MORE ABOUT THE SUN.
Not among the least of the wondrous things of creation are the
tremendous disturbances taking place upon the surface of the sun--that
raging, roaring sea of flame.
A good many explanations have been from time to time offered as to the
dark spots seen to move across the face of the sun. Some one or more of
these explanations may be true; but we know little about the matter.
A sun-spot does not commonly consist of merely one black patch. There
is the dark center, called the _umbra_--plural, _umbræ_. There is the
grayish part surrounding the umbra, called the _penumbra_. Also, in
the center of the umbra there is sometimes observable an intensely
black spot, called the _nucleus_. Sometimes a spot is made up of
nucleus and umbra alone, without any penumbra. Sometimes it is made
of penumbra alone, without any umbra. Sometimes in one spot there are
several umbræ, with the gray penumbra round the whole, and gray bridges
dividing the umbræ.
The enormous size of these spots has been already described in an
earlier chapter. Fifty thousand to one hundred thousand miles across
is nothing unusual. In the year 1839 a spot was seen which measured no
less than one hundred and eighty-six thousand miles in diameter.
One explanation proposed was, that the sun might be a cool body,
covered over with different envelopes or dense layers of cloudy form,
one above another. The inside envelope--or, as some say, the inside
atmosphere--would then be thick and dull-colored, protecting the solid
globe within and reflecting light, but having none of its own. The next
envelope would be one mass of raging, burning gases--the _photosphere_,
in fact. The outer envelope would be a transparent, surrounding
atmosphere, lighted up by the sea of fire within. A sun-spot would then
consist of the tearing open of one or more of these envelopes, so as to
give glimpses of the gray, inner atmosphere, or even of the dark, cool
globe at the center.
There may be some truth in this explanation; but the notion of a
cool and dark body within is now pretty well given up. The apparent
blackness of a spot-nucleus does not prove actual blackness or absence
of heat. A piece of white-hot iron, held up against the sun, looks
black; and it may be merely the contrast of the glowing photosphere
which makes the nucleus seem so dark. It is even believed that the
blackest parts may be the most intensely hot of all.
Another proposed explanation was of dark clouds floating in the sun’s
atmosphere. There, again, are found difficulties, particularly in
the fact that, as the spots move across the sun, the changes which
regularly take place in their appearance make it pretty clear that
their shape is not flat, but hollow and cave-like. The changes here
spoken of are seeming changes of shape, caused by change of position.
There are also real changes constantly taking place. Although the spots
often keep their general outlines long enough to be watched across the
face of the sun, and even to be known again after spending nearly a
fortnight hidden on the other side, still they are far from being fixed
in form.
[Illustration: A TYPICAL SUN-SPOT.]
The alterations are at times not only very great, but very rapid.
Sometimes in a single hour of watching, an astronomer can see marked
movement going on--as you or I might in an hour observe movements
slowly taking place in a high layer of clouds. For movement to show at
all in one hour, at so immense a distance, proves that the actual rate
of motion must be very great.
The first great fact which was got from the study of these spots was
this, that this great sun is very much like our own earth, in so far
as it rotates on an axis in exactly the same way that our earth does.
Not only do the spots change their position on the face of the sun, in
consequence of the sun’s rotation on its axis, but they change very
much from day to day, and even from hour to hour; so that we have
evidence not only that the sun is rotating like our earth, but that the
atmosphere of the sun is subjected to most tremendous storms--storms
so tremendous, in fact, that the fiercest cyclones on our own earth
are not for one moment to be compared with them. The fact that the
spots do really move with the sun, and are really indentations,
saucer-like hollows, in the photosphere, is shown by the appearance,
which is always presented by a spot when it is near the edge of the
sun. You know if you take a dinner-plate, and look it full in the face,
it is round; but if you look at it edgeways, it is not round. Take
two different views of the same spot: In one, you look the sun-spot
straight in the face, and you see into it and can learn all about it;
but in the other, when it has nearly gone round the corner, and is
disappearing on the sun’s edge, you see it in the same way that you
would see a plate looked at edgeways.
These sun-cyclones must indeed be of terrific force and extent,
compared with anything we see on earth. It was calculated that the
speed of movement perceived in one spot was about three hundred and
sixty-three miles each second.
