by the rotation of the sun on its axis. It is evident, from Fig. 201,
that the sun will need to make more than a complete rotation in order to
bring a spot again upon the same part of the disk as seen from the
earth. _S_ represents the sun, and _E_ the earth. The arrows indicate
the direction of the sun's rotation. When the earth is at _E_, a spot at
_a_ would be seen at the centre of the solar disk. While the sun is
turning on its axis, the earth moves in its orbit from _E_ to _E'_:
hence the sun must make a complete rotation, and turn from _a_ to _a'_
in addition, in order to bring the spot again to the centre of the disk.
To carry the spot entirely around, and then on to _a'_, requires about
twenty-seven days. From this _synodical period_ of the spot, as it might
be called, it has been calculated that the sun must rotate on its axis
in about twenty-five days.
[Illustration: Fig. 202.]
185. _The Inclination of the Sun's Axis._--The paths described by
sun-spots across the solar disk vary with the position of the earth in
its orbit, as shown in Fig. 202. We therefore conclude that the sun's
axis is not perpendicular to the plane of the earth's orbit. The sun
rotates on its axis from west to east, and the axis leans about seven
degrees from the perpendicular to the earth's orbit.
186. _The Proper Motion of the Spots._--When the period of the sun's
rotation is deduced from the motion of spots in different solar
latitudes, there is found to be considerable variation in the results
obtained. Thus spots near the equator indicate that the sun rotates in
about twenty-five days; while those in latitude 20° indicate a period
about eighteen hours longer; and those in latitude 30° a period of
twenty-seven days and a half. Strictly speaking, the sun, as a whole,
has no single period of rotation; but different portions of its surface
perform their revolutions in different times. The equatorial regions not
only move more rapidly in miles per hour than the rest of the solar
surface, but they _complete the entire rotation in shorter time_.
[Illustration: Fig. 203.]
There appears to be a peculiar surface-drift in the equatorial regions
of the sun, the cause of which is unknown, but which gives the spots a
_proper_ motion; that is, a motion of their own, independent of the
rotation of the sun.
[Illustration: Fig. 204.]
187. _Distribution of the Sun-Spots._--The sun-spots are not distributed
uniformly over the sun's surface, but occur mainly in two zones on each
side of the equator, and between the latitudes of 10° and 30°, as shown
in Fig. 203. On and near the equator itself they are comparatively rare.
There are still fewer beyond 35° of latitude, and only a single spot has
ever been recorded more than 45° from the solar equator.
Fig. 204 shows the distribution of the sun-spots observed by Carrington
during a period of eight years. The irregular line on the left-hand side
of the figure indicates by its height the comparative frequency with
which the spots occurred in different latitudes. In Fig. 205 the same
thing is indicated by different degrees of darkness in the shading of
the belts.
[Illustration: Fig. 205.]
188. _The Periodicity of the Spots._--Careful observations of the solar
spots indicate a period of about eleven years in the spot-producing
activity of the sun. During two or three years the spots increase in
number and in size; then they begin to diminish, and reach a minimum
five or six years after the maximum. Another period of about six years
brings the return of the maximum. The intervals are, however, somewhat
irregular.
[Illustration: Fig. 206.]
Fig. 206 gives a graphic representation of the periodicity of the
sun-spots. The height of the curve shows the frequency of the sun-spots
in the years given at the bottom of the figure. It appears, from an
examination of this sun-spot curve, that the average interval from a
minimum to the next following maximum is only about four years and a
half, while that from a maximum to the next following minimum is six
years and six-tenths. The disturbance which produces the sun-spots is
developed suddenly, but dies away gradually.
189. _Connection between Sun-Spots and Terrestrial Magnetism._--The
magnetic needle does not point steadily in the same direction, but is
subject to various disturbances, some of which are regular, and others
irregular.
(1) One of the most noticeable of the regular magnetic changes is the
so-called _diurnal oscillation_. During the early part of the day the
north pole of the needle moves toward the west in our latitude,
returning to its mean position about ten P.M., and remaining nearly
stationary during the night. The extent of this oscillation in the
United States is about fifteen minutes of arc in summer, and not quite
half as much in winter; but it differs very much in different localities
and at different times, and the average diurnal oscillation in any
locality increases and decreases pretty regularly during a period of
about eleven years. The maximum and minimum of this period of magnetic
disturbance are found to coincide with the maximum and minimum of the
sun-spot period. This is shown in Fig. 206, in which the dotted lines
indicate the variations in the intensity of the magnetic disturbance.
