sounding body is moving rapidly towards us, the pitch of its note
becomes somewhat higher than when the body is stationary; and, when such
a body is moving rapidly from us, the pitch of its note is lowered
somewhat. We have a good illustration of this change of pitch at a
country railway station on the passage of an express-train. The pitch of
the locomotive whistle is considerably higher when the train is
approaching the station than when it is leaving it.
167. _Explanation of the Change of Pitch produced by Motion._--The pitch
of sound depends upon the rapidity with which the pulsations of sound
beat upon the drum of the ear. The more rapidly the pulsations follow
each other, the higher is the pitch: hence the shorter the sound-waves
(provided the sound is all the while travelling at the same rate), the
higher the pitch of the sound. Any thing, then, which tends to shorten
the waves of sound tends also to raise its pitch, and any thing which
tends to lengthen these waves tends to lower its pitch.
When a sounding body is moving rapidly forward, the sound-waves are
crowded together a little, and therefore shortened; when it is moving
backward, the sound-waves are drawn out, or lengthened a little.
The effect of the motion of a sounding body upon the length of its
sonorous waves will be readily seen from the following illustration:
Suppose a number of persons stationed at equal intervals in a line
on a long platform capable of moving backward and forward. Suppose
the men are four feet apart, and all walking forward at the same
rate, and that the platform is stationary, and that, as the men
leave the platform, they keep on walking at the same rate: the men
will evidently be four feet apart in the line in front of the
platform, as well as on it. Suppose next, that the platform is
moving forward at the rate of one foot in the interval between two
men's leaving the platform, and that the men continue to walk as
before: it is evident that the men will then be three feet apart in
the line after they have left the platform. The forward motion of
the platform has the effect of crowding the men together a little.
Were the platform moving backward at the same rate, the men would be
five feet apart after they had left the platform. The backward
motion of the platform has the effect of separating the men from one
another.
The distance between the men in this illustration corresponds to the
length of the sound-wave, or the distance between its two ends. Were
a person to stand beside the line, and count the men that passed him
in the three cases given above, he would find that more persons
would pass him in the same time when the platform is moving forward
than when it is stationary, and fewer persons would pass him in the
same time when the platform is moving backward than when it is
stationary. In the same way, when a sounding body is moving rapidly
forward, the sound-waves beat more rapidly upon the ear of a person
who is standing still than when the body is at rest, and less
rapidly when the sounding body is moving rapidly backward.
Were the platform stationary, and were the person who is counting
the men to be walking along the line, either towards or away from
the platform, the effect upon the number of men passing him in a
given time would be precisely the same as it would be were the
person stationary, and the platform moving either towards or away
from him at the same rate. So the change in the rapidity with which
pulsations of sound beat upon the ear is precisely the same whether
the ear is stationary and the sounding body moving, or the sounding
body is stationary and the ear moving.
168. _Change of Refrangibility due to the Motion of a Luminous
Body._--Refrangibility in light corresponds to pitch in sound, and
depends upon the length of the luminous waves. The shorter the luminous
waves, the greater the refrangibility of the waves. Very rapid motion of
a luminous body has the same effect upon the length of the luminous
waves that motion of a sounding body has upon the length of the sonorous
waves. When a luminous body is moving very rapidly towards us, its
luminous waves are shortened a little, and its light becomes a little
more refrangible; when the luminous body is moving rapidly from us, its
luminous waves are lengthened a little, and its light becomes a little
less refrangible.
[Illustration: Fig. 185.]
