THE SUN.
The vast Size of the Sun--Hotter than Melting Platinum--Is the Sun
the Source of Heat for the Earth?--The Sun is 92,900,000 miles
distant--How to realise the magnitude of this distance--Day and
Night--Luminous and Non-Luminous Bodies--Contrast between the Sun
and the Stars--The Sun a Star--Granulated Appearance of the
Sun--The Spots on the Sun--Changes in the Form of a Spot--The
Faculae--The Rotation of the Sun on its Axis--View of a Typical
Sun-Spot--Periodicity of the Sun-Spots--Connection between the
Sun-Spots and Terrestrial Magnetism--Principles of Spectrum
Analysis--Substances present in the Sun--Spectrum of a Spot--The
Prominences surrounding the Sun--Total Eclipse of the Sun--Size and
Movement of the Prominences--Their connection with the
Spots--Spectroscopic Measurement of Motion on the Sun--The Corona
surrounding the Sun--Constitution of the Sun.
In commencing our examination of the orbs which surround us, we
naturally begin with our peerless sun. His splendid brilliance gives him
the pre-eminence over all other celestial bodies.
The dimensions of our luminary are commensurate with his importance.
Astronomers have succeeded in the difficult task of ascertaining the
exact figures, but they are so gigantic that the results are hard to
realise. The diameter of the orb of day, or the length of the axis,
passing through the centre from one side to the other, is 866,000 miles.
Yet this bare statement of the dimensions of the great globe fails to
convey an adequate idea of its vastness. If a railway were laid round
the sun, and if we were to start in an express train moving sixty miles
an hour, we should have to travel for five years without intermission
night or day before we had accomplished the journey.
When the sun is compared with the earth the bulk of our luminary becomes
still more striking. Suppose his globe were cut up into one million
parts, each of these parts would appreciably exceed the bulk of our
earth. Fig. 10 exhibits a large circle and a very small one, marked S
and E respectively. These circles show the comparative sizes of the two
bodies. The mass of the sun does not, however, exceed that of the earth
in the same proportion. Were the sun placed in one pan of a mighty
weighing balance, and were 300,000 bodies as heavy as our earth placed
in the other, the luminary would turn the scale.
[Illustration: Fig. 10.--Comparative Size of the Earth and the Sun.]
The sun has a temperature far surpassing any that we artificially
produce, either in our chemical laboratories or our metallurgical
establishments. We can send a galvanic current through a piece of
platinum wire. The wire first becomes red hot, then white hot; then it
glows with a brilliance almost dazzling until it fuses and breaks. The
temperature of the melting platinum wire could hardly be surpassed in
the most elaborate furnaces, but it does not attain the temperature of
the sun.
It must, however, be admitted that there is an apparent discrepancy
between a fact of common experience and the statement that the sun
possesses the extremely high temperature that we have just tried to
illustrate. "If the sun were hot," it has been said, "then the nearer we
approach to him the hotter we should feel; yet this does not seem to be
the case. On the top of a high mountain we are nearer to the sun, and
yet everybody knows that it is much colder up there than in the valley
beneath. If the mountain be as high as Mont Blanc, then we are certainly
two or three miles nearer the glowing globe than we were at the
sea-level; yet, instead of additional warmth, we find eternal snow." A
simple illustration may help to lessen this difficulty. In a greenhouse
on a sunshiny day the temperature is much hotter than it is outside. The
glass will permit the hot sunbeams to enter, but it refuses to allow
them out again with equal freedom, and consequently the temperature
rises. The earth may, from this point of view, be likened to a
greenhouse, only, instead of the panes of glass, our globe is enveloped
by an enormous coating of air. On the earth's surface, we stand, as it
were, inside the greenhouse, and we benefit by the interposition of the
atmosphere; but when we climb very high mountains, we gradually pass
through some of the protecting medium, and then we suffer from the cold.
If the earth were deprived of its coat of air, it seems certain that
eternal frost would reign over whole continents as well as on the tops
of the mountains.
The actual distance of the sun from the earth is about 92,900,000 miles;
but by merely reciting the figures we do not receive a vivid impression
of the real magnitude. It would be necessary to count as quickly as
possible for three days and three nights before one million was
completed; yet this would have to be repeated nearly ninety-three times
before we had counted all the miles between the earth and the sun.
Every clear night we see a vast host of stars scattered over the sky.
Some are bright, some are faint, some are grouped into remarkable forms.
With regard to this multitude of brilliant points we have now to ask an
important question. Are they bodies which shine by their own light like
the sun, or do they only shine with borrowed light like the moon? The
answer is easily stated. Most of those bodies shine by their own light,
and they are properly called _stars_.
Suppose that the sun and the multitude of stars, properly so called, are
each and all self-luminous brilliant bodies, what is the great
distinction between the sun and the stars? There is, of course, a vast
and obvious difference between the unrivalled splendour of the sun and
the feeble twinkle of the stars. Yet this distinction does not
necessarily indicate that our luminary has an intrinsic splendour
superior to that of the stars. The fact is that we are nestled up
comparatively close to the sun for the benefit of his warmth and light,
while we are separated from even the nearest of the stars by a mighty
abyss. If the sun were gradually to retreat from the earth, his light
would decrease, so that when he had penetrated the depths of space to a
distance comparable with that by which we are separated from the stars,
his glory would have utterly departed. No longer would the sun seem to
be the majestic orb with which we are familiar. No longer would he be a
source of genial heat, or a luminary to dispel the darkness of night.
Our great sun would have shrunk to the insignificance of a star, not so
bright as many of those which we see every night.
Momentous indeed is the conclusion to which we are now led. That myriad
host of stars which studs our sky every night has been elevated into
vast importance. Each one of those stars is itself a mighty sun,
actually rivalling, and in many cases surpassing, the splendour of our
own luminary. We thus open up a majestic conception of the vast
dimensions of space, and of the dignity and splendour of the myriad
globes by which that space is tenanted.
There is another aspect of the picture not without its utility. We must
from henceforth remember that our sun is only a star, and not a
particularly important star. If the sun and the earth, and all which it
contains, were to vanish, the effect in the universe would merely be
that a tiny star had ceased its twinkling. Viewed simply as a star, the
sun must retire to a position of insignificance in the mighty fabric of
the universe. But it is not as a star that we have to deal with the sun.
To us his comparative proximity gives him an importance incalculably
transcending that of all the other stars. We imagined ourselves to be
withdrawn from the sun to obtain his true perspective in the universe;
let us now draw near, and give him that attention which his supreme
importance to us merits.
[Illustration: Fig. 11.--The Sun, photographed on September 22, 1870.]
To the unaided eye the sun appears to be a flat circle. If, however, it
be examined with the telescope, taking care of course to interpose a
piece of dark-coloured glass, or to employ some similar precaution to
screen the eye from injury, it will then be perceived that the sun is
not a flat surface, but a veritable glowing globe.
The first question which we must attempt to answer enquires whether the
glowing matter which forms the globe is a solid mass, or, if not solid,
which is it, liquid or gaseous? At the first glance we might think that
the sun cannot be fluid, and we might naturally imagine that it was a
solid ball of some white-hot substance. But this view is not correct;
for we can show that the sun is certainly not a solid body in so far at
least as its superficial parts are concerned.
A general view of the sun as shown by a telescope of moderate dimensions
may be seen in Fig. 11, which is taken from a photograph obtained by Mr.
Rutherford at New York on the 22nd of September, 1870. It is at once
seen that the surface of the luminary is by no means of uniform texture
or brightness. It may rather be described as granulated or mottled. This
appearance is due to the luminous clouds which float suspended in a
somewhat less luminous layer of gas. It is needless to say that these
solar clouds are very different from the clouds which we know so well in
our own atmosphere. Terrestrial clouds are, of course, formed from
minute drops of water, while the clouds at the surface of the sun are
composed of drops of one or more chemical elements at an exceedingly
high temperature.