And still all this is nothing, or almost nothing, in comparison with
the real power of the sun! The liquid state of the ocean; the gaseous
state of the atmosphere; the currents of the sea; the raising of the
clouds, the rains, storms, streams, rivers; the calorific value of
all the forests of the globe and all the coal-mines of the earth; the
motion of all living beings; the heat of all humanity; the stored-up
power in all human muscles, in all the manufactories, in all the
guns,--all that is almost nothing compared with that of which the
sun is capable. Do we think that we have measured the solar power
by enumerating the effects which it produces on the earth? Error!
profound, tremendous, foolish error! This would be to believe still
that this star has been created on purpose to illuminate terrestrial
humanity. In reality, what an infinitesimal fraction of the sun’s total
radiation the earth receives and utilizes! In order to appreciate
it, let us consider the distance of ninety-three millions of miles
which separates us from the central star, and at this distance let us
see what effect our little globe produces, what heat it intercepts.
Let us imagine an immense sphere _traced at this distance from the
sun_, and entirely surrounding it. Well, on this gigantic sphere,
the spot intercepted by our little earth is only equivalent to the
fraction 1/2,138,000,000; that is to say, that the dazzling solar
hearth radiates all round it through immensity a quantity of light
and heat two thousand one hundred and thirty-eight million times more
than that which we receive, and of which we have just now estimated
the stupendous effects. The earth only stops in its passage the _two
thousand millionth part of the total radiation_.
Sometimes the storms or outbursts come in the shape of a bright spot
instead of a dark one.
[Illustration: A SOLAR ERUPTION.]
Two astronomers were one day watching the sun from two different
observatories, when they saw such an event take place. An intense
and dazzling spot of light burst out upon the surface of the sun--so
intense, so dazzling, as to stand quite apart from the radiant
photosphere. To one astronomer it looked like a single spot, while the
other saw two spots close together. In about a minute the light grew
more dim, and in five minutes all was over. But in those five minutes
the spot or spots had traveled a distance of thirty-five thousand
miles.
It was a very remarkable thing that the _magnets_ on earth--those
delicate little needles which point so steadily yet perseveringly
towards the North Pole--seemed to be strongly agitated by the distant
solar outburst. This brings us to another interesting fact.
The spots on the sun are not always the same in number. Sometimes they
are many, sometimes they are few. Long and close watching has made it
clear that they pass through a regular _order_ of changes: some years
of many spots being followed by other years of less and less spots;
then some years of very few spots being followed by other years of more
and more spots,--decrease and increase being seemingly regular and
alternate.
This turn or _cycle_ of changes--from more to less, and then from less
to more again--is found to run its course about once in every eleven
years, with some variations.
Now, it has long been known that the magnetic-needle goes through
curious variations. Though we speak of it as pointing always north,
yet it does not always so point exactly. Every day the needle is found
to make certain tiny, delicate motions, as if faintly struggling to
follow the daily movements of the sun--just a little towards the east,
or just a little towards the west. These tiny motions, having been
long watched and measured, were found to go through a regular course
of changes--some years more, and some years less, waxing and waning by
turns. It was discovered that the course of changes from more to less,
and from less to more again, took place in about eleven years.
These two things, you see, were quite independent of one another.
Those who watched the sun-spots were not thinking of the magnets, and
those who watched the magnets were not thinking of the sun-spots. But
somebody did at last happen to think of both together. He was laughed
at, yet he took the trouble carefully to compare the two. And, strange
to say, he found that these two periods in the main agreed--the eleven
years of alternate changes in the number of sun-spots, and the eleven
years of alternate changes in the movements of the magnetic-needle.
When the spots are most, the needle moves most. When the spots are
least, the needle moves least. So much we know. But to explain the why
and the wherefore is beyond our power.
