AN HOUR WITH THE SUN
[Illustration]
Before beginning upon the subject of our lecture to-day I want to tell
you the story of a great puzzle which presented itself to me when I was
a very young child. I happened to come across a little book--I can see
it now as though it were yesterday--a small square green book called
_World without End_, which had upon the cover a little gilt picture of a
stile with trees on each side of it. That was all. I do not know what
the book was about, indeed I am almost sure I never opened it or saw it
again, but that stile and the title "World without End" puzzled me
terribly. What was on the other side of the stile? If I could cross over
it and go on and on should I be in a world which had no ending, and what
would be on the other side? But then there could be no other side if it
was a world without any end. I was very young, you must remember, and I
grew confused and bewildered as I imagined myself reaching onwards and
onwards beyond that stile and never, never resting. At last I consulted
my greatest friend, an old man who did the weeding in my father's
garden, and whom I believed to be very wise. He looked at first almost
as bewildered as I was, but at last light dawned upon him. "I tell you
what it is, Master Arthur," said he, "I do not rightly know what happens
when there is no end, but I do know that there is a mighty lot to be
found out in this world, and I'm thinking we had better learn first all
about that, and perhaps it may teach us something which will help us to
understand the other."
I daresay you will wonder what this anecdote can have to do with a
lecture on the sun--I will tell you. Last night I stood on the balcony
and looked out far and farther away into the star-depths of the midnight
sky, marvelling what could be the history of those countless suns of
which we see ever more and more as we increase the power of our
telescopes, or catch the faint beams of those we cannot see and make
them print their image on the photographic plate. And, as I grew
oppressed at the thought of this never-ending expanse of suns and at my
own littleness, I remembered all at once the little square book of my
childish days with its gilt stile, and my old friend's advice to learn
first all we can of that which lies nearest.
So to-day, before we travel away to the stars, we had better inquire
what is known about the one star in the heavens which is comparatively
near to us, our own glorious sun, which sends us all our light and heat,
causes all the movements of our atmosphere, draws up the moisture from
the ground to return in refreshing rain, ripens our harvests, awakens
the seeds and sleeping plants into vigorous growth, and in a word
sustains all the energy and life upon our earth. Yet even this star,
which is more than a million times as large as our earth, and bound so
closely to us that a convulsion on its surface sends a thrill right
through our atmosphere, is still so far off that it is only by
questioning the sunbeams it sends to us, that we can know anything about
it.
You have already learnt[1] a good deal as to the size, the intense heat
and light, and the photographic power of the sun, and also how his white
beams of light are composed of countless coloured rays which we can
separate in a prism. Now let us pass on to the more difficult problem of
the nature of the sun itself, and what we know of the changes and
commotions going on in that blazing globe of light.
[1] _Fairyland of Science_, Chapter II.
We will try first what we can see for ourselves. If you take a card and
make a pin-hole in it, you can look through this hole straight at the
sun without injuring your eye, and you will see a round shining disc on
which, perhaps, you may detect a few dark spots. Then if you take your
hand telescopes, which I have shaded by putting a piece of smoked glass
inside the eye-piece, you will find that this shining disc is really a
round globe, and moreover, although the object-glass of your telescopes
measures only two-and-a-half inches across, you will be able to see the
dark spots very distinctly and to observe that they are shaded, having a
deep spot in the centre with a paler shadow round it.
[Illustration: Fig. 45.
Face of the sun projected on a sheet of cardboard C. T, Telescope. _f_,
Finder. _og_, Object-glass. _ep_, Eye-piece. S, Screen shutting off the
diffused light from the window.]
As, however, you cannot all use the telescopes, and those who can will
find it difficult to point them truly on to the sun, we will adopt still
another plan. I will turn the object-glass of my portable telescope full
upon the sun's face, and bringing a large piece of cardboard on an easel
near to the other end, draw it slowly backward till the eye-piece forms
a clear sharp image upon it (see Fig. 45). This you can all see
clearly, especially as I have passed the eye-piece of the telescope
through a large screen _s_, which shuts off the light from the window.
You have now an exact image of the face of the sun and the few dark
spots which are upon it, and we have brought, as it were, into our room
that great globe of light and heat which sustains all the life and
vigour upon our earth.