(2) Occasionally so-called _magnetic storms_ occur, during which the
compass-needle is sometimes violently disturbed, oscillating five
degrees, or even ten degrees, within an hour or two. These storms are
generally accompanied by an aurora, and an aurora is _always_
accompanied by magnetic disturbance. A careful comparison of aurora
observations with those of sun-spots shows an almost perfect parallelism
between the curves of auroral and sun-spot frequency.
(3) A number of observations render it very probable that every intense
disturbance of the solar surface is propagated to our terrestrial
magnetism with the speed of light.
[Illustration: Fig. 207.]
Fig. 207 shows certain of the solar lines as they were observed by
Professor Young on Aug. 3, 1872. The contortions of the _F_ line
indicated an intense disturbance in the atmosphere of the sun. There
were three especially notable paroxysms in this distortion, occurring at
a quarter of nine, half-past ten, and ten minutes of twelve, A.M.
[Illustration: Fig. 208.]
Fig. 208 shows the curve of magnetic disturbance as traced at Greenwich
on the same day. It will be seen from the curve that it was a day of
general magnetic disturbance. At the times of the three paroxysms, which
are given at the bottom of the figure, it will be observed that there is
a peculiar shivering of the magnetic curve.
190. _The Spots are Depressions in the Photosphere._--This fact was
first clearly brought out by Dr. Wilson of Glasgow, in 1769, from
observations upon the penumbra of a spot in November of that year. He
found, that when the spot appeared at the eastern limb, or edge of the
sun, just moving into sight, the penumbra was well marked on the side of
the spot nearest to the edge of the disk; while on the other edge of the
spot, towards the centre of the sun, there was no penumbra visible at
all, and the umbra itself was almost hidden, as if behind a bank. When
the spot had moved a day's journey toward the centre of the disk, the
whole of the umbra came into sight, and the penumbra on the inner edge
of the spot began to be visible as a narrow line. After the spot was
well advanced upon the disk, the penumbra was of the same width all
around the spot. When the spot approached the sun's western limb, the
same phenomena were repeated, but in the inverse order. The penumbra on
the _inner_ edge of the spot narrowed much faster than that on the
outer, disappeared entirely, and finally seemed to hide from sight much
of the umbra nearly a whole day before the spot passed from view around
the limb. This is precisely what would occur (as Fig. 209 clearly shows)
if the spot were a saucer-shaped depression in the solar surface, the
bottom of the saucer corresponding to the umbra, and the sloping sides
to the penumbra.
[Illustration: Fig. 209.]
[Illustration: Fig. 210.]
191. _Sun-Spot Spectrum._--When the image of a sun-spot is thrown upon
the slit of the spectroscope, the spectrum is seen to be crossed
longitudinally by a continuous dark band, showing an increased general
absorption in the region of the sun-spot. Many of the spectral lines are
greatly thickened, as shown in Fig. 210. This thickening of the lines
shows that the absorption is taking place at a greater depth. New lines
and shadings often appear, which indicate, that, in the cooler nucleus
of the spot, certain compound vapors exist, which are dissociated
elsewhere on the sun's surface. These lines and shadings are shown in
Fig. 211.
[Illustration: Fig. 211.]
It often happens that certain of the spectral lines are reversed in the
spectrum of the spot, a thin bright line appearing over the centre of a
thick dark one, as shown in Fig. 212. These reversals are due to very
bright vapors floating over the spot.
[Illustration: Fig. 212.]
At times, also, the spectrum of a spot indicates violent motion in the
overlying gases by distortion and displacement of the lines. This
phenomenon occurs oftener at points near the outer edge of the penumbra
than over the centre of the spot; but occasionally the whole
neighborhood is violently agitated. In such cases, lines in the spectrum
side by side are often affected in entirely different ways, one being
greatly displaced while its neighbor is not disturbed in the least,
showing that the vapors which produce the lines are at different levels
in the solar atmosphere, and moving independently of each other.
[Illustration: Fig. 213.]