The granulated appearance of the solar surface is beautifully shown in
the remarkable photographs on a large scale which M. Janssen, of Meudon,
has succeeded in obtaining during the last twenty years. We are enabled
to reproduce one of them in Fig. 12. It will be observed that the
interstices between the luminous dots are of a greyish tint, the general
effect (as remarked by Professor Young) being much like that of rough
drawing paper seen from a little distance. We often notice places over
the surface of such a plate where the definition seems to be
unsatisfactory. These are not, however, the blemishes that might at
first be supposed. They arise neither from casual imperfections of the
photographic plate nor from accidents during the development; they
plainly owe their origin to some veritable cause in the sun itself, nor
shall we find it hard to explain what that cause must be. As we shall
have occasion to mention further on, the velocities with which the
glowing gases on the sun are animated must be exceedingly great. Even in
the hundredth part of a second (which is about the duration of the
exposure of this plate) the movements of the solar clouds are
sufficiently great to produce the observed indistinctness.
[Illustration: Fig. 12.--Photograph of the Solar Surface. (_By
Janssen._)]
Irregularly dispersed over the solar surface small dark objects called
sun-spots are generally visible. These spots vary greatly both as to
size and as to number. Sun-spots were first noticed in the beginning of
the seventeenth century, shortly after the invention of the telescope.
Their general appearance is shown in Fig. 13, in which the dark central
nucleus appears in sharp contrast with the lighter margin or penumbra.
Fig. 16 shows a small spot developing out of one of the pores or
interstices between the granules.
[Illustration: Fig. 13.--An Ordinary Sun-spot.]
The earliest observers of these spots had remarked that they seem to
have a common motion across the sun. In Fig. 14 we give a copy of a
remarkable drawing by Father Scheiner, showing the motion of two spots
observed by him in March, 1627. The figure indicates the successive
positions assumed by the spots on the several days from the 2nd to the
16th March. Those marks which are merely given in outline represent the
assumed positions on the 11th and the 13th, on which days it happened
that the weather was cloudy, so that no observations could be made. It
is invariably found that these objects move in the same
direction--namely, from the eastern to the western limb[3] of the sun.
They complete the journey across the face of the sun in twelve or
thirteen days, after which they remain invisible for about the same
length of time until they reappear at the eastern limb. These early
observers were quick to discern the true import of their discovery. They
deduced from these simple observations the remarkable fact that the sun,
like the earth, performs a rotation on its axis, and in the same
direction. But there is the important difference between these rotations
that whereas the earth takes only twenty-four hours to turn once round,
the solar globe takes about twenty-six days to complete one of its much
more deliberate rotations.
[Illustration: PLATE III.
SPOTS AND FACULAE ON THE SUN.
(FROM A PHOTOGRAPH BY MR. WARREN DE LA RUE, 20TH SEPT., 1861.)]
If we examine sun-spots under favourable atmospheric conditions and
with a telescope of fairly large aperture, we perceive a great amount of
interesting detail which is full of information with regard to the
structure of the sun. The penumbra of a spot is often found to be made
up of filaments directed towards the middle of the spot, and generally
brighter at their inner ends, where they adjoin the nucleus. In a
regularly formed spot the outline of the penumbra is of the same general
form as that of the nucleus, but astronomers are frequently deeply
interested by witnessing vast spots of very irregular figure. In such
cases the bright surface-covering of the sun (the photosphere, as it is
called) often encroaches on the nucleus and forms a peninsula stretching
out into, or even bridging across, the gloomy interior. This is well
shown in Professor Langley's fine drawing (Plate II.) of a very
irregular spot which he observed on December 23-24, 1873.
The details of a spot vary continually; changes may often be noticed
even from day to day, sometimes from hour to hour. A similar remark may
be made with respect to the bright streaks or patches which are
frequently to be observed especially in the neighbourhood of spots.
These bright marks are known by the name of _faculae_ (little torches).
They are most distinctly seen near the margin of the sun, where the
light from its surface is not so bright as it is nearer to the centre of
the disc. The reduction of light at the margin is due to the greater
thickness of absorbing atmosphere round the sun, through which the light
emitted from the regions near the margin has to pass in starting on its
way towards us.
None of the markings on the solar disc constitute permanent features on
the sun. Some of these objects may no doubt last for weeks. It has,
indeed, occasionally happened that the same spot has marked the solar
globe for many months; but after an existence of greater or less
duration those on one part of the sun may disappear, while as frequently
fresh marks of the same kind become visible in other places. The
inference from these various facts is irresistible. They tell us that
the visible surface of the sun is not a solid mass, is not even a liquid
mass, but that the globe, so far as we can see it, consists of matter in
the gaseous, or vaporous, condition.
[Illustration: Fig. 14.--Scheiner's Observations on Sun-spots.]
It often happens that a large spot divides into two or more separate
portions, and these have been sometimes seen to fly apart with a
velocity in some cases not less than a thousand miles an hour. "At
times, though very rarely" (I quote here Professor Young,[4] to whom I
am frequently indebted), "a different phenomenon of the most surprising
and startling character appears in connection with these objects:
patches of intense brightness suddenly break out, remaining visible for
a few minutes, moving, while they last, with velocities as great as one
hundred miles _a second_."
[Illustration: Fig. 15.--Zones on the Sun's Surface in which Spots
appear.]
"One of these events has become classical. It occurred on the forenoon
(Greenwich time) of September 1st, 1859, and was independently witnessed
by two well-known and reliable observers--Mr. Carrington and Mr.
Hodgson--whose accounts of the matter may be found in the Monthly
Notices of the Royal Astronomical Society for November, 1859. Mr.
Carrington at the time was making his usual daily observations upon the
position, configuration, and size of the spots by means of an image of
the solar disc upon a screen--being then engaged upon that eight years'
series of observations which lie at the foundation of so much of our
present solar science. Mr. Hodgson, at a distance of many miles, was at
the same time sketching details of sun-spot structure by means of a
solar eye-piece and shade-glass. They simultaneously saw two luminous
objects, shaped something like two new moons, each about eight thousand
miles in length and two thousand wide, at a distance of some twelve
thousand miles from each other. These burst suddenly into sight at the
edge of a great sun-spot with a dazzling brightness at least five or six
times that of the neighbouring portions of the photosphere, and moved
eastward over the spot in parallel lines, growing smaller and fainter,
until in about five minutes they disappeared, after traversing a course
of nearly thirty-six thousand miles."
The sun-spots do not occur at all parts of the sun's surface
indifferently. They are mainly found in two zones (Fig. 15) on each side
of the solar equator between the latitudes of 10 deg. and 30 deg.. On the
equator the spots are rare except, curiously enough, near the time when
there are few spots elsewhere. In high latitudes they are never seen.
Closely connected with these peculiar principles of their distribution
is the remarkable fact that spots in different latitudes do not indicate
the same values for the period of rotation of the sun. By watching a
spot near the sun's equator Carrington found that it completed a
revolution in twenty-five days and two hours. At a latitude of 20 deg. the
period is about twenty-five days and eighteen hours, at 30 deg. it is no
less than twenty-six days and twelve hours, while the comparatively few
spots observed in the latitude of 45 deg. require twenty-seven and a half
days to complete their circuit.