169. _Displacement of Spectral Lines._--In examining the spectra of the
stars, we often find that certain of the dark lines are _displaced_
somewhat, either towards the red or the violet end of the spectrum. As
the dark lines are in the same position as the bright lines of the
absorbing vapor would be, a displacement of the lines towards the red
end of the spectrum indicates a lowering of the refrangibility of the
rays, due to a motion of the luminous vapor away from us; and a
displacement of the lines towards the violet end of the spectrum
indicates an increase of refrangibility, due to a motion of the luminous
vapor towards us. From the amount of the displacement of the lines, it
is possible to calculate the velocity at which the luminous gas is
moving. In Fig. 185 is shown the displacement of the _F_ line in the
spectrum of Sirius. This is one of the hydrogen lines. _RV_ is the
spectrum, _R_ being the red, and _V_ the violet end. The long vertical
line is the bright _F_ line of hydrogen, and the short dark line to the
left of it is the position of the _F_ line in the spectrum of Sirius. It
is seen that this line is displaced somewhat towards the red end of the
spectrum. This indicates that Sirius must be moving from us; and the
amount of the displacement indicates that the star must be moving at the
rate of some twenty-five or thirty miles a second.
[Illustration: Fig. 186.]
170. _Contortion of Lines on the Disk of the Sun._--Certain of the dark
lines seen on the centre of the sun's disk often appear more or less
distorted, as shown in Fig. 186, which represents the contortion of the
hydrogen line as seen at various times. 1 and 2 indicate a rapid motion
of hydrogen away from us, or a _down-rush_ at the sun; 3 and 4 (in which
the line at the centre is dark on one side, and bent towards the red end
of the spectrum, and bright on the other side with a distortion towards
the violet end of the spectrum) indicate a _down-rush_ of _cool_
hydrogen side by side with an _up-rush_ of _hot and bright_ hydrogen; 5
indicates local _down-rushes_ associated with _quiescent_ hydrogen.
The contorted lines, which indicate a violently agitated state of the
sun's atmosphere, appear in the midst of other lines which indicate a
quiescent state. This is owing to the fact that the absorption which
produces the dark lines takes place at various depths in the solar
atmosphere. There may be violent commotion in the lower layers of the
sun's atmosphere, and comparative quiet in the upper layers. In this
case, the lines which are due to absorption in the lower layers would
indicate this disturbance by their contortions; while the lines produced
by absorption in the upper layers would be free from contortion.
It often happens, too, that the contortions are confined to one set of
lines of an element, while other lines of the same element are entirely
free from contortions. This is undoubtedly due to the fact that
different layers of the solar atmosphere differ greatly in temperature;
so that the same element would give one set of lines at one depth, and
another set at another depth: hence commotion in the solar atmosphere at
any particular depth would be indicated by the contortion of those lines
of the element only which are produced by the temperature at that
particular depth.
A remarkable case of contortion witnessed by Professor Young is shown in
Fig. 187. Three successive appearances of the _C_ line are shown. The
second view was taken three minutes after the first, and the third five
minutes after the second. The contortion in this case indicated a
velocity ranging from two hundred to three hundred miles a second.
[Illustration: Fig. 187.]
171. _Contortion of Lines on the Sun's Limb._--When the spectroscope is
directed to the centre of the sun's disk, the distortion of the lines
indicates only vertical motion in the sun's atmosphere; but, when the
spectroscope is directed to the limb of the sun, displacements of the
lines indicate horizontal motions in the sun's atmosphere. When a
powerful spectroscope is directed to the margin of the sun's disk, so
that the slit of the collimator tube shall be perpendicular to the sun's
limb, one or more of the dark lines on the disk are seen to be prolonged
by a bright line, as shown in Fig. 188. But this prolongation, instead
of being straight and narrow, as shown in the figure, is often widened
and distorted in various ways, as shown in Fig. 189. In the left-hand
portion of the diagram, the line is deflected towards the red end of the
spectrum; this indicates a violent wind on the sun's surface blowing
away from us. In the right-hand portion of the diagram, the line is
deflected towards the violet end of the spectrum; this indicates a
violent wind blowing towards us. In the middle portion of the figure,
the line is seen to be bent both ways; this indicates a cyclone, on one
side of which the wind would be blowing from us, and on the other side
towards us.
[Illustration: Fig. 188.]
[Illustration: Fig. 189.]
The distortions of the solar lines indicate that the wind at the surface
of the sun often blows with a velocity of _from one hundred to three
hundred miles a second_. The most violent wind known on the earth has
velocity of a hundred miles an hour.
III. THE PHOTOSPHERE AND SUN SPOTS.
The Photosphere.