192. _The Cause and Nature of Sun-Spots._--According to Professor Young,
the arrangement and relations of the photospheric clouds in the
neighborhood of a spot are such as are represented in Fig. 213. "Over
the sun's surface generally, these clouds probably have the form of
vertical columns, as at _aa_. Just outside the spot, the level of the
photosphere is the most part, overtopped by eruptions of hydrogen and
usually raised into faculæ, as at _bb_. These faculæ are, for metallic
vapors, as indicated by the shaded clouds.... While the great clouds of
hydrogen are found everywhere upon the sun, these spiky, vivid outbursts
of metallic vapors seldom occur except just in the neighborhood of a
spot, and then only during its season of rapid change. In the penumbra
of the spot the photospheric filaments become more or less nearly
horizontal, as at _pp_; in the umbra at _u_ it is quite uncertain what
the true state of affairs may be. We have conjecturally represented the
filaments there as vertical also, but depressed and carried down by a
descending current. Of course, the cavity is filled by the gases which
overlie the photosphere; and it is easy to see, that, looked at from
above, such a cavity and arrangement of the luminous filaments would
present the appearances actually observed."
Professor Young also suggests that the spots may be depressions in the
photosphere caused "by the _diminution of upward pressure_ from below,
in consequence of eruptions in the neighborhood; the spots thus being,
so to speak, _sinks_ in the photosphere. Undoubtedly the photosphere is
not a strictly continuous shell or crust; but it is _heavy_ as compared
with the uncondensed vapors in which it lies, just as a rain-cloud in
our terrestrial atmosphere is heavier than the air; and it is probably
continuous enough to have its upper level affected by any diminution of
pressure below. The gaseous mass below the photosphere supports its
weight and the weight of the products of condensation, which must always
be descending in an inconceivable rain and snow of molten and
crystallized material. To all intents and purposes, though nothing but a
layer of clouds, the photosphere thus forms a constricting shell, and
the gases beneath are imprisoned and compressed. Moreover, at a high
temperature the viscosity of gases is vastly increased, so that quite
probably the matter of the solar nucleus resembles pitch or tar in its
consistency more than what we usually think of as a gas. Consequently,
any sudden diminution of pressure would propagate itself slowly from the
point where it occurred. Putting these things together, it would seem,
that, whenever a free outlet is obtained through the photosphere at any
point, thus decreasing the inward pressure, the result would be the
sinking of a portion of the photosphere somewhere in the immediate
neighborhood, to restore the equilibrium; and, if the eruption were kept
up for any length of time, the depression in the photosphere would
continue till the eruption ceased. This depression, filled with the
overlying gases, would constitute a spot. Moreover, the line of fracture
(if we may call it so) at the edges of the sink would be a region of
weakness in the photosphere, so that we should expect a series of
eruptions all around the spot. For a time the disturbance, therefore,
would grow, and the spot would enlarge and deepen, until, in spite of
the viscosity of the internal gases, the equilibrium of pressure was
gradually restored beneath. So far as we know the spectroscopic and
visual phenomena, none of them contradict this hypothesis. There is
nothing in it, however, to account for the distribution of the spots in
solar latitudes, nor for their periodicity."
IV. THE CHROMOSPHERE AND PROMINENCES.
193. _The Sun's Outer Atmosphere._--What we see of the sun under
ordinary circumstances is but a fraction of his total bulk. While by far
the greater portion of the solar _mass_ is included within the
photosphere, the larger portion of his _volume_ lies without, and
constitutes a gaseous envelope whose diameter is at least double, and
its bulk therefore sevenfold, that of the central globe.
This outer envelope, though mainly gaseous, is not spherical, but has an
exceedingly irregular and variable outline. It seems to be made up, not
of regular strata of different density, like our atmosphere, but rather
of flames, beams, and streamers, as transient and unstable as those of
the aurora borealis. It is divided into two portions by a boundary as
definite, though not so regular, as that which separates them both from
the photosphere. The outer and far more extensive portion, which in
texture and rarity seems to resemble the tails of comets, is known as
the _coronal atmosphere_, since to it is chiefly due the _corona_, or
glory, which surrounds the darkened sun during an eclipse.
194. _The Chromosphere._--At the base of the coronal atmosphere, and in
contact with the photosphere, is what resembles a sheet of scarlet fire.