As the sun, so far at least as its outer regions are concerned, is a
mass of gas and not a solid body, there would be nothing incredible in
the supposition that spots are occasionally endowed with movements of
their own like ships on the ocean. It seems, however, from the facts
before us that the different zones on the sun, corresponding to what we
call the torrid and temperate zones on the earth, persist in rotating
with velocities which gradually decrease from the equator towards the
poles. It seems probable that the interior parts of the sun do not
rotate as if the whole were a rigidly connected mass. The mass of the
sun, or at all events its greater part, is quite unlike a rigid body,
and the several portions are thus to some extent free for independent
motion. Though we cannot actually see how the interior parts of the sun
rotate, yet here the laws of dynamics enable us to infer that the
interior layers of the sun rotate more rapidly than the outer layers,
and thus some of the features of the spot movements can be accounted
for. But at present it must be confessed that there are great
difficulties in the way of accounting for the distribution of spots and
the law of rotation of the sun.
In the year 1826 Schwabe, a German astronomer, commenced to keep a
regular register of the number of spots visible on the sun. After
watching them for seventeen years he was able to announce that the
number of spots seemed to fluctuate from year to year, and that there
was a period of about ten years in their changes. Subsequent
observations have confirmed this discovery, and old books and
manuscripts have been thoroughly searched for information of early
date. Thus a more or less complete record of the state of the sun as
regards spots since the beginning of the seventeenth century has been
put together. This has enabled astronomers to fix the period of the
recurring maximum with greater accuracy.
The course of one of the sun-spot cycles may be described as follows:
For two or three years the spots are both larger and more numerous than
on the average; then they begin to diminish, until in about six or seven
years from the maximum they decline to a minimum; the number of the
spots then begins to increase, and in about four and a half years the
maximum is once more attained. The length of the cycle is, on an
average, about eleven years and five weeks, but both its length and the
intensity of the maxima vary somewhat. For instance, a great maximum
occurred in the summer of 1870, after which a very low minimum occurred
in 1879, followed by a feeble maximum at the end of 1883; next came an
average minimum about August, 1889, followed by the last observed
maximum in January, 1894. It is not unlikely that a second period of
about sixty or eighty years affects the regularity of the eleven-year
period. Systematic observations carried on through a great many years to
come will be required to settle this question, as the observations of
sun-spots previous to 1826 are far too incomplete to decide the issues
which arise.
A curious connection seems to exist between the periodicity of the spots
and their distribution over the surface of the sun. When a minimum is
about to pass away the spots generally begin to show themselves in
latitudes about 30 deg. north and south of the sun's equator; they then
gradually break out somewhat nearer to the equator, so that at the time
of maximum frequency most of them appear at latitudes not greater than
16 deg.. This distance from the sun's equator goes on decreasing till the
time of minimum. Indeed, the spots linger on very close to the equator
for a couple of years more, until the outbreak signalising the
commencement of another period has commenced in higher latitudes.
We have still to note an extraordinary feature which points to an
intimate connection between the phenomena of sun-spots and the purely
terrestrial phenomena of magnetism. It is of course well known that the
needle of a compass does not point exactly to the north, but diverges
from the meridian by an angle which is different in different places and
is not even constant at the same place. For instance, at Greenwich the
needle at present points in a direction 17 deg. West of North, but this
amount is subject to very slow and gradual changes, as well as to very
small daily oscillations. It was found about fifty years ago by Lamont
(a Bavarian astronomer, but a native of Scotland) that the extent of
this daily oscillation increases and decreases regularly in a period
which he gave as 10-1/3 years, but which was subsequently found to be
11-1/10 years, exactly the same as the period of the spots on the sun.
From a diligent study of the records of magnetic observations it has
been found that the time of sun-spot maximum always coincides almost
exactly with that of maximum daily oscillation of the compass needle,
while the minima agree similarly. This close relationship between the
periodicity of sun-spots and the daily movements of the magnetic needle
is not the sole proof we possess that there is a connection of some sort
between solar phenomena and terrestrial magnetism. A time of maximum
sun-spots is a time of great magnetic activity, and there have even been
special cases in which a peculiar outbreak on the sun has been
associated with remarkable magnetic phenomena on the earth. A very
interesting instance of this kind is recorded by Professor Young, who,
when observing at Sherman on the 3rd August, 1872, perceived a very
violent disturbance of the sun's surface. He was told the same day by a
member of his party, who was engaged in magnetic observations and who
was quite in ignorance of what Professor Young had seen, that he had
been obliged to desist from his magnetic work in consequence of the
violent motion of his magnet. It was afterwards found from the
photographic records at Greenwich and Stonyhurst that the magnetic
"storm" observed in America had simultaneously been felt in England. A
similar connection between sun-spots and the aurora borealis has also
been noticed, this fact being a natural consequence of the well-known
connection between the aurora and magnetic disturbances. On the other
hand, it must be confessed that many striking magnetic storms have
occurred without any corresponding solar disturbance,[5] but even those
who are inclined to be sceptical as to the connection between these two
classes of phenomena in particular cases can hardly doubt the remarkable
parallelism between the general rise and fall in the number of sun-spots
and the extent of the daily movements of the compass needle.
[Illustration: Fig. 16.--The Texture of the Sun and a small Spot.]
We have now described the principal solar phenomena with which the
telescope has made us acquainted. But there are many questions connected
with the nature of the sun which not even the most powerful telescope
would enable us to solve, but which the spectroscope has given us the
means of investigating.
What we receive from the sun is warmth and light. The intensely heated
mass of the sun radiates forth its beams in all directions with
boundless prodigality. Each beam we feel to be warm, and we see to be
brilliantly white, but a more subtle analysis than mere feeling or mere
vision is required. Each sunbeam bears marks of its origin. These marks
are not visible until a special process has been applied, but then the
sunbeam can be made to tell its story, and it will disclose to us much
of the nature of the constitution of the great luminary.
We regard the sun's light as colourless, just as we speak of water as
tasteless, but both of those expressions relate rather to our own
feelings than to anything really characteristic of water or of sunlight.
We regard the sunlight as colourless because it forms, as it were, the
background on which all other colours are depicted. The fact is, that
white is so far from being colourless that it contains every known hue
blended together in certain proportions. The sun's light is really
extremely composite; Nature herself tells us this if we will but give
her the slightest attention. Whence come the beautiful hues with which
we are all familiar? Look at the lovely tints of a garden; the red of
the rose is not in the rose itself. All the rose does is to grasp the
sunbeams which fall upon it, extract from these beams the red which they
contain, and radiate that red light to our eyes. Were there not red rays
conveyed with the other rays in the sunbeam, there could be no red rose
to be seen by sunlight.
The principle here involved has many other applications; a lady will
often say that a dress which looks very well in the daylight does not
answer in the evening. The reason is that the dress is intended to show
certain colours which exist in the sunlight; but these colours are not
contained to the same degree in gaslight, and consequently the dress has
a different hue. The fault is not in the dress, the fault lies in the
gas; and when the electric light is used it sends forth beams more
nearly resembling those from the sun, and the colours of the dress
appear with all their intended beauty.
The most glorious natural indication of the nature of the sunlight is
seen in the rainbow. Here the sunbeams are refracted and reflected from
tiny globes of water in the clouds; these convey to us the sunlight, and
in doing so decompose the white beams into the seven primary hues--red,
orange, yellow, green, blue, indigo, and violet.
[Illustration: PLATE A.
THE SUN.
_Royal Observatory, Greenwich, July 8, 1892._]
[Illustration: Fig. 17.--The Prism.]