[Illustration: Fig. 190.]
172. _The Granulation of the Photosphere._--When the surface of the sun
is examined with a good telescope under favorable atmospheric
conditions, it is seen to be composed of minute grains of intense
brilliancy and of irregular form, floating in a darker medium, and
arranged in streaks and groups, as shown in Fig. 190. With a rather low
power, the general effect of the surface is much like that of rough
drawing-paper, or of curdled milk seen from a little distance. With a
high power and excellent atmospheric conditions, the _grains_ are seen
to be irregular, rounded masses, some hundreds of miles in diameter,
sprinkled upon a less brilliant background, and appearing somewhat like
snow-flakes sparsely scattered over a grayish cloth. Fig. 191 is a
representation of these grains according to Secchi.
[Illustration: Fig. 191.]
With a very powerful telescope and the very best atmospheric conditions,
the grains themselves are resolved into _granules_, or little luminous
dots, not more than a hundred miles or so in diameter, which, by their
aggregation, make up the grains, just as they, in their turn, make up
the coarser masses of the solar surface. Professor Langley estimates
that these granules constitute about one-fifth of the sun's surface,
while they emit at least three-fourths of its light.
173. _Shape of the Grains._--The grains differ considerably in shape at
different times and on different parts of the sun's surface. Nasmyth, in
1861, described them as _willow-leaves_ in shape, several thousand miles
in length, but narrow and with pointed ends. He figured the surface of
the sun as a sort of basket-work formed by the interweaving of such
filaments. To others they have appeared to have the form of
_rice-grains_. On portions of the sun's disk the elementary structure is
often composed of long, narrow, blunt-ended filaments, not so much like
willow-leaves as like bits of straw lying roughly parallel to each
other,--a _thatch-straw_ formation, as it has been called. This is
specially common in the immediate neighborhood of the spots.
174. _Nature of the Grains._--The grains are, undoubtedly, incandescent
_clouds_ floating in the sun's atmosphere, and composed of partially
condensed metallic vapors, just as the clouds of our atmosphere are
composed of partially condensed aqueous vapor. Rain on the sun is
composed of white-hot drops of molten iron and other metals; and these
drops are often driven with the wind with a velocity of over a hundred
miles a second.
As to the forms of the grains, Professor Young says, "If one were to
speculate as to the explanation of the grains and thatch-straws, it
might be that the grains are the upper ends of long filaments of
luminous cloud, which, over most of the sun's surface, stand
approximately vertical, but in the neighborhood of a spot are inclined
so as to lie nearly horizontal. This is not certain, though: it may be
that the cloud-masses over the more quiet portions of the solar surface
are really, as they seem, nearly globular, while near the spots they are
drawn out into filamentary forms by atmospheric currents."
175. _Faculæ._--The _faculæ_ are irregular streaks of greater brightness
than the general surface, looking much like the flecks of foam on the
surface of a stream below a waterfall. They are sometimes from five to
twenty thousand miles in length, covering areas immensely larger than a
terrestrial continent.
These faculæ are _elevated regions_ of the solar surface, ridges and
crests of luminous matter, which rise above the general level of the
sun's surface, and protrude through the denser portions of the solar
atmosphere. When one of these passes over the edge of the sun's disk, it
can be seen to project, like a little tooth. Any elevation on the sun to
be perceptible at all must measure at least half a second of an arc, or
two hundred and twenty-five miles.
The faculæ are most numerous in the neighborhood of the spots, and much
more conspicuous near the limb of the sun than near the centre of the
disk. Fig. 192 gives the general appearance of the faculæ, and the
darkening of the limb of the sun. Near the spots, the faculæ often
undergo very rapid change of form, while elsewhere on the disk they
change rather slowly, sometimes undergoing little apparent alteration
for several days.
[Illustration: Fig. 192.]
176. _Why the Faculæ are most Conspicuous near the Limb of the
Sun._--The reason why the faculæ are most conspicuous near the limb of
the sun is this: The luminous surface of the sun is covered with an
atmosphere, which, though not very thick compared with the diameter of
the sun, is still sufficient to absorb a good deal of light. Light
coming from the centre of the sun's disk penetrates this atmosphere
under the most favorable conditions, and is but slightly reduced in
amount. The edges of the disk, on the other hand, are seen through a
much greater thickness of atmosphere; and the light is reduced by
absorption some seventy-five per cent. Suppose, now, a facula were
sufficiently elevated to penetrate quite through this atmosphere. Its
light would be undimmed by absorption on any part of the sun's disk; but
at the centre of the disk it would be seen against a background nearly
as bright as itself, while at the margin it would be seen against one
only a quarter as bright. It is evident that the light of any facula,
owing to the elevation, would be reduced less rapidly as we approach the
edge of the disk than that of the general surface of the sun, which lies
at a lower level.
Sun-Spots.
177. _General Appearance of Sun-Spots._--The general appearance of a
well-formed sun-spot is shown in Fig. 193. The spot consists of a very
dark central portion of irregular shape, called the _umbra_, which is
surrounded by a less dark fringe, called the _penumbra_. The penumbra is
made up, for the most part, of filaments directed radially inward.
[Illustration: Fig. 193.]
There is great variety in the details of form in different sun-spots;
but they are generally nearly circular during the middle period of their
existence. During the period of their development and of their
disappearance they are much more irregular in form.
There is nothing like a gradual shading-off of the penumbra, either
towards the umbra on the one side, or towards the photosphere on the
other. The penumbra is separated from both the umbra and the photosphere
by a sharp line of demarcation. The umbra is much brighter on the inner
than on the outer edge, and frequently the photosphere is excessively
bright at the margin of the penumbra. The brightness of the inner
penumbra seems to be due to the crowding together of the penumbral
filaments where they overhang the edge of the umbra.
There is a general antithesis between the irregularities of the outer
and inner edges of the penumbra. Where an angle of the penumbral matter
crowds in upon the umbra, it is generally matched by a corresponding
outward extension into the photosphere, and _vice versa_.
The umbra of the spot is far from being uniformly dark. Many of the
penumbral filaments terminate in little detached grains of luminous
matter; and there are also fainter veils of a substance less brilliant,
but sometimes rose-colored, which seem to float above the umbra. The
umbra itself is made up of masses of clouds which are really intensely
brilliant, and which appear dark only by contrast with the intenser
brightness of the solar surface. Among these clouds are often seen one
or more minute circular spots much darker than the rest of the umbra.
These darker portions are called _nuclei_. They seem to be the mouths of
tubular orifices penetrating to unknown depths. The faint veils
mentioned above continually melt away, and are replaced by others in
some different position. The bright granules at the tips of the
penumbral filaments seem to sink and dissolve, while fresh portions
break off to replace them. There is a continual indraught of luminous
matter over the whole extent of the penumbra.
At times, though very rarely, patches of intense brightness suddenly
break out, remain visible for a few minutes, and move over the spot with
velocities as great as a hundred miles _a second_.
The spots change their form and size quite perceptibly from day to day,
and sometimes even from hour to hour.
178. _Duration of Sun-Spots._--The average life of a sun-spot is two or
three months: the longest on record is that of a spot observed in 1840
and 1841, which lasted eighteen months. There are cases, however, where
the disappearance of a spot is very soon followed by the appearance of
another at the same point; and sometimes this alternate disappearance
and re-appearance is several times repeated. While some spots are thus
long-lived, others endure only a day or two, and sometimes only a few
hours.
179. _Groups of Spots._--The spots usually appear not singly, but in
groups. A large spot is often followed by a train of smaller ones to the
east of it, many of which are apt to be irregular in form and very
imperfect in structure, sometimes with no umbra at all, often with a
penumbra only on one side. In such cases, when any considerable change
of form or structure shows itself in the principal spot, it seems to
rush westward over the solar surface, leaving its attendants trailing
behind. When a large spot divides into two or more, as often happens,
the parts usually seem to repel each other, and fly apart with great
velocity.
180. _Size of the Spots._--The spots are sometimes of enormous size.
