THE SUN
So far we have referred to the sun just so much as was necessary to show
the planets rotating round him, and to acknowledge him as the source of
all our light and heat; but we have not examined in detail this
marvellous furnace that nourishes all the life on our planet and burns
on with undiminished splendour from year to year, without thought or
effort on our part. To sustain a fire on the earth much time and care
and expense are necessary; fuel has to be constantly supplied, and men
have to stoke the fire to keep it burning. Considering that the sun is
not only vastly larger than all the fires on the earth put together, but
also than the earth itself, the question very naturally occurs to us,
Who supplies the fuel, and who does the stoking on the sun? Before we
answer this we must try to get some idea of the size of this stupendous
body. It is not the least use attempting to understand it by plain
figures, for the figures would be too great to make any impression on
us--they would be practically meaningless; we must turn to some other
method. Suppose, for instance, that the sun were a hollow ball; then, if
the earth were set at the centre, the moon could revolve round her at
the same distance she is now, and there would be as great a distance
between the moon and the shell of the sun as there is between the moon
and the earth. This gives us a little idea of the size of the sun.
Again, if we go back to that solar system in which we represented the
planets by various objects from a pea to a football, and set a lamp in
the centre to do duty for the sun, what size do you suppose that lamp
would have to be really to represent the sun in proportion to the
planets? Well, if our greengage plum which did duty for the earth were
about three-quarters of an inch in diameter we should want a lamp with a
flame as tall as the tallest man you know, and even then it would not
give a correct idea unless you imagined that man extending his arms
widely, and you drew round him a circle and filled in all the circle
with flame! If this glorious flame burnt clear and fair and bright,
radiating beams of light all around, the little greengage plum would not
have to be too near, or it would be shrivelled up as in the blast of a
furnace. To place it at anything resembling the distance it is from the
sun in reality you would have to walk away from the flaming light for
about three hundred steps, and set it down there; then, after having
done all this, you would have some little idea of the relative sizes of
the sun and the earth, and of the distance between them.
Of course, all the other planets would have to be at corresponding
distances. On this same scale, Neptune, the furthest out, would be three
miles from our artificial sun! It seems preposterous to think that some
specks so small as to be quite invisible, specks that crawl about on
that plum, have dared to weigh and measure the gigantic sun; but yet
they have done it, and they have even decided what he is made of. The
result of the experiments is that we know the sun to be a ball of
glowing gas at a temperature so high that nothing we have on earth could
even compare with it. Of his radiating beams extending in all directions
few indeed fall on our little plum, but those that do are the source of
all life, whether animal or vegetable. If the sun's rays were cut off
from us, we should die at once. Even the coal we use to keep us warm is
but sun's heat stored up ages ago, when the luxuriant tropical
vegetation sprang up in the warmth and then fell down and was buried in
the earth. At night we are still enjoying the benefit of the sun's
rays--that is, of those which are retained by our atmosphere; for if
none remained even the very air itself would freeze, and by the next
morning not one inhabitant would be left alive to tell the awful tale.
Yet all this life and growth and heat we receive on the whole earth is
but one part in two thousand two hundred millions of parts that go out
in all directions into space. It has been calculated that the heat which
falls on to all the planets together cannot be more than one part in one
hundred millions and the other millions of parts seem to us to be simply
wasted.
For untold ages the sun has been pouring out this prodigal profusion of
glory, and as we know that this cannot go on without some sort of
compensation, we want to understand what keeps up the fires in the sun.
It is true that the sun is so enormous that he might go on burning for a
very long time without burning right away; but, then, even if he is
huge, his expenditure is also huge. If he had been made of solid coal he
would have been all used up in about six thousand years, burning at the
pace he does. Now, we know that the ancient Egyptians kept careful note
of the heavenly bodies, and if the sun were really burning away he must
have been very much larger in their time; but we have no record of this;
on the contrary, all records of the sun even to five thousand years ago
show that he was much the same as at present. It is evident that we must
search elsewhere for an explanation. It has been suggested that his
furnace is supplied by the number of meteors that fall into him. Meteors
are small bodies of the same materials as the planets, and may be
likened to the dust of the solar system. It is not difficult to
calculate the amount of matter he would require on this assumption to
keep him going, and the amount required is so great as to make it
practically impossible that this is the source of his supply. We have
seen that all matter influences all other matter, and the quantity of
meteoric stuff that would be required to support the sun's expenditure
would be enough to have a serious effect on Mercury, an effect that
would certainly have been noticed. There can, therefore, be no such mass
of matter near the sun, and though there is no doubt a certain number of
meteors do fall into his furnaces day by day, it is not nearly enough to
account for his continuous radiation. It seems after this as if nothing
else could be suggested; but yet an answer has been found, an answer so
wonderful that it is more like a fairy tale than reality.