The bow set in the cloud is typical of that great department of modern
science of which we shall now set forth the principles. The globes of
water decompose the solar beams; and we follow the course suggested by
the rainbow, and analyse the sunlight into its constituents. We are
enabled to do this with scientific accuracy when we employ that
remarkable key to Nature's secrets known as the spectroscope. The beams
of white sunlight consist of innumerable beams of every hue in intimate
association. Every shade of red, of yellow, of blue, and of green, can
be found in a sunbeam. The magician's wand, with which we strike the
sunbeam and sort the tangled skein into perfect order, is the simple
instrument known as the glass prism. We have represented this instrument
in its simplest form in the adjoining figure (Fig. 17). It is a piece of
pure and homogeneous glass in the shape of a wedge. When a ray of light
from the sun or from any source falls upon the prism, it passes through
the transparent glass and emerges on the other side; a remarkable change
is, however, impressed on the ray by the influence of the glass. It is
bent by refraction from the path it originally pursued, and is compelled
to follow a different path. If, however, the prism bent all rays of
light equally, then it would be of no service in the analysis of light;
but it fortunately happens that the prism acts with varying efficiency
on the rays of different hues. A red ray is not refracted so much as a
yellow ray; a yellow ray is not refracted so much as a blue one. It
consequently happens that when the composite beam of sunlight, in which
all the different rays are blended, passes through the prism, they
emerge in the manner shown in the annexed figure (Fig. 18). Here then we
have the source of the analysing power of the prism; it bends the
different hues unequally and consequently the beam of composite
sunlight, after passing through the prism, no longer shows mere white
light, but is expanded into a coloured band of light, with hues like the
rainbow, passing from deep red at one end through every intermediate
grade to the violet.
[Illustration: Fig. 18.--Dispersion of Light by the Prism.]
We have in the prism the means of decomposing the light from the sun, or
the light from any other source, into its component parts. The
examination of the quality of the light when analysed enables us to
learn something of the constitution of the body from which this light
has emanated. Indeed, in some simple cases the mere colour of a light
will be sufficient to indicate the source from which it has come. There
is, for instance, a splendid red light sometimes seen in displays of
fireworks, due to the metal strontium. The eye can identify the element
by the mere colour of the flame. There is also a characteristic yellow
light produced by the flame of common salt burned with spirits of wine.
Sodium is the important constituent of salt, so here we recognise
another substance merely by the colour it emits when burning. We may
also mention a third substance, magnesium, which burns with a brilliant
white light, eminently characteristic of the metal.
[Illustration: PLATE XIII.
SPECTRA OF THE SUN AND STARS.
I. SUN.
II. SIRIUS.
III. ALDEBARAN.
IV. BETELGEUZE.]
The three metals, strontium, sodium, and magnesium, may thus be
identified by the colours they produce when incandescent. In this simple
observation lies the germ of the modern method of research known as
spectrum analysis. We may now examine with the prism the colours of the
sun and the colours of the stars, and from this examination we can learn
something of the materials which enter into their composition. We are
not restricted to the use of merely a single prism, but we may arrange
that the light which it is desired to analyse shall pass through several
prisms in succession in order to increase the _dispersion_ or the
spreading out of the different colours. To enter the spectroscope the
light first passes through a narrow slit, and the rays are then rendered
parallel by passing through a lens; these parallel rays next pass
through one or more prisms, and are finally viewed through a small
telescope, or they may be intercepted by a photographic plate on which a
picture will then be made. If the beam of light passing through the slit
has radiated from an incandescent solid or liquid body, or from a gas
under high pressure, the coloured band or _spectrum_ is found to contain
all the colours indicated on Plate XIII., without any interruption
between the colours. This is known as a continuous spectrum. But if we
examine light from a gas under low pressure, as can be done by placing a
small quantity of the gas in a glass tube and making it glow by an
electric current, we find that it does not emit rays of all colours, but
only rays of certain distinct colours which are different for different
gases. The spectrum of a gas, therefore, consists of a number of
detached luminous lines.
When we study the sunlight through the prism, it is found that the
spectrum does not extend quite continuously from one end to the other,
but is shaded over by a multitude of dark lines, only a few of which are
shown in the adjoining plate. (Plate XIII.) These lines are a permanent
feature in the solar spectrum. They are as characteristic of the
sunlight as the prismatic colours themselves, and are full of interest
and information with regard to the sun. These lines are the characters
in which the history and the nature of the sun are written. Viewed
through an instrument of adequate power, dark lines are to be found
crossing the solar spectrum in hundreds and in thousands. They are of
every variety of strength and faintness; their distribution seems guided
by no simple law. At some parts of the spectrum there are but few lines;
in other regions they are crowded so closely together that it is
difficult to separate them. They are in some places exquisitely fine and
delicate, and they never fail to excite the admiration of every one who
looks at this interesting spectacle in a good instrument.
There can be no better method of expounding the rather difficult subject
of spectrum analysis than by actually following the steps of the
original discovery which first gave a clear demonstration of the
significance of the dark "Fraunhofer" lines. Let us concentrate our
attention specially upon that line of the solar spectrum marked D. This,
when seen in the spectroscope, is found to consist of two lines, very
delicately separated by a minute interval, one of these lines being
slightly thicker than the other. Suppose that while the attention is
concentrated on these lines the flame of an ordinary spirit-lamp
coloured by common salt be held in front of the instrument, so that the
ray of direct solar light passes through the flame before entering the
spectroscope. The observer sees at once the two lines known as D flash
out with a greatly increased blackness and vividness, while there is no
other perceptible effect on the spectrum. A few trials show that this
intensification of the D lines is due to the vapour of sodium arising
from the salt burning in the lamp through which the sunlight has passed.
It is quite impossible that this marvellous connection between sodium
and the D lines of the spectrum can be merely casual. Even if there were
only a single line concerned, it would be in the highest degree unlikely
that the coincidence should arise by accident; but when we find the
sodium affecting both of the two close lines which form D, our
conviction that there must be some profound connection between these
lines and sodium rises to absolute certainty. Suppose that the sunlight
be cut off, and that all other light is excluded save that emanating
from the glowing vapour of sodium in the spirit flame. We shall then
find, on looking through the spectroscope, that we no longer obtain all
the colours of the rainbow; the light from the sodium is concentrated
into two bright yellow lines, filling precisely the position which the
dark D lines occupied in the solar spectrum, and the darkness of which
the sodium flame seemed to intensify.
We must here endeavour to remove what may at first sight appear to be a
paradox. How is it, that though the sodium flame produces two _bright_
lines when viewed in the absence of other light, yet it actually appears
to intensify the two _dark_ lines in the sun's spectrum? The explanation
of this leads us at once to the cardinal doctrine of spectrum analysis.
The so-called dark lines in the solar spectrum are only dark _by
contrast_ with the brilliant illumination of the rest of the spectrum. A
good deal of solar light really lies in the dark lines, though not
enough to be seen when the eye is dazzled by the brilliancy around. When
the flame of the spirit-lamp charged with sodium intervenes, it sends
out a certain amount of light, which is entirely localised in these two
lines. So far it would seem that the influence of the sodium flame ought
to be manifested in diminishing the darkness of the lines and rendering
them less conspicuous. As a matter of fact, they are far more
conspicuous with the sodium flame than without it. This arises from the
fact that the sodium flame possesses the remarkable property of cutting
off the sunlight which was on its way to those particular lines; so
that, though the sodium contributes some light to the lines, yet it
intercepts a far greater quantity of the light that would otherwise have
illuminated those lines, and hence they became darker with the sodium
flame than without it.
We are thus conducted to a remarkable principle, which has led to the
interpretation of the dark lines in the spectrum of the sun. We find
that when the sodium vapour is heated, it gives out light of a very
particular type, which, viewed through the prism, is concentrated in two
lines. But the sodium vapour possesses also this property, that light
from the sun can pass through it without any perceptible absorption,
except of those particular rays which are of the same characters as the
two lines in question. In other words, we say that if the heated vapour
of a substance gives a spectrum of bright lines, corresponding to lights
of various kinds, this same vapour will act as an opaque screen to
lights of those special kinds, while remaining transparent to light of
every other description.