It appears as if countless jets of heated gas were issuing through vents
over the whole surface, clothing it with flame, which heaves and tosses
like the blaze of a conflagration. This is the _chromosphere_, or
color-sphere. It owes its vivid redness to the predominance of hydrogen
in the flames. The average depth of the chromosphere is not far from ten
or twelve seconds, or five thousand or six thousand miles.
195. _The Prominences._--Here and there masses of this hydrogen, mixed
with other substances, rise far above the general level into the coronal
regions, where they float like clouds, or are torn to pieces by
conflicting currents. These cloud-masses are known as solar
_prominences_, or _protuberances_.
196. _Magnitude and Distribution of the Prominences._--The prominences
differ greatly in magnitude. Of the 2,767 observed by Secchi, 1,964
attained an altitude of eighteen thousand miles; 751, or nearly a fourth
of the whole, reached a height of twenty-eight thousand miles; several
exceeded eighty-four thousand miles. In rare instances they reach
elevations as great as a hundred thousand miles. A few have been seen
which exceeded a hundred and fifty thousand miles; and Secchi has
recorded one of three hundred thousand miles.
[Illustration: Fig. 214.]
The irregular lines on the right-hand side of Fig. 214 show the
proportion of the prominences observed by Secchi, that were seen in
different parts of the sun's surface. The outer line shows the
distribution of the smaller prominences, and the inner dotted line that
of the larger prominences. By comparing these lines with those on the
opposite side of the circle, which show the distribution of the spots,
it will be seen, that, while the spots are confined mainly to two belts,
the prominences are seen in all latitudes.
197. _The Spectrum of the Chromosphere._--The spectrum of the
chromosphere is comparatively simple. There are eleven lines only which
are always present; and six of these are lines of hydrogen, and the
others, with a single exception, are of unknown elements. There are
sixteen other lines which make their appearance very frequently. Among
these latter are lines of sodium, magnesium, and iron.
Where some special disturbance is going on, the spectrum at the base of
the chromosphere is very complicated, consisting of hundreds of bright
lines. "The majority of the lines, however, are seen only occasionally,
for a few minutes at a time, when the gases and vapors, which generally
lie low (mainly in the interstices of the clouds which constitute the
photosphere), and below its upper surface, are elevated for the time
being by some eruptive action. For the most part, the lines which appear
only at such times are simply _reversals_ of the more prominent dark
lines of the ordinary solar spectrum. But the selection of the lines
seems most capricious: one is taken, and another left, though belonging
to the same element, of equal intensity, and close beside the first."
Some of the main lines of the chromosphere and prominences are shown in
Fig. 215.
[Illustration: Fig. 215.]
198. _Method of Studying the Chromosphere and Prominences._--Until
recently, the solar atmosphere could be seen only during a total eclipse
of the sun; but now the spectroscope enables us to study the
chromosphere and the prominences with nearly the same facility as the
spots and faculæ.
The protuberances are ordinarily invisible, for the same reason that the
stars cannot be seen in the daytime; they are hidden by the intense
light reflected from our own atmosphere. If we could only get rid of
this aerial illumination, without at the same time weakening the light
of the prominences, the latter would become visible. This the
spectroscope enables us to accomplish. Since the air-light is reflected
sunshine, it of course presents the same spectrum as sunlight,--a
continuous band of color crossed by dark lines. Now, this sort of
spectrum is weakened by increase of dispersive power (159), because the
light is spread out into a longer ribbon, and made to cover a greater
area. On the other hand, the spectrum of the prominences, being composed
of bright lines, undergoes no such diminution by increased dispersion.
[Illustration: Fig. 216.]
When the spectroscope is used as a means of examining the prominences,
the slit is more or less widened. The telescope is directed so that the
image of that portion of the solar limb which is to be examined shall be
tangent to the opened slit, as in Fig. 216, which represents the
slit-plate of the spectroscope of its actual size, with the image of the
sun in the proper position for observation.
[Illustration: Fig. 217.]
