied by
brilliant auroral displays, were observed in Europe. A connection
between the terrestrial magnetism and the auroral phenomena is further
proved by the fact that, before the appearance of an aurora, the
magnetic intensity of our globe considerably increases, but diminishes
as soon as the first flashes show themselves.
The auroral phenomena are also connected in some way with electricity,
and generate serious disturbances in the electric currents traversing
our telegraphic lines, which are thus often rendered useless for the
transmission of messages during great auroral displays. It sometimes
happens, however, during such displays, that the telegraphic lines can
be operated for a long distance, without the assistance of a battery;
the aurora, or at least its cause, furnishing the necessary electric
current for the working of the line. During auroras, the telephonic
lines are also greatly affected, and all kinds of noises and
crepitations are heard in the instruments.
Two observations of mine, which may have a bearing on the subject,
present some interest, as they seem to indicate the action of the aurora
on some of the clouds of our atmosphere. On January 6th, 1872, after I
had been observing a brilliant aurora for over one hour, an isolated
black cumulus cloud appeared at a little distance from the western
extremity of the dark auroral segment. This cloud, probably driven by
the wind, rapidly advanced eastward, and was soon followed by a
succession of similar clouds, all starting from the same point. All
these black clouds apparently followed the same path, which was not a
straight line, but parallel to and concentric with the border of the
dark auroral segment. When the first cloud arrived in the vicinity of
the magnetic meridian passing through the middle of the auroral arc, it
very rapidly dissolved, and on reaching this meridian became invisible.
The same phenomenon was observed with the succession of black clouds
following, each rapidly dissolving as it approached the magnetic
meridian. This phenomenon of black clouds vanishing like phantoms in
crossing the magnetic meridian, was observed for nearly an hour. On June
17th, 1879, I observed a similar phenomenon during a fine auroral
display. About midway between the horizon and the polar star, but a
little to the west of the magnetic meridian, there was a large black
cumulo-stratus cloud which very slowly advanced eastward. As it
progressed in that direction, its eastern extremity was dissolved in
traversing the magnetic meridian; while, at the same time, several short
and quite bright auroral rays issued from its western extremity, which
in its turn dissolved rapidly, as if burned or melted away in the
production of the auroral flame.
It seems to be a well observed fact, that during auroras, a strong
sulphurous odor prevails in high northern latitudes. According to Dr.
Hjaltalin, during these phenomena, "the ozone of the atmosphere
increases considerably, and men and animals exposed out of doors emit a
sulphurous odor when entering a heated room." The Esquimaux and other
inhabitants of the northern regions assert that great auroras are
sometimes accompanied by crepitations and crackling noises of various
sorts. Although these assertions have been denied by several travelers
who have visited the regions of these phenomena, they are confirmed by
many competent observers. Dr. Hjaltalin, who has heard these noises
about six times in a hundred observations, says that they are especially
audible when the weather is clear and calm; but that when the atmosphere
is agitated they are not heard. He compares them to the peculiar sound
produced by a silk cloth when torn asunder, or to the crepitations of
the electric machine when its motion is accelerated. "When the auroral
light is much agitated and the streamers show great movements, it is
then that these noises are heard at different places in the atmosphere."
The spectrum of the auroral light, although it varies with almost every
aurora, always shows a bright green line on a faint continuous spectrum.
In addition to this green line I have frequently observed four broad
diffused bands of greater refrangibility in the spectra of some auroras.
In two cases, when the auroras appeared red towards the west, the
spectrum showed a bright red line, in addition to the green line and the
broad bands described. These facts evidently show that the light of the
aurora is due to the presence of luminous vapors in our atmosphere; and
it may reasonably be supposed that these vapors are rendered luminous by
the passage of electric discharges through them.
THE ZODIACAL LIGHT
PLATE V
In our northern latitudes may be seen, on every clear winter and spring
evening, a column of faint, whitish, nebulous light, rising obliquely
above the western horizon. A similar phenomenon may also be observed in
the east, before day-break, on any clear summer or autumn night. To this
pale, glimmering luminosity the name of "Zodiacal Light" has been given,
from the fact that it lies in the zodiac along the ecliptic.
In common with all the celestial bodies, the zodiacal light participates
in the diurnal motion of the sky, and rises and sets with the
constellations in which it appears. Aside from this apparent motion, it
is endowed with a motion of its own, accomplished from west to east, in
a period of a year. In its motion among the stars, the zodiacal light
always keeps pace with the Sun, and appears as if forming two faint
luminous wings, resting on opposite sides of this body. In reality it
extends on each side of the Sun, its axis lying very nearly in the plane
of the ecliptic.
In our latitudes the phenomena can be observed most advantageously
towards the equinoxes, in March and September, when twilight is of short
duration. As we proceed southward it becomes more prominent, and
gradually increases in size and brightness. It is within the tropical
regions that the zodiacal light acquires all its splendor: there it is
visible all the year round, and always appears very nearly perpendicular
to the horizon, while at the same time its proportions and brilliancy
are greatly increased.
[PLATE V.--THE ZODIACAL LIGHT.
Observed February 20, 1876]