This principle is of such importance in the theory of spectrum analysis
that we add a further example. Let us take the element iron, which in a
very striking degree illustrates the law in question. In the solar
spectrum some hundreds of the dark lines are known to correspond with
the spectrum of iron. This correspondence is exhibited in a vivid manner
when, by a suitable contrivance, the light of an electric spark from
poles of iron is examined in the spectroscope side by side with the
solar spectrum. The iron lines in the sun are identical in position with
the lines in the spectrum of glowing iron vapour. But the spectrum of
iron, as here described, consists of bright lines; while those with
which it is compared in the sun are dark on a bright background. They
can be completely understood if we suppose the vapour arising from
intensely heated iron to be present in the atmosphere which surrounds
the luminous strata on the sun. This vapour would absorb or stop
precisely the same rays as it emits when incandescent, and hence we
learn the important fact that iron, no less than sodium, must, in one
form or another, be a constituent of the sun.
Such is, in brief outline, the celebrated discovery of modern times
which has given an interpretation to the dark lines of the solar
spectrum. The spectra of a large number of terrestrial substances have
been examined in comparison with the solar spectrum, and thus it has
been established that many of the elements known on the earth are
present in the sun. We may mention calcium, iron, hydrogen, sodium,
carbon, nickel, magnesium, cobalt, aluminium, chromium, strontium,
manganese, copper, zinc, cadmium, silver, tin, lead, potassium. Some of
the elements which are of the greatest importance on the earth would
appear to be missing from the sun. Sulphur, phosphorus, mercury, gold,
nitrogen may be mentioned among the elements which have hitherto given
no indication of their being solar constituents.
It is also possible that the lines of a substance in the sun's
atmosphere may be so very bright that the light of the continuous
spectrum, on which they are superposed, is not able to "reverse"
them--_i.e._ turn them into dark lines. We know, for instance, that the
bright lines of sodium vapour may be made so intensely bright that the
spectrum of an incandescent lime-cylinder placed behind the sodium
vapour does not reverse these lines. If, then, we make the sodium lines
fainter, they may be reduced to exactly the intensity prevailing in that
part of the spectrum of the lime-light, in which case the lines, of
course, could not be distinguished. The question as to what elements are
really missing from the sun must therefore, like many other questions
concerning our great luminary, at present be considered an open one. We
shall shortly see that an element previously unknown has actually been
discovered by means of a line representing it in the solar spectrum.
Let us now return to the sun-spots and see what the spectroscope can
teach us as to their nature. We attach a powerful spectroscope to the
eye-end of a telescope in order to get as much light as possible
concentrated on the slit; the latter has therefore to be placed exactly
at the focus of the object-glass. The instrument is then pointed to a
spot, so that its image falls on the slit, and the presence of the dark
central part called the _umbra_ reveals itself by a darkish stripe which
traverses the ordinary sun-spectrum from end to end. It is bordered on
both sides by the spectrum of the _penumbra_, which is much brighter
than that of the umbra, but fainter than that of the adjoining regions
of the sun.
From the fact that the spectrum is darkened we learn that there is
considerable general absorption of light in the umbra. This absorption
is not, however, such as would be caused by the presence of volumes of
minute solid or liquid particles like those which constitute smoke or
cloud. This is indicated by the fact, first discovered by Young in
1883, that the spectrum is not uniformly darkened as it would be if the
absorption were caused by floating particles. In the course of
examination of many large and quiescent spots, he perceived that the
middle green part of the spectrum was crossed by countless fine, dark
lines, generally touching each other, but here and there separated by
bright intervals. Each line is thicker in the middle (corresponding to
the centre of the spot) and tapers to a fine thread at each end; indeed,
most of these lines can be traced across the spectrum of the penumbra
and out on to that of the solar surface. The absorption would therefore
seem to be caused by gases at a much lower temperature than that of the
gases present outside the spot.
In the red and yellow parts of the spot-spectrum, which have been
specially studied for many years by Sir Norman Lockyer at the South
Kensington Observatory, interesting details are found which confirm this
conclusion. Many of the dark lines are not thicker and darker in the
spot than they are in the ordinary sun-spectrum, while others are very
much thickened in the spot-spectrum, such as the lines of iron, calcium,
and sodium. The sodium lines are sometimes both widened and doubly
reversed--that is, on the thick dark line a bright line is superposed.
The same peculiarity is not seldom seen in the notable calcium lines H
and K at the violet end of the spectrum. These facts indicate the
presence of great masses of the vapours of sodium and calcium over the
nucleus. The observations at South Kensington have also brought to light
another interesting peculiarity of the spot-spectra. At the time of
minimum frequency of spots the lines of iron and other terrestrial
elements are prominent among the most widened lines; at the maxima these
almost vanish, and the widening is found only amongst lines of unknown
origin.
The spectroscope has given us the means of studying other interesting
features on the sun, which are so faint that in the full blaze of
sunlight they cannot be readily observed with a mere telescope. We can,
however, see them easily enough when the brilliant body of the sun is
obscured during the rare occurrence of a total eclipse. The conditions
necessary for the occurrence of an eclipse will be more fully
considered in the next chapter. For the present it will be sufficient to
observe that by the movement of the moon it may so happen that the moon
completely hides the sun, and thus for certain parts of the earth
produces what we call a total eclipse. The few minutes during which a
total eclipse lasts are of much interest to the astronomer. Darkness
reigns over the landscape, and in that darkness rare and beautiful
sights are witnessed.
[Illustration: Fig. 19.--Prominences seen in Total Eclipse.]
We have in Fig. 19 a diagram of a total eclipse, showing some of the
remarkable objects known as prominences (_a_, _b_, _c_, _d_, _e_) which
project from behind the dark body of the moon. That they do not belong
to the moon, but are solar appendages of some sort, is easily
demonstrated. They first appear on the eastern limb at the commencement
of totality. Those first seen are gradually more or less covered by the
advancing moon, while others peep out behind the western limb of the
moon, until totality is over and the sunlight bursts out again, when
they all instantly vanish.
The first total eclipse which occurred after the spectroscope had been
placed in the hands of astronomers was in 1868. On the 18th August in
that year a total eclipse was visible in India. Several observers, armed
with spectroscopes, were on the look-out for the prominences, and were
able to announce that their spectrum consisted of detached bright lines,
thus demonstrating that these objects were masses of glowing gas. On the
following day the illustrious astronomer, Janssen, one of the observers
of the eclipse, succeeded in seeing the lines in full sunlight, as he
now knew exactly where to look for them. Many months before the eclipse
Sir Norman Lockyer had been preparing to search for the prominences, as
he expected them to yield a line spectrum which would be readily
visible, if only the sun's ordinary light could be sufficiently winnowed
away. He proposed to effect this by using a spectroscope of great
dispersion, which would spread out the continuous spectrum considerably
and make it fainter. The effect of the great dispersion on the isolated
bright lines he expected to see would be only to widen the intervals
between them without interfering with their brightness. The new
spectroscope, which he ordered to be constructed for this purpose, was
not completed until some weeks after the eclipse was over, though before
the news of Janssen's achievement reached Europe from India. When that
news did arrive Sir N. Lockyer had already found the spectrum of unseen
prominences at the sun's limb. The honour of the practical application
of a method of observing solar prominences without the help of an
eclipse must therefore be shared between the two astronomers.