This small image can, however, tell us very little. Let us next see what
photography can show us. The diagram (Fig. 46) shows a photograph of the
sun taken by Mr. Selwyn in October 1860. Let me describe how this is
done. You will remember that there is a point in the telescope tube
where the rays of light form a real image of the object at which the
telescope is pointed (see p. 44). Now an astronomer who wishes to take a
photograph of the sun takes away the eye-piece of his telescope and puts
a photographic plate in the tube exactly at the place where this real
image is formed. He takes care to blacken the frame of the plate and
shuts up this end of the telescope and the plate in a completely dark
box, so that no diffused light from outside can reach it. Then he turns
his telescope upon the sun that it may print its image.
But the sun's light is so strong that even in a second of time it would
print a great deal too much, and all would be black and confused. To
prevent this he has a strip of metal which slides across the tube of the
telescope in front of the plate, and in the upper part of this strip a
very fine slit is cut. Before he begins, he draws the metal up so that
the slit is outside the tube and the solid portion within, and he
fastens it in this position by a thread drawn through and tied to a bar
outside. Then he turns his telescope on the sun, and as soon as he
wishes to take the photograph he cuts the thread. The metal slides
across the tube with a flash, the slit passing across it and out again
below in the hundredth part of a second, and in that time the sun has
printed through the slit the picture before you.
[Illustration: Fig. 46.
Photograph of the face of the sun, taken by Mr. Selwyn, October 1860,
showing spots, faculae, and mottled surface.]
In it you will observe at least two things not visible on our
card-image. The spots, though in a different position from where we see
them to-day, look much the same, but round them we see also some bright
streaks called _faculae_, or torches, which often appear in any region
where a spot is forming, while the whole face of the sun appears mottled
with bright and darker spaces intermixed. Those of you who have the
telescopes can see this mottling quite distinctly through them if you
look at the sun. The bright points have been called by many names, and
are now generally known as "light granules," as good a name, perhaps, as
any other.
This is all our photograph can tell us, but the round disc there shown,
which is called the _photosphere_, or light-giving sphere, is by no
means the whole of the sun, though it is all we see daily with the naked
eye. Whenever a total eclipse of the sun takes place--by the dark body
of the moon coming between us and it, so as to shut out the whole of
this disc--a brilliant white halo, called the crown or _corona_, is seen
to extend for many thousands of miles all round the darkened globe. It
varies very much in shape, sometimes forming a kind of irregular square,
sometimes a circle with off-shoots, as in Fig. 47, which shows what
Major Tennant saw in India during the total eclipse of August 18, 1868,
and at other times it shoots out in long pearly white jets and sheets of
light with dark spaces between. On the whole it varies periodically. At
the time of few sun-spots its extensions are equatorial; but when the
sun's face is much covered with spots, they are diagonal, stretching
away from the spot-zones, but not nearly so far.
[Illustration: Fig. 47.
Total eclipse of the sun, as drawn by Major Tennant at Guntoor in India,
August 18, 1868, showing corona and the protuberances seen at the
beginning of totality.]
And besides this corona there are seen very curious flaming projections
on the edge of the sun, which begin to appear as soon as the moon covers
the bright disc. In our diagram (Fig. 47) you see them on the left side
where the moon is just creeping over the limits of the photosphere and
shutting out the strong light of the sun as the eclipse becomes total. A
very little later they are better seen on the other side just before the
bright edge of the sun is uncovered as the moon passes on its way. These
projections in the real sun are of a bright red colour, and they take on
all manners of strange shapes, sometimes looking like ranges of fiery
hills, sometimes like gigantic spikes and scimitars, sometimes even like
branching fiery trees. They were called _prominences_ before their
nature was well understood, and will probably always keep that name. It
would be far better, however, if some other name such as "glowing
clouds" or "red jets" could be used, for there is now no doubt that they
are jets of gases, chiefly hydrogen, constantly playing over the face of
the sun, though only seen when his brighter light is quenched. They have
been found to shoot up 20,000, 80,000, and even as much as 350,000 miles
beyond the edge of the shining disc; and this last means that the flames
were so gigantic that if they had started from our earth they would have
reached beyond the moon. We shall see presently that astronomers are now
able by the help of the spectroscope to see the prominences even when
there is no eclipse, and we know them to be permanent parts of the
bright globe.
This gives us at last the whole of the sun, so far as we know. There is,
indeed, a strange faint zodiacal light, a kind of pearly glow seen after
sunset or before sunrise extending far beyond the region of the corona;
but we understand so little about this that we cannot be sure that it
actually belongs to the sun.
And now how shall I best give you an idea of what little we do know
about this great surging monster of light and heat which shines down
upon us? You must give me all your attention, for I want to make the
facts quite clear, that you may take a firm hold upon them.
Our first step is to question the sunlight which comes to us; and this
we do with the spectroscope. Let me remind you how we read the story of
light through this instrument. Taking in a narrow beam of light through
a fine slit, we pass the beam through a lens to make the rays parallel,
and then throw it upon a prism or row of prisms, so that each set of
waves of coloured light coming through the slit is bent on its own road
and makes an upright image of the slit on any screen or telescope put to
receive it (see Fig. 21, p. 52). Now when the light we examine comes
from a glowing solid, like white-hot iron, or a glowing liquid, or a gas
under such enormous pressure that it behaves like a liquid, then the
images of the slit always overlap each other, so that we see a
continuous unbroken band of colour. However much you spread out the
light you can never break up or separate the spectrum in any part.[1]
But when you send the light, of a glowing gas such as hydrogen through
the spectroscope, or of a substance melted into gas or vapour, such as
sodium or iron vaporised by great heat, then it is a different story.
Such gases give only a certain number of bright lines quite separate
from each other on the dark background, and each kind of gas gives its
own peculiar lines; so that even when several are glowing together there
is no confusion, but when you look at them through the spectroscope you
can detect the presence of each gas by its own lines in the spectrum.
[1] Two rare earths, Erbia and Didymium, form an exception to this,
but they do not concern us here.
[Illustration: TABLE OF SPECTRA. Plate I.]
To make quite sure of this we will close the shutters and put a pinch of
salt in a spirit-flame. Salt is chloride of sodium, and in the flame the
sodium glows with a bright yellow light. Look at this light through your
small direct-vision spectroscopes[1] and you see at once the bright
yellow double-line of sodium (No. 3, Plate I.) start into view across
the faint continuous spectrum given by the spirit-flame. Next I will
show you glowing hydrogen. I have here a glass tube containing hydrogen,
so arranged that by connecting two wires fastened to it with the
induction coil of our electric battery it will soon glow with a bright
red colour. Look at this through your spectroscopes and you will see
three bright lines, one red, one greenish blue, and one indigo blue,
standing out on the dark background (No. 4, Plate I.)
[1] A direct-vision spectroscope is like a small telescope with
prisms arranged inside the tube. The object-glass end is covered by
two pieces of metal, which slide backwards and forwards by means of
a screw, so that a narrow or broad slit can be opened.
Think for a moment what a grand power this gives you of reading as in a
book the different gases which are glowing in the sky even billions of
miles away. You would never mistake the lines of hydrogen for the line
of sodium, but when looking at a nebula or any mass of glowing gas you
could say at once "sodium is glowing there," or "that cloud must be
composed of hydrogen."
Now, opening the shutters, look at the sunlight through your
spectroscopes. Here you have something different from either the
continuous spectrum of solids, or the bright separate lines of gases,
for while you have a bright-coloured band you have also some dark lines
crossing it (No. 2, Plate I.) It is those dark lines which enable us to
guess what is going on in the sun before the light comes to us. In 1859
Professor Kirchhoff made an experiment which explained those dark lines,
and we will repeat it now. Take a good look at the sunlight spectrum, to
fix the lines in your memory, and then close the shutters again.
[Illustration: Fig. 48.
Kirchhoff's experiment, explaining the dark lines in sunlight.
A, Limelight dispersed through a prism. _s_, Slit through which the beam
of light comes. _l_, Lens bringing it to a focus on the prism _p_. _sp_,
Continuous spectrum thrown on the wall. B, The same light, with the
flame _f_ containing glowing sodium placed in front of it. D, Dark
sodium line appearing in the spectrum.]
I have here our magic-lantern with its lime-light, in which the solid
lime glows with a white heat, in consequence of the jets of oxygen and
hydrogen burning round it. This was the light Kirchhoff used, and you
know it will give a continuous bright band in the spectroscope. I put a
cap with a narrow slit in it over the lantern tube, so as to get a
narrow beam of light; in front of this I put a lens _l_, and in front of
this again the prism _p_. The slit and the prism act exactly like your
spectroscopes, and you can all see the continuous spectrum on the screen
(_sp_, A, Fig. 48). Next I put a lighted lamp of very weak spirit in
front of the slit, and find that it makes no difference, for whatever
light it gives only strengthens the spectrum. But now notice carefully.
I am going to put a little salt into the flame, and you would expect
that the sodium in it, when turned to glowing vapour, causing it to look
yellow, would strengthen the yellow part of the spectrum and give a
bright line. This is what Kirchhoff expected, but to his intense
surprise he saw as you do now a _dark line_ D start out where the bright
line should have been.
What can have happened? It is this. The oxyhydrogen light is very hot
indeed, the spirit flame with the sodium is comparatively weak and cool.
So when those special coloured waves of the oxyhydrogen light which
agree with those of the sodium light reached the flame, they spent all
their energy in heating up those waves to their own temperature, and
while all the other coloured rays travelled on and reached the screen,
these waves were stopped or _absorbed_ on the way, and consequently
there was a blank, black space in the spectrum where they should have
been. If I could put a hydrogen flame cooler than the original light in
the road, then there would be three dark lines where the bright hydrogen
lines should be, and so with every other gas. _The cool vapour in front
of the hot light cuts off from the white ray exactly those waves which
it gives out itself when burning._
Thus each black line of the sun-spectrum (No. 2, Plate I.), tells us
that some particular ray of sunlight has been absorbed by a cooler
vapour _of its own kind_ somewhere between the sun and us, and it must
be in the sun itself, for when we examine other stars we often find dark
lines in their spectrum different from those in the sun, and this shows
that the missing rays must have been stopped close at home, for if they
were stopped in our atmosphere they would all be alike.
There are, by the bye, some lines which we know are caused by our
atmosphere, especially when it is full of invisible water vapour, and
these we easily detect, because they show more distinctly when the sun
is low and shines through a thicker layer of air than when he is high up
and shines through less.
But to return to the sun. In your small spectroscopes you see very few
dark lines, but in larger and more perfect ones they can be counted by
thousands, and can be compared with the bright lines of glowing gases
burnt here on earth. In the spectrum of glowing iron vapour 460 lines
are found to agree with dark lines in the sun-spectrum, and other gases
have nearly as many. Still, though thousands of lines can now be
explained, by matching them with the bright lines of known gases, the
whole secret of sunlight is not yet solved, for the larger number of
lines still remain a riddle to be read.
We see then that the spectroscope teaches us that the round light-giving
disc or photosphere of the sun consists of a bright and intensely hot
light shining behind a layer of cooler though still very hot vapours,
which form a kind of shell of luminous clouds around it, and in this
shell, or _reversing layer_--as it is often called, because it turns
light to darkness--we have proved that iron, lead, copper, zinc,
aluminum, magnesium, potassium, sodium, carbon, hydrogen, and many other
substances common to our earth, exist in a state of vapour for a depth
of perhaps 1000 miles.
You will easily understand that when the spectroscope had told so much,
astronomers were eager to learn what it would reveal about the
prominences or red jets seen during eclipses, and they got an answer in
India during that same eclipse of August 1868 which is shown in our
diagram (Fig. 47). Making use of the time during which the prominences
were seen, they turned the telescope upon them with a spectroscope
attached to it, and saw a number of bright lines start out, of which the
chief were the three bright lines of hydrogen, showing that these
curious appearances are really flames of glowing gas.
In the same year Professor Jannsen and Mr. Lockyer succeeded in seeing
the bright lines of the prominences in full sunlight. This was done in a
very simple way, when once it was discovered to be possible, and though
my apparatus (Fig. 49) is very primitive compared with some now made, it
will serve to explain the method.
[Illustration: Fig. 49.
The spectroscope attached to the telescope for the examination of the
sun. (Lockyer.)
P, Pillar of Telescope. T, Telescope. S, Finder or small telescope for
pointing the telescope in position. _a_, _a_, _b_, Supports fastening
the spectroscope to the telescope. _d_, Collimator or tube carrying the
slit at the end nearest the telescope, and a lens at the other end to
render the rays parallel. _c_, Plate on which the prisms are fixed. _e_,
Small telescope through which the observer examines the spectrum after
the ray has been dispersed in the prisms. _h_, Micrometer for measuring
the relative distance of the lines.]
When an astronomer wishes to examine the spectrum of any special part of
the sun, he takes off the eye-piece of his telescope and screws the
spectroscope upon the draw-tube. The spectroscope is made exactly like
the large one for ordinary work. The tube _d_ (Fig. 49) carries the slit
at the end nearest the telescope, and this slit must be so placed as to
stand precisely at the principal focus of the lens where the sun's image
is formed (see _i_, _i_, p. 44). This comes to exactly the same thing as
if we could put the slit close against the face of the sun, so as to
show only the small strip which it covers, and by moving it to one part
or another of the image we can see any point that we wish and no other.
The light then passes through the tube _d_ into the round of prisms
standing on the tray _c_, and the observer looking through the small
telescope _e_ sees the spectrum as it emerges from the last prism. In
this way astronomers can examine the spectrum of a spot, or part of a
spot, or of a bright streak, or any other mark on the sun's face.
Now in looking at the prominences we have seen that the difficulty is
caused by the sunlight, between us and them, overpowering the bright
lines of the gas, nor could we overcome this if it were not for a
difference which exists between the two kinds of light. The more you
disperse or spread out the continuous sun-spectrum the fainter it
becomes, but in spreading out the bright lines of the gas you only send
them farther and farther apart; they themselves remain almost as bright
as ever. So, when the telescope forms an image of the red flame in front
of the slit, though the glowing gas and the sunlight both send rays into
the spectroscope, you have only to use enough prisms and arrange them in
such a way that the sunlight is dispersed into a very long faint
spectrum, and then the bright lines of the flames will stand out bright
and clear. Of course only a small part of the long spectrum can be seen
at once, and the lines must be studied separately. On the other hand, if
you want to compare the strong light of the sun with the bright lines of
the prominences, you place the slit just at the edge of the sun's image
in the telescope, so that half the slit is on the sun's face and half on
the prominence. The prisms then disperse the sunlight between you and
the prominences, while they only lessen the strong light of the sun
itself, which still shows clearly. In this way the two spectra are seen
side by side and the dark and bright lines can be compared accurately
together (see Fig. 50).
[Illustration: Fig. 50.
Bright lines of prominences.
Sun-spectrum with dark lines.]
Wherever the telescope is turned all round the sun the lines of luminous
gas are seen, showing that they form a complete layer outside the
photosphere, or light-giving mass, of the sun. This layer of luminous
gases is called the _chromosphere_, or coloured sphere. It lies between
the photosphere and the corona, and is supposed to be at least 5000
miles deep, while, as we have seen, the flames shoot up from it to
fabulous heights.
The quiet red flames are found to be composed of hydrogen and another
new metal called helium; but lower down, near the sun's edge, other
bright lines are seen, showing that sodium, magnesium, and other metals
are there, and when violent eruptions occur these often surge up and
mingle with the purer gas above. At other times the eruptions below
fling the red flames aloft with marvellous force, as when Professor
Young saw a long low-lying cloud of hydrogen, 100,000 miles long, blown
into shreds and flung up to a height of 200,000 miles, when the
fragments streamed away and vanished in two hours. Yet all these violent
commotions and storms are unseen by us on earth unless we look through
our magic glasses.
You will wonder no doubt how the spectroscope can show the height and
the shape of the flames. I will explain to you, and I hope to show them
you one day. You must remember that the telescope makes a small real
image of the flame at its focus, just as in one of our earlier
experiments you saw the exact image of the candle-flame upside down on
the paper (see p. 33). The reason why we only see a strip of the flame
in the spectroscope is because the slit is so narrow. But when once the
sunlight was dispersed so as no longer to interfere, Dr. Huggins found
that it is possible to open the slit wide enough to take in the image of
the whole flame, and then, by turning the spectroscope so as to bring
one of the bright hydrogen lines into view, the actual shape of the
prominence is seen, only it will look a different colour, either red,
greenish-blue, or indigo-blue, according to the line chosen. As the
image of the whole sun and its appendages in the telescope is so very
small, you will understand that even a very narrow slit will really take
in a very large prominence several thousand miles in length. Fig 51
shows a drawing by Mr. Lockyer of a group of flames he observed very
soon after Dr. Huggins suggested the open slit, and these shapes did not
last long, for in another picture he drew ten minutes later their
appearance had already changed.