If, now, a prominence exists at this part of the solar limb, and if the
spectroscope itself is so adjusted that the _C_ line falls in the centre
of the field of view, then one will see something like Fig. 217. "The
red portion of the spectrum will stretch athwart the field of view like
a scarlet ribbon with a darkish band across it; and in that band will
appear the prominences, like scarlet clouds, so like our own terrestrial
clouds, indeed, in form and texture, that the resemblance is quite
startling. One might almost think he was looking out through a
partly-opened door upon a sunset sky, except that there is no variety or
contrast of color; all the cloudlets are of the same pure scarlet hue.
Along the edge of the opening is seen the chromosphere, more brilliant
than the clouds which rise from it or float above it, and, for the most
part, made up of minute tongues and filaments."
199. _Quiescent Prominences._--The prominences differ as widely in form
and structure as in magnitude. The two principal classes are the
_quiescent_, _cloud-formed_, or _hydrogenous_, and the _eruptive_, or
_metallic_.
[Illustration: Plate 3.]
The _quiescent_ prominences resemble almost exactly our terrestrial
clouds, and differ among themselves in the same manner. They are often
of enormous dimensions, especially in horizontal extent, and are
comparatively permanent, often undergoing little change for hours and
days. Near the poles they sometimes remain during a whole solar
revolution of twenty-seven days. Sometimes they appear to lie upon the
limb of the sun, like a bank of clouds in the terrestrial horizon,
probably because they are so far from the edge that only their upper
portions are in sight. When fully seen, they are usually connected to
the chromosphere by slender columns, generally smallest at the base, and
often apparently made up of separate filaments closely intertwined, and
expanding upward. Sometimes the whole under surface is fringed with
pendent filaments. Sometimes they float entirely free from the
chromosphere; and in most cases the larger clouds are attended by
detached cloudlets. Various forms of quiescent prominences are shown in
Plate III. Other forms are given in Figs. 218 and 219.
[Illustration: Fig. 218.]
Their spectrum is usually very simple, consisting of the four lines of
hydrogen and the orange _D_^3: hence the appellation _hydrogenous_.
Occasionally the sodium and magnesium lines also appear, even near the
tops of the clouds.
[Illustration: Fig. 219.]
200. _Eruptive Prominences._--The _eruptive_ prominences ordinarily
consist of brilliant spikes or jets, which change very rapidly in form
and brightness. As a rule, their altitude is not more than twenty
thousand or thirty thousand miles; but occasionally they rise far higher
than even the largest of the quiescent protuberances. Their spectrum is
very complicated, especially near their base, and often filled with
bright lines. The most conspicuous lines are those of sodium, magnesium,
barium, iron, and titanium: hence Secchi calls them _metallic_
prominences.
[Illustration: Fig. 220.]
They usually appear in the immediate vicinity of a spot, never very near
the solar poles. They change with such rapidity, that the motion can
almost be seen with the eye. Sometimes, in the course of fifteen or
twenty minutes, a mass of these flames, fifty thousand miles high, will
undergo a total transformation; and in some instances their complete
development or disappearance takes no longer time. Sometimes they
consist of pointed rays, diverging in all directions, as represented in
Fig. 220. "Sometimes they look like flames, sometimes like sheaves of
grain, sometimes like whirling water-spouts capped with a great cloud;
occasionally they present most exactly the appearance of jets of liquid
fire, rising and falling in graceful parabolas; frequently they carry on
their edges spirals like the volutes of an Ionic column; and continually
they detach filaments, which rise to a great elevation, gradually
expanding and growing fainter as they ascend, until the eye loses them."
[Illustration: Fig. 221.]
201. _Change of Form in Prominences._--Fig. 221 represents a prominence
as seen by Professor Young, Sept. 7, 1871. It was an immense quiescent
cloud, a hundred thousand miles long and fifty-four thousand miles high.
At _a_ there was a brilliant lump, somewhat in the form of a
thunder-head. On returning to the spectroscope less than half an hour
afterwards, he found that the cloud had been literally blown into shreds
by some inconceivable uprush from beneath. The prominence then presented
the form shown in Fig. 222. The _débris_ of the cloud had already
attained a height of a hundred thousand miles. While he was watching
them for the next ten minutes, they rose, with a motion almost
perceptible to the eye, till the uppermost reached an altitude of two
hundred thousand miles. As the filaments rose, they gradually faded away
like a dissolving cloud.
[Illustration: Fig. 222.]