When a spectroscope is pointed to the margin of the sun so that the slit
is radial, certain short luminous lines become visible which lie exactly
in the prolongation of the corresponding dark lines in the solar
spectrum. From due consideration of the circumstances it can be shown
that the gases which form the prominences are also present as a
comparatively shallow atmospheric layer all round the great luminary.
This layer is about five or six thousand miles deep, and is situated
immediately above the dense layer of luminous clouds which forms the
visible surface of the sun and which we call the photosphere. The
gaseous envelope from which the prominences spring has been called the
chromosphere on account of the coloured lines displayed in its spectrum.
Such lines are very numerous, but those pertaining to the single
substance, hydrogen, predominate so greatly that we may say the
chromosphere consists chiefly of this element. It is, however, to be
noted that calcium and one other element are also invariably present,
while iron, manganese and magnesium are often apparent. The remarkable
element, of which we have not yet mentioned the name, has had an
astonishing history.
During the eclipse of 1868 a fine yellow line was noticed among the
lines of the prominence spectrum, and it was not unnaturally at first
assumed that it must be the yellow sodium line. But when careful
observations were afterwards made without hurry in full sunshine, and
accurate measures were obtained, it was at once remarked that this line
was not identical with either of the components of the double sodium
line. The new line was, no doubt, quite close to the sodium lines, but
slightly towards the green part of the spectrum. It was also noticed
there was not generally any corresponding line to be seen among the dark
lines in the ordinary solar spectrum, though a fine dark one has now and
then been detected, especially near a sun-spot. Sir Norman Lockyer and
Sir Edward Frankland showed that this was not produced by any known
terrestrial element. It was, therefore, supposed to be caused by some
hitherto unknown body to which the name of _helium_, or the sun element,
was given. About a dozen less conspicuous lines were gradually
identified in the spectrum of the prominences and the chromosphere,
which appeared also to be caused by this same mysterious helium. These
same remarkable lines have in more recent years also been detected in
the spectra of various stars.
This gas so long known in the heavens was at last detected on earth. In
April, 1895, Professor Ramsay, who with Lord Rayleigh had discovered the
new element argon, detected the presence of the famous helium line in
the spectrum of the gas liberated by heating the rare mineral known as
cleveite, found in Norway. Thus this element, the existence of which had
first been detected on the sun, ninety-three million miles away, has at
last been proved to be a terrestrial element also.
When it was announced by Runge that the principal line in the spectrum
of the terrestrial helium had a faint and very close companion line on
the red-ward side, some doubt seemed at first to be cast on the identity
of the new terrestrial gas discovered by Ramsay with the helium of the
chromosphere. The helium line of the latter had never been noticed to be
double. Subsequently, however, several observers provided with very
powerful instruments found that the famous line in the chromosphere
really had a very faint companion line. Thus the identity between the
celestial helium and the gas found on our globe was established in the
most remarkable manner. Certain circumstances have seemed to indicate
that the new gas might possibly be a mixture of two gases of different
densities, but up to the present this has not been proved to be the
case.
After it had been found possible to see the spectra of prominences
without waiting for an eclipse, Sir W. Huggins, in an observation on the
13th of February, 1869, successfully applied a method for viewing the
remarkable solar objects themselves instead of their mere spectra in
full sunshine. It is only necessary to adjust the spectroscope so that
one of the brightest lines--_e.g._ the red hydrogen line--is in the
middle of the field of the viewing telescope, and then to open wide the
slit of the spectroscope. A red image of the prominence will then be
displayed instead of the mere line. In fact, when the slit is opened
wide, the prisms produce a series of detached images of the prominence
under observation, one for each kind of light which the object emits.
We have spoken of the spectroscope as depending upon the action of glass
prisms. It remains to be added that in the highest class of
spectroscopes the prisms are replaced by ruled gratings from which the
light is reflected. The effect of the ruling is to produce by what is
known as diffraction the required breaking up of the beam of light into
its constituent parts.
[Illustration: PLATE IV.
SOLAR PROMINENCES.
(DRAWN BY TROUVELOT AT HARVARD COLLEGE, CAMBRIDGE, U.S., IN 1872.)]
Majestic indeed are the proportions of some of those mighty prominences
which leap from the luminous surface; yet they flicker, as do our
terrestrial flames, when we allow them time comparable to their gigantic
dimensions. Drawings of the same prominence made at intervals of a few
hours, or even less, often show great changes. The magnitude of the
displacements that have been noticed sometimes attains many thousands of
miles, and the actual velocity with which such masses move frequently
exceeds 100 miles a second. Still more violent are the convulsions when,
from the surface of the chromosphere, as from a mighty furnace, vast
incandescent masses of gas are projected upwards. Plate IV. gives a view
of a number of prominences as seen by Trouvelot at Harvard College
Observatory, Cambridge, U.S.A. Trouvelot has succeeded in exhibiting in
the different pictures the wondrous variety of aspect which these
objects assume. The dimensions of the prominences may be inferred from
the scale appended to the plate. The largest of those here shown is
fully 80,000 miles high; and trustworthy observers have recorded
prominences of an altitude even much greater. The rapid changes which
these objects sometimes undergo are well illustrated in the two sketches
on the left of the lowest line, which were drawn on April 27th, 1872.
These are both drawings of the same prominence taken at an interval no
greater than twenty minutes. This mighty flame is so vast that its
length is ten times as great as the diameter of the earth, yet in this
brief period it has completely changed its aspect; the upper part of the
flame has, indeed, broken away, and is now shown in that part of the
drawing between the two figures on the line above. The same plate also
shows various instances of the remarkable spike-like objects, taken,
however, at different times and at various parts of the sun. These
spikes attain altitudes not generally greater than 20,000 miles, though
sometimes they soar aloft to stupendous distances.
We may refer to one special object of this kind, the remarkable history
of which has been chronicled by Professor Young. On October 7th, 1880, a
prominence was seen, at about 10.30 a.m., on the south-east limb of the
sun. It was then about 40,000 miles high, and attracted no special
attention. Half an hour later a marvellous transformation had taken
place. During that brief interval the prominence became very brilliant
and doubled its length. For another hour the mighty flame still soared
upwards, until it attained the unprecedented elevation of 350,000
miles--a distance more than one-third the diameter of the great luminary
itself. At this climax the energy of the mighty outbreak seems to have
at last become exhausted: the flame broke up into fragments, and by
12.30--an interval of only two hours from the time when it was first
noticed--the phenomenon had completely faded away.
No doubt this particular eruption was exceptional in its vehemence, and
in the vastness of the changes of which it was an indication. The
velocity of upheaval must have been at least 200,000 miles an hour, or,
to put it in another form, more than fifty miles a second. This mighty
flame leaped from the sun with a velocity more than 100 times as great
as that of the swiftest bullet ever fired from a rifle.
The prominences may be generally divided into two classes. We have first
those which are comparatively quiescent, and in form somewhat resemble
the clouds which float in our earth's atmosphere. The second class of
prominences are best described as eruptive. They are, in fact, thrown up
from the chromosphere like gigantic jets of incandescent material. These
two classes of objects differ not only in appearance but also in the
gases of which they are composed. The cloud-like prominences consist
mainly of hydrogen, with helium and calcium, while many metals are
present in the eruptive discharges. The latter are never seen in the
neighbourhood of the sun's poles, but generally appear close to a
sun-spot, thus confirming the conclusion that the spots are associated
with violent disturbances on the surface of the sun. When a spot has
reached the limb of the sun it is frequently found to be surrounded by
prominences. It has even been possible in a few instances to detect
powerful gaseous eruptions in the neighbourhood of a spot, the
spectroscope rendering them visible against the background of the solar
surface just as the prominences are observed at the limb against the
background of the sky.