[Illustration: Fig. 51.
Red prominences, as drawn by Mr. Lockyer during the total eclipse of
March 14, 1869.]
These then are some of the facts revealed to us by our magic glasses. I
scarcely expect you to remember all the details I have given you, but
you will at least understand now how astronomers actually penetrate into
the secrets of the sun by bringing its image into their observatory, as
we brought it to-day on the card-board, and then making it tell its own
tale through the prisms of the spectroscope; and you will retain some
idea of the central light of the sun with its surrounding atmosphere of
cooler gases and its layer of luminous lambent gases playing round it
beyond.
Of the corona I cannot tell you much, except that it is far more subtle
than anything we have spoken of yet; that it is always strongest when
the sun is most spotted; that it is partly made up of self-luminous
gases whose bright lines we can see, especially an unknown green ray;
while it also shines partly by reflected light from the sun, for we can
trace in it faint dark lines; lastly it fades away into the mysterious
zodiacal light, and so the sun ends in mystery at its outer fringe as it
began at its centre.
And now at last, having learnt something of the material of the sun, we
can come back to the spots and ask what is known about them. As I have
said, they are not always the same on the sun's face. On the contrary,
they vary very much both in number and size. In some years the sun's
face is quite free from them, at others there are so many that they form
two wide belts on each side of the sun's equator, with a clear space of
about six degrees between. No spots ever appear near the poles. Herr
Schwabe, who watched the sun's face patiently for more than thirty
years, has shown that it is most spotted about every eleven years, then
the spots disappear very quickly and reappear slowly till the full-spot
time comes round again.
Some spots remain a very short time and then break up and disappear, but
others last for days, weeks, and even months, and when we watch these,
we find that a spot appears to travel slowly across the face of the sun
from east to west and then round the western edge so that it disappears.
It is when it reaches the edge that we can convince ourselves that the
spot is really part of the sun, for there is no space to be seen between
them, the edge and the spot are one, as the last trace of the dark
blotch passes out of sight. In fact, it is not the spot which has
crossed the sun's face, but the sun itself which has turned, like our
earth, upon its axis, carrying the spot round with it. As some spots
remain long enough to reappear, after about twelve or thirteen days, on
the opposite edge, and even pass round two or three times, astronomers
can reckon that the sun takes about twenty-five days and five hours in
performing one revolution. You will wonder why I say only _about_
twenty-five, but I do so because all spots do not come round in exactly
the same time, those farthest from the equator lag rather more than a
day behind those nearer to it, and this is explained by the layer of
gases in which they are formed, drifting back in higher latitudes as the
sun turns.
It is by watching a spot as it travels across the sun, that we are able
to observe that the centre partlies deeper in the sun's face than the
outer rim. There are many ways of testing this, and you can try one
yourselves with a telescope if you watch day after day. I will explain
it by a simple experiment. I have here a round lump of stiff dough, in
which I have made a small hollow and blackened the bottom with a drop of
ink. As I turn this round, so that the hollow facing you moves from
right to left, you will see that after it passes the middle of the face,
the hole appears narrower and narrower till it disappears, and if you
observe carefully you will note that the dark centre is the first thing
you lose sight of, while the edges of the cup are still seen, till just
before the spot disappears altogether. But now I will stick a wafer on,
and a pea half into, the dough, marking the centre of each with ink.
Then I turn the ball again. This time you lose sight of the foremost
edge first, and the dark centre is seen almost to the last moment. This
shows that if the spots were either flat marks, or hillocks, on the
sun's face, the dark centre would remain to the last, but as a fact it
disappears before the rim. Father Secchi has tried to measure the depth
of a spot-cavity, and thinks they vary from 1000 to 3000 miles deep. But
there are many difficulties in interpreting the effects of light and
shadow at such an enormous distance, and some astronomers still doubt
whether spots are really depressions.