Groups have often been observed covering areas of more than a hundred
thousand miles square, and single spots occasionally measure from forty
to fifty thousand miles in diameter, the umbra being twenty-five or
thirty thousand miles across. A spot, however, measuring thirty thousand
miles over all, may be considered a large one. Such a spot can easily be
seen without a telescope when the brightness of the sun's surface is
reduced by clouds or nearness to the horizon, or by the use of colored
glass. During the years 1871 and 1872 spots were visible to the naked
eye for a considerable portion of the time. The largest spot yet
recorded was observed in 1858. It had a breadth of more than a hundred
and forty-three thousand miles, or nearly eighteen times the diameter of
the earth, and covered about a thirty-sixth of the whole surface of the
sun.
[Illustration: Fig. 194.]
Fig. 194 represents a group of sun-spots observed by Professor Langley,
and drawn on the same scale as the small circle in the upper left-hand
corner, which represents the surface of half of our globe.
[Illustration: Fig. 195.]
181. _The Penumbral Filaments._--Not unfrequently the penumbral
filaments are curved spirally, indicating a cyclonic action, as shown in
Fig. 195. In such cases the whole spot usually turns slowly around,
sometimes completing an entire revolution in a few days. More
frequently, however, the spiral motion lasts but a short time; and
occasionally, after continuing for a while in one direction, the motion
is reversed. Very often in large spots we observe opposite spiral
movements in different portions of the umbra, as shown in Figs. 196 and
197.
[Illustration: Fig. 196.]
Neighboring spots show no tendency to rotate in the same direction. The
number of spots in which a decided cyclonic motion (like that shown in
Fig. 198) appears is comparatively small, not exceeding two or three per
cent of the whole.
[Illustration: Fig. 197.]
[Illustration: Fig. 198.]
[Illustration: Plate 2.]
Plate II. represents a typical sun-spot as delineated by Professor
Langley. At the left-hand and upper portions of this great spot the
filaments present the ordinary appearance, while at the lower edge, and
upon the great overhanging branch, they are arranged very differently.
The feathery brush below the branch, closely resembling a frost-crystal
on a window-pane, is as rare as it is curious, and has not been
satisfactorily explained.
[Illustration: Fig. 199.]
182. _Birth and Decay of Sun-Spots._--The formation of a spot is
sometimes gradual, requiring days or even weeks for its full
development; and sometimes a single day suffices. Generally, for some
time before its appearance, there is an evident disturbance of the solar
surface, indicated especially by the presence of many brilliant faculæ,
among which _pores_, or minute black dots, are scattered. These enlarge,
and between them appear grayish patches, in which the photospheric
structure is unusually evident, as if they were caused by a dark mass
lying below a thin veil of luminous filaments. This veil seems to grow
gradually thinner, and finally breaks open, giving us at last the
complete spot with its penumbra. Some of the pores coalesce with the
principal spot, some disappear, and others form the attendant train
before described (179). The spot when once formed usually assumes a
circular form, and remains without striking change until it disappears.
As its end approaches, the surrounding photosphere seems to crowd in,
and overwhelm the penumbra. Bridges of light (Fig. 199), often much
brighter than the average of the solar surface, push across the umbra;
the arrangement of the penumbra filaments becomes confused; and, as
Secchi expresses it, the luminous matter of the photosphere seems to
tumble pell-mell into the chasm, which disappears, and leaves a
disturbed surface marked with faculæ, which, in their turn, gradually
subside.
[Illustration: Fig. 200.]
183. _Motion of Sun-Spots._--The spots have a regular motion across the
disk of the sun from east to west, occupying about twelve days in the
transit. A spot generally appears first on or near the east limb, and,
after twelve or fourteen days, disappears at the west limb. At the end
of another fourteen days, or more, it re-appears at the east limb,
unless, in the mean time, it has vanished from sight entirely. This
motion of the spots is indicated by the arrow in Fig. 200. The interval
between two successive appearances of the same spot on the eastern edge
of the sun is about twenty-seven days.
[Illustration: Fig. 201.]