In order to photograph a prominence we have, of course, to substitute a
photographic plate for the observer's eye. Owing, however, to the
difficulty of preventing the feeble light from the prominence from being
overpowered by extraneous light, the photography of these bodies was not
very successful until Professor Hale, of Chicago, designed his
spectro-heliograph. In this instrument there is (in addition to the
usual slit through which the light falls on the prisms, or grating,) a
second slit immediately in front of the photographic plate through which
the light of a given wave-length can be permitted to pass to the
exclusion of all the rest. The light chosen for producing an image of
the prominences is that radiated in the remarkable "K line," due to
calcium. This lies at the extreme end of the violet. The light from that
part of the spectrum, though it is invisible to the eye, is much more
active photographically than the light from the red, yellow, or green
parts of the spectrum. The front slit is adjusted so that the K line
falls upon the second slit, and as the front slit is slowly swept by
clockwork over the whole of a prominence, the second slit keeps pace
with it by a mechanical contrivance.
If the image of the solar disc is hidden by a screen of exactly the
proper size, the slits may be made to sweep over the whole sun, thus
giving us at one exposure a picture of the chromospheric ring round the
sun's limb with its prominences. The screen may now be withdrawn, and
the slits may be made to sweep rapidly over the disc itself. They reveal
the existence of glowing calcium vapours in many parts of the surface of
the sun. Thus we get a striking picture of the sun as drawn by this
particular light. In this manner Professor Hale confirmed the
observation made long before by Professor Young, that the spectra of
faculae always show the two great calcium bands.
The velocity with which a prominence shoots upward from the sun's limb
can, of course, be measured directly by observations of the ordinary
kind with a micrometer. The spectroscope, however, enables us to
estimate the speed with which disturbances at the surface of the sun
travel in the direction towards the earth or from the earth. We can
measure this speed by watching the peculiar behaviour of the spectral
lines representing the rapidly moving masses. This opens up a remarkable
line of investigation with important applications in many branches of
astronomy.
It is, of course, now generally understood that the sensation of light
is caused by waves or undulations which impinge on the retina of the
eye after having been transmitted through that medium which we call the
ether. To the different colours correspond different wave-lengths--that
is to say, different distances between two successive waves. A beam of
white light is formed by the union of innumerable different waves whose
lengths have almost every possible value lying between certain limits.
The wave-length of red light is such that there are 33,000 waves in an
inch, while that of violet light is but little more than half that of
red light. The position of a line in the spectrum depends solely on the
wave-length of the light to which it is due. Suppose that the source of
light is approaching directly towards the observer; obviously the waves
follow each other more closely than if the source were at rest, and the
number of undulations which his eye receives in a second must be
proportionately increased. Thus the distance between two successive
ether waves will be very slightly diminished. A well-known phenomenon of
a similar character is the change of pitch of the whistle of a
locomotive engine as it rushes past. This is particularly noticeable if
the observer happens to be in a train which is moving rapidly in the
opposite direction. In the case of sound, of course, the vibrations or
waves take place in the air and not in the ether. But the effect of
motion to or from the observer is strictly analogous in the two cases.
As, however, light travels 186,000 miles a second, the source of light
will also have to travel with a very high velocity in order to produce
even the smallest perceptible change in the position of a spectral line.
We have already seen that enormously high velocities are by no means
uncommon in some of these mighty disturbances on the sun; accordingly,
when we examine the spectrum of a sun-spot, we often see that some of
the lines are shifted a little towards one end of the spectrum and
sometimes towards the other, while in other cases the lines are seen to
be distorted or twisted in the most fantastic manner, indicating very
violent local commotions. If the spot happens to be near the centre of
the sun's disc, the gases must be shooting upwards or downwards to
produce these changes in the lines. The velocities indicated in
observations of this class sometimes amount to as much as two or even
three hundred miles per second. We find it difficult to conceive the
enormous internal pressures which are required to impel such mighty
masses of gases aloft from the photosphere with speeds so terrific, or
the conditions which bring about the downrush of such gigantic masses of
vapour from above. In the spectra of the prominences on the sun's limb
also we often see the bright lines bent or shifted to one side. In such
cases what we witness is evidently caused by movements along the surface
of the chromosphere, conveying materials towards us or away from us.
An interesting application of this beautiful method of measuring the
speed of moving bodies has been made in various attempts to determine
the period of rotation of the sun spectroscopically. As the sun turns
round on its axis, a point on the eastern limb is moving towards the
observer and a point on the western limb is moving away from him. In
each case the velocity is a little over a mile per second. At the
eastern limb the lines in the solar spectrum are very slightly shifted
towards the violet end of the spectrum, while the lines in the spectrum
of the western limb are equally shifted towards the red end. By an
ingenious optical contrivance it is possible to place the spectra from
the two limbs side by side, which doubles the apparent displacement, and
thus makes it much more easy to measure. Even with this contrivance the
visual quantities to be measured remain exceedingly minute. All the
parts of the instrument have to be most accurately adjusted, and the
observations are correspondingly delicate. They have been attempted by
various observers. Among the most successful investigations of this kind
we may mention that of the Swedish astronomer, Duner, who, by pointing
his instrument to a number of places on the limb, found values in good
agreement with the peculiar law of rotation which has been deduced from
the motion of sun-spots. This result is specially interesting, as it
shows that the atmospheric layers, in which that absorption takes place
which produces the dark lines in the spectrum, shares in the motion of
the photosphere at the same latitude.
[Illustration: Fig. 20.--View of the Corona (and a Comet) in a Total
Eclipse.]
[Illustration: PLATE V.
TOTAL SOLAR ECLIPSE, JULY 29TH, 1878.
THE CORONA FROM THE PHOTOGRAPHS.
(HARKNESS.)]
We have yet to mention one other striking phenomenon which is among the
chief attractions to observers of total eclipses, and which it has
hitherto not been found possible to see in full daylight. This is the
corona or aureole of light which is suddenly seen to surround the sun in
an eclipse when the moon has completely covered the last remaining
crescent of the sun. A general idea of the appearance of the corona is
given in Fig. 20, and we further present in Plate V. the drawing of the
corona made by Professor Harkness from a comparison of a large number of
photographs obtained at different places in the United States during the
total eclipse of July 29th, 1878. In Fig. 21 we are permitted by the
kindness of Mr. and Mrs. Maunder to reproduce the remarkable photograph
of the corona which they obtained in India during the eclipse of January
22nd, 1898.
[Illustration: Fig. 21.--View of Corona during the Eclipse of Jan. 22nd,
1898 (_Reproduced by kind permission of Mr. and Mrs. Maunder and of the
proprietors of "Knowledge._")]
The part of the corona nearest the sun is very bright, though not so
brilliant as the prominences, which (as Professor Young says) blaze
through it like carbuncles. This inner portion is generally of fairly
regular outline, forming a white ring about a tenth part of the solar
diameter in width. The outer parts of the corona are usually very
irregular and very extensive. They are often interrupted by narrow
"rifts," or narrow dark bands, which reach from the limb of the sun
through the entire corona. On the other hand, there are also sometimes
narrow bright streamers, inclined at various angles to the limb of the
sun and not seldom curved. In the eclipses of 1867, 1878, and 1889, all
of which occurred at periods of sun-spot minimum, the corona showed long
and faint streamers nearly in the direction of the sun's equator, and
short but distinct brushes of light near the poles. In the eclipses of
1870, 1882, and 1893, near sun-spot maxima, the corona was more
regularly circular, and chiefly developed over the spot zones. We have
here another proof (if one were necessary) of the intimate connection
between the periodicity of the spots and the development of all other
solar phenomena.