For many centuries now the spots have been watched forming and
dispersing, and this is roughly speaking what is seen to happen. When
the sun is fairly clear and there are few spots, these generally form
quietly, several black dots appearing and disappearing with bright
streaks or _faculae_ round their edge, till one grows bigger than the
rest, and forms a large dark nucleus, round which, after a time, a
half-shadow or _penumbra_ is seen and we have a sun-spot complete, with
bright edges, dark shadow, and deep black centre (Fig. 52). This lasts
for a certain time and then it becomes bridged over with light streaks,
the dark spot breaks up and disappears, and last of all the half-shadow
dies away.
[Illustration: Fig. 52.
A quiet sun-spot. (Secchi.)]
But things do not always take place so quietly. When the sun's face is
very troubled and full of spots, the bright _faculae_, which appear with
a spot, seem to heave and wave, and generally several dark centres form
with whirling masses of light round them, while in some of them tongues
of fire appear to leap up from below (Fig. 53). Such spots change
quickly from day to day, even if they remain for a long time, until at
last by degrees the dark centres become less distinct, the half-shadows
disappear, leaving only the bright streaks, which gradually settle down
into luminous points or _light granules_. These light granules are in
fact supposed by astronomers to be the tips of glowing clouds heaving up
everywhere, while the dark spaces between them are cooler currents
passing downwards.
[Illustration: Fig. 53.
A tumultuous sun-spot. (Langley.)]
Below these clouds, no doubt, the great mass of the sun is in a violent
state of heat and commotion, and when from time to time, whether
suddenly or steadily, great upheavals and eruptions take place, bright
flames dart up and luminous clouds gather and swell, so that long
streaks or _faculae_ surge upon the face of the sun.
Now these hot gases rising up thus on all sides would leave room below
for cooler gases to pour down from above, and these, as we know, would
cut off, or absorb, much of the light coming from the body of the sun,
so that the centre, where the down current was the strongest, would
appear black even though some light would pass through. This is the best
explanation we have as yet of the formation of a sun-spot, and many
facts shown in the spectroscope help to confirm it, as for example the
thickening of the dark lines of the spectrum when the slit is placed
over the centre of a spot, and the flashing out of bright lines when an
uprush of streaks occurs either across the spots or round it.
And now, before you go, I must tell you of one of these wonderful
uprushes, which sent such a thrill through our own atmosphere, as to
tell us very plainly the power which the sun has over our globe. The
year 1859 was remarkable for sun-spots, and on September 1, when two
astronomers many miles apart were examining them, they both saw, all at
once, a sudden cloud of light far brighter than the general surface of
the sun burst out in the midst of a group of spots. The outburst began
at eight minutes past eleven in the forenoon, and in five minutes it was
gone again, but in that time it had swept across a space of 35,000 miles
on the sun! Now both before and after this violent outburst took place
a magnetic storm raged all round the earth, brilliant auroras were seen
in all parts of the world, sparks flashed from the telegraph wires, and
the telegraphic signalmen at Washington and Philadelphia received severe
electric shocks. Messages were interrupted, for the storm took
possession of the wires and sent messages of its own, the magnetic
needles darting to and fro as though seized with madness. At the
very instant when the bright outburst was seen in the sun, the
self-registering instruments at Kew marked how three needles jerked all
at once wildly aside; and the following night the skies were lit up with
wondrous lights as the storm of electric agitation played round the
earth.
We are so accustomed to the steady glow of sunshine pouring down upon us
that we pay very little heed to daylight, though I hope none of us are
quite so ignorant as the man who praised the moon above the sun, because
it shone in the dark night, whereas the sun came in the daytime when
there was light enough already! Yet probably many of us do not actually
realise how close are the links which bind us to our brilliant star as
he carries us along with him through space. It is only when an unusual
outburst occurs, such as I have just described, that we feel how every
thrill which passes through our atmosphere, through the life-current of
every plant, and through the fibre and nerve of every animal has some
relation to the huge source of light, heat, electricity, and magnetism
at which we are now gazing across a space of more than 93,000,000 miles.
Yet it is well to remember that the sudden storm and the violent
eruption are the exceptional occurrences, and that their use to us as
students is chiefly to lead us to understand the steady and constant
thrill which, never ceasing, never faltering, fulfils the great purpose
of the unseen Lawgiver in sustaining all movement and life in our little
world.