To begin at the beginning, we must go back to the time when the sun was
only a great gaseous nebula filling all the space included in the orbit
of Neptune. This nebula was not in itself hot, but as it rotated it
contracted. Now, heat is really only a form of energy, and energy and
heat can be interchanged easily. This is a very startling thing when
heard for the first time, but it is known as surely as we know anything
and has been proved again and again. When a savage wants to make a fire
he turns a piece of hard wood very very quickly between his
palms--twiddles it, we should say expressively--into a hole in another
piece of wood, until a spark bursts out. What is the spark? It is the
energy of the savage's work turned to heat. When a horse strikes his
iron-shod hoofs hard on the pavement you see sparks fly; that is caused
by the energy of the horse's leg. When you pump hard at your bicycle you
feel your pump getting quite hot, for part of the energy you are putting
into your work is transformed into heat; and so on in numberless
instances. No energetic action of any kind in this world takes place
without some of the energy being turned into heat, though in many
instances the amount is so small as to be unnoticeable. Nothing falls
to the ground without some heat being generated. Now, when this great
nebula first began its remarkable career, by the action of gravity all
the particles in it were drawn toward the centre; little by little they
fell in, and the nebula became smaller. We are not now concerned with
the origin of the planets--we leave that aside; we are only
contemplating the part of the nebula which remained to become the sun.
Now these particles being drawn inward each generated some heat, so as
the nebula contracted its temperature rose. Throughout the ages, over
the space of millions and millions of miles, it contracted and grew
hotter. It still remained gaseous, but at last it got to an immense
temperature, and is the sun as we know it. What then keeps it shining?
It is still contracting, but slowly, so slowly that it is quite
imperceptible to our finest instruments. It has been calculated that if
it contracts two hundred and fifty feet in diameter in a year, the
energy thus gained and turned into heat is quite sufficient to account
for its whole yearly output. This is indeed marvellous. In comparison
with the sun's size two hundred and fifty feet is nothing. It would take
nine thousand years at this rate before any diminution could be noticed
by our finest instruments! Here is a source of heat which can continue
for countless ages without exhaustion. Thus to all intents and purposes
we may say the sun's shining is inexhaustible. Yet we must follow out
the train of reasoning, and see what will happen in the end, in eras and
eras of time, if nothing intervenes. Well, some gaseous bodies are far
finer and more tenuous than others, and when a gaseous body contracts it
is all the time getting denser; as it grows denser and denser it at last
becomes liquid, and then solid, and then it ceases to contract, as of
course the particles of a solid body cannot fall freely toward the
centre, as those of a gaseous body can. Our earth has long ago reached
this stage. When solid the action ceases, and the heat is no more kept
up by this source of energy, therefore the body begins to cool--surface
first, and lastly the interior; it cools more quickly the smaller it is.
Our moon has parted with all her heat long ago, while the earth still
retains some internally. In the sun, therefore, we have an object-lesson
of the stages through which all the planets must have passed. They have
all once been glowing hot, and some may be still hot even on the
surface, as we have seen there is reason to believe is the case with
Jupiter.
By this marvellous arrangement for the continued heat of the sun we can
see that the warmth of our planets is assured for untold ages. There is
no need to fear that the sun will wear out by burning. His brightness
will continue for ages beyond the thoughts of man.
Besides this, a few other things have been discovered about him. He is,
of course, exceptionally difficult to observe; for though he is so
large, which should make it easy, he is so brilliant that anyone
regarding him through a telescope without the precaution of prepared
glasses to keep off a great part of the light would be blinded at once.
One most remarkable fact about the sun is that his surface is flecked
with spots, which appear sometimes in greater numbers and sometimes in
less, and the reason and shape of these spots have greatly exercised
men's minds. Sometimes they are large enough to be seen without a
telescope at all, merely by looking through a piece of smoked or
coloured glass, which cuts off the most overpowering rays. When they are
visible like this they are enormous, large enough to swallow many earths
in their depths. At other times they may be observed by the telescope,
then they may be about five thousand miles across. Sometimes one spot
can be followed by an astronomer as it passes all across the sun,
disappears at the edge, and after a lapse of time comes back again round
the other edge. This first showed men that the sun, like all the
planets, rotated on his axis, and gave them the means of finding out how
long he took in doing so. But the spots showed a most surprising result,
for they took slightly different times in making their journey round the
sun, times which differed according to their position. For instance, a
spot near the equator of the sun took twenty-five days to make the
circuit, while one higher up or lower down took twenty-six days, and one
further out twenty-seven; so that if these spots are, as certainly
believed, actually on the surface, the conclusion is that the sun does
not rotate all in one piece, but that some parts go faster than others.
No one can really explain how this could be, but it is certainly more
easily understood in the case of a body of gas than of a solid body,
when it would be simply impossible to conceive. The spots seem to keep
principally a little north and a little south of the equator; there are
very few actually at it, and none found near the poles, but no reason
for this distribution has been discovered. It has been noted that about
every eleven years the greatest number of spots appears, and that
they become fewer again, mounting up in number to the next eleven years,
and so on. All these curious facts show there is much yet to be solved
about the sun. The spots were supposed for long to be eruptions bursting
up above the surface, but now they are generally held to be deep
depressions like saucers, probably caused by violent tempests, and it is
thought that the inrush of cooler matter from above makes them look
darker than the other parts of the sun's surface. But when we use the
words 'cooler' and 'darker,' we mean only by comparison, for in reality
the dark parts of the spots are brighter than electric light.
[Illustration: _Royal Observatory, Greenwich._
SUN-SPOTS.]
The fact that the spots are in reality depressions or holes is shown by
their change of appearance as they pass over the face of the sun toward
the edge; for the change of shape is exactly that which would be caused
by foreshortening.
It sounds odd to say that the best time for observing the sun is during
a total eclipse, for then the sun's body is hidden by the moon. But yet
to a certain extent this is true, and the reason is that the sun's own
brilliance is our greatest hindrance in observing him, his rays are so
dazzling that they light up our own atmosphere, which prevents us seeing
the edges. Now, during a total eclipse, when nearly all the rays are
cut off, we can see marvellous things, which are invisible at other
times. But total eclipses are few and far between, and so when one is
approaching astronomers make great preparations beforehand.
A total eclipse is not visible from all parts of the world, but only
from that small part on which the shadow of the moon falls, and as the
earth travels, this shadow, which is really a round spot, passes along,
making a dark band. In this band astronomers choose the best
observatories, and there they take up their stations. The dark body of
the moon first appears to cut a little piece out of the side of the sun,
and as it sails on, gradually blotting out more and more, eager
telescopes follow it; at last it covers up the whole sun, and then a
marvellous spectacle appears, for all round the edges of the black moon
are seen glorious red streamers and arches and filaments of marvellous
shapes, continually changing. These are thrown against a background of
pale green light that surrounds the black moon and the hidden sun. In
early days astronomers thought these wonderful coloured streamers
belonged to the moon; but it was soon proved that they really are part
of the sun, and are only invisible at ordinary times, because our
atmosphere is too bright to allow them to be seen. An instrument has
now been invented to cut off most of the light of the sun, and when this
is attached to a telescope these prominences, as they are called, can be
seen at any time, so that there is no need to wait for an eclipse.
[Illustration: THE EARTH AS IT WOULD APPEAR IN COMPARISON WITH THE
FLAMES SHOOTING OUT FROM THE SUN.]
What are these marvellous streamers and filaments? They are what they
seem, eruptions of fiery matter discharged from the ever-palpitating sun
thousands of miles into surrounding space. They are for ever shooting
out and bursting and falling back, fireworks on a scale too enormous for
us to conceive. Some of these brilliant flames extend for three hundred
thousand miles, so that in comparison with one of them the whole world
would be but a tiny ball, and this is going on day and night without
cessation. Look at the picture where the artist has made a little black
ball to represent the earth as she would appear if she could be seen in
the midst of the flames shooting out from the sun. Do not make a mistake
and think the earth really could be in this position; she is only shown
there so that you may see how tiny she is in comparison with the sun.
All the time you have lived and your father, and grandfather, and right
back to the beginnings of English history, and far, far further into the
dim ages, this stupendous exhibition of energy and power has continued,
and only of late years has anyone known anything about it; even now a
mere handful of people do know, and the rest, who are warmed and fed and
kept alive by the gracious beams of this great revolving glowing
fireball, never give it a thought.
I said just now a pale green halo surrounded the sun, extending far
beyond the prominences; this is called the corona and can only be seen
during an eclipse. It surrounds the sun in a kind of shell, and there is
reason to believe that it too is made of luminous stuff ejected by the
sun in its burning fury. It is composed of large streamers or filaments,
which seem to shoot out in all directions; generally these are not much
larger than the apparent width of the sun, but sometimes they extend
much further. The puzzle is, this corona cannot be an atmosphere in any
way resembling that of our earth; for the gravitational force of the
sun, owing to its enormous size, is so great that it would make any such
atmosphere cling to it much more densely near to the surface, while it
would be thinner higher up, and the corona is not dense in any way, but
thin and tenuous throughout. This makes it very difficult to explain; it
is supposed that some kind of electrical force enters into the problem,
but what it is exactly we are far from knowing yet.