In the spectrum of the corona there is a mysterious line in the green,
as to the origin of which nothing is at present certainly known. It is
best seen during eclipses occurring near the time of sun-spot maximum.
It is presented in the ordinary solar spectrum as a very thin, dark
line, which generally remains undisturbed even when lines of hydrogen
and other substances are twisted and distorted by the violent rush of
disturbed elements. The line is always present among the bright lines of
the chromosphere spectrum. In addition to it the corona shows a few
other bright lines, belonging, no doubt, to the same unknown element
("coronium"), and also a faint continuous spectrum, in which even a few
of the more prominent dark lines of the solar spectrum have been
sometimes detected. This shows that in addition to glowing gas
(represented by the bright lines) the corona also contains a great deal
of matter like dust, or fog, the minute particles of which are capable
of reflecting the sunlight and thereby producing a feeble continuous
spectrum. This matter seems to form the principal constituent of the
long coronal rays and streamers, as the latter are not visible in the
detached images of the corona which appear instead of the bright lines
when the corona is viewed, or photographed, during an eclipse, in a
spectroscope without a slit. If the long rays were composed of the gas
or gases which constitute the inner corona, it is evident that they
ought to appear in these detached images. As to the nature of the forces
which are continually engaged in shooting out these enormously long
streamers, we have at present but little information. It is, however,
certain that the extensive atmospheric envelope round the sun, which
shows itself as the inner corona, must be extremely attenuated. Comets
have on several occasions been known to rush through this coronal
atmosphere without evincing the slightest appreciable diminution in
their speed from the resistance to which they were exposed.
We have accumulated by observation a great number of facts concerning
the sun, but when we try to draw from these facts conclusions as to the
physical constitution of that great body, it cannot be denied that the
difficulties seem to be very great indeed. We find that the best
authorities differ considerably in the opinions they entertain as to its
nature. We shall here set forth the principal conclusions as to which
there is little or no controversy.
We shall see in a following chapter that astronomers have been able to
determine the relative densities of the bodies in the solar system; in
other words, they have found the relation between the quantities of
matter contained in an equally large volume of each. It has thus been
ascertained that the average density of the sun is about a quarter that
of the earth. If we compare the weight of the sun with that of an
equally great globe of water, we find that the luminary would be barely
one and a half times as heavy as the water. Of course, the actual mass
of the sun is very enormous; it is no less than 330,000 times as great
as that of the earth. The solar material itself is, however, relatively
light, so that the sun is four times as big as it would have to be if,
while its weight remained the same, its density equalled that of the
earth. Bearing in mind this lightness of the sun, and also the
exceedingly high temperature which we know to prevail there, no other
conclusion seems possible than that the body of the sun must be in a
gaseous state. The conditions under which such gases exist in the sun
are, no doubt, altogether different from those with which we are
acquainted on the earth. At the surface of the sun the force of gravity
is more than twenty-seven times as great as it is on the earth. A person
who on the earth could just lift twenty-seven equal pieces of metal
would, if he were transferred to the sun, only be able to lift one of
the pieces at a time. The pressure of the gases below the surface must
therefore be very great, and it might be supposed that they would become
liquefied in consequence. It was, however, discovered by Andrews that so
long as a gas is kept at a temperature higher than a certain point,
known as the "critical temperature" (which is different for different
gases), the gas will not be turned into a liquid however great be the
pressure to which it is submitted. The temperature on the sun cannot be
lower than the critical temperatures of the gases there existing; so it
would seem that even the enormous pressure can hardly reduce the gases
in the great luminary to the liquid form.
Of the interior of the sun we can, of course, expect to learn little or
nothing. What we observe is the surface-layer, the so-called
photosphere, in which the cold of space produces the condensation of the
gases into those luminous clouds which we see in our drawings and
photographs as "rice grains" or "willow leaves." It has been suggested
by Dr. Johnstone Stoney (and afterwards by Professor Hastings, of
Baltimore) that these luminous clouds are mainly composed of carbon with
those of the related elements silicon and boron, the boiling points of
which are much higher than those of other elements which might be
considered likely to form the photospheric clouds. The low atomic weight
of carbon must also have the effect of giving the molecules of this
element a very high velocity, and thereby enabling them to work their
way into the upper regions, where the temperature has so fallen that the
vapour becomes chilled into cloud. A necessary consequence of the rapid
cooling of these clouds, and the consequent radiation of heat on a
large scale, would be the formation of what we may perhaps describe as
smoke, which settles by degrees through the intervals between the clouds
(making these intervals appear darker) until it is again volatilised on
reaching a level of greater heat below the clouds. This same smoke is
probably the cause of the well-known fact that the solar limb is
considerably fainter than the middle of the disc. This seems to arise
from the greater absorption caused by the longer distance which a ray of
light from a point near the limb has to travel through this layer of
smoke before reaching the earth. It is shown that this absorption cannot
be attributed to a gaseous atmosphere, since this would have the effect
of producing more dark absorption lines in the spectrum. There would
thus be a marked difference between the solar spectrum from a part near
the middle of the disc and the spectrum from a part near the limb. This,
however, we do not find to be the case.
With regard to the nature of sun-spots, the idea first suggested by
Secchi and Lockyer, that they represent down rushes of cooler vapours
into the photosphere (or to its surface), seems on the whole to accord
best with the observed phenomena. We have already mentioned that the
spots are generally accompanied by faculae and eruptive prominences in
their immediate neighbourhood, but whether these eruptions are caused by
the downfall of the vapour which makes the photospheric matter "splash
up" in the vicinity, or whether the eruptions come first, and by
diminishing the upward pressure from below form a "sink," into which
overlying cooler vapour descends, are problems as to which opinions are
still much divided.
A remarkable appendage to the sun, which extends to a distance very much
greater than that of the corona, produces the phenomenon of the zodiacal
light. A pearly glow is sometimes seen in the spring to spread over a
part of the sky in the vicinity of the point where the sun has
disappeared after sunset. The same spectacle may also be witnessed
before sunrise in the autumn, and it would seem as if the material
producing the zodiacal light, whatever it may be, had a lens-shaped
form with the sun in the centre. The nature of this object is still a
matter of uncertainty, but it is probably composed of a kind of dust, as
the faint spectrum it affords is of a continuous type. A view of the
zodiacal light is shown in Fig. 22.
In all directions the sun pours forth, with the most prodigal
liberality, its torrents of light and of heat. The earth can only grasp
the merest fraction, less than the 2,000,000,000th part of the whole.
Our fellow planets and the moon also intercept a trifle; but how small
is the portion of the mighty flood which they can utilise! The sip that
a flying swallow takes from a river is as far from exhausting the water
in the river as are the planets from using all the heat which streams
from the sun.
The sun's gracious beams supply the magic power that enables the corn to
grow and ripen. It is the heat of the sun which raises water from the
ocean in the form of vapour, and then sends down that vapour as rain to
refresh the earth and to fill the rivers which bear our ships down to
the ocean. It is the heat of the sun beating on the large continents
which gives rise to the breezes and winds that waft our vessels across
the deep; and when on a winter's evening we draw around the fire and
feel its invigorating rays, we are only enjoying sunbeams which shone on
the earth countless ages ago. The heat in those ancient sunbeams
developed the mighty vegetation of the coal period, and in the form of
coal that heat has slumbered for millions of years, till we now call it
again into activity. It is the power of the sun stored up in coal that
urges on our steam-engines. It is the light of the sun stored up in coal
that beams from every gaslight in our cities.
For the power to live and move, for the plenty with which we are
surrounded, for the beauty with which nature is adorned, we are
immediately indebted to one body in the countless hosts of space, and
that body is the sun.
[Illustration: Fig. 22.--The Zodiacal Light in 1874.]