This study of the magnetism of our wandering planet is very
interesting, and one which is still very little known. Here is a weak
needle, a slip of magnetic iron, which, with its restless and agitated
finger, incessantly seeks a region near the north. Carry this needle
in a balloon up to the higher aerial regions, where human life begins
to be extinguished; shut it up in a tomb closely separated from the
light of day; take it down into the pit of a mine, to more than a
thousand yards in depth,--and incessantly, day and night, without
fatigue and without rest, it watches, trembles, throbs, seeks the
point which attracts it across the sky, through the earth, and through
the night. Now--and here is a coincidence truly filled with notes of
interrogation--the years when the oscillation of this innocent little
steel wire is strongest are the years when there are more spots, more
eruptions, more tempests in the sun; and the years when its daily
fluctuations are weakest are those when we see in the day-star neither
spots, eruptions, nor storms. Does there exist, then, a magnetic bond
between the immense solar globe and our wandering abode? Is the sun
magnetic? But the magnetic currents disappear at the temperature of
red-hot iron, and the incandescent focus of light is at a temperature
incomparably higher still. Is it an electrical influx which is
transmitted from the sun to the earth across a space of ninety-three
millions of miles? On September 1, 1859, two astronomers--Carrington
and Hodgson--were observing the sun, independently of each other;
the first on a screen which received the image; the second directly
through a telescope,--when, in a moment, a dazzling flash blazed
out in the midst of a group of spots. This light sparkled for five
minutes above the spots without modifying their form, as if it were
completely independent, and yet it must have been the effect of a
terrible conflagration occurring in the solar atmosphere. Each observer
ascertained the fact separately, and was for an instant dazzled. Now,
here is a surprising coincidence: at the very moment when the sun
appeared inflamed in this region the magnetic instruments of the Kew
Observatory, near London, where they were observing, manifested a
strange agitation; the magnetic needle jumped for more than an hour
as if infatuated. Moreover, a part of the world was on that day and
the following one enveloped in the fires of an aurora borealis, in
Europe as well as in America. It was seen almost everywhere,--at Rome,
at Calcutta, in Cuba, in Australia, and in South America. Violent
magnetic perturbations were manifested, and at several points the
telegraph-lines ceased to act. Why should these two curious events not
be associated with each other? A similar coincidence was observed on
August 3, 1872, by Professor Charles A. Young, of Princeton, N. J.: a
paroxysm in the solar chromosphere, magnetic disturbances everywhere.
There is a very singular appearance seen upon the sun which must not
be passed over without mention. Some astronomers speak of the whole
surface as being _mottled_ all over with a curious rough look when
examined through a powerful telescope. This “mottling” is described by
various observers in various ways. One speaks of “luminous spots shaped
like rice-grains.” Another, of “luminous spots resembling strokes
made with a camel’s-hair pencil.” Another, of “luminous objects or
granules.” Others, of “multitudes of leaves,” “nodules,” “crystalline
shapes,” “leaves or scales, crossing one another in all directions,
like what are called spills in the game of spillikens.” They have also
been pictured as “certain luminous objects of an exceedingly definite
shape and general uniformity of size, whose form is that of the oblong
leaves of a willow-tree.” These cover the whole disk of the sun,
excepting the space occupied by the spots, in countless millions, and
lie crossing each other in every imaginable direction.
In size they are said to be about one thousand miles long, by two
or three hundred broad, but they vary a good deal. Where there is a
spot, the willow-leaves at its edge are said to point pretty regularly
towards the center. Whether they are, as they seem to be, solid in
form; whether they are, as some suppose, the chief source of the
sun’s light and heat; whether they lie on his surface or float in
his atmosphere; what is their real nature, and what is their real
use,--about these questions we are at present quite in the dark.
We have next to think a little more about the edge or limb of the sun,
and the stormy flames and outburst there seen. Until quite lately
the only time for observing such appearances was during a total
eclipse of the sun. Lately, by means of the new instrument called the
“spectroscope,” it has been found possible to take observations when no
eclipse is going on.
[Illustration: SUN-FLAMES, MAY 3, 1892.]
A few words of explanation as to eclipses of the sun seem needful,
before going further. An eclipse of the sun is caused simply by the
round body of the moon passing exactly between the sun and the earth,
so as to hide the sun from us.
Let there be a candle on the table, while you stand near. The rays of
light from the candle fall upon your face. Now move slowly, to and
fro, a round ball between you and the candle. So long as it is not
precisely in the line between--so long as it is a little higher, or
a little lower, or a little to one side--then you can still see the
flame. Once you let the ball come just between the light and your
eyes, and you see it no more. In other words the candle-flame is
eclipsed--hidden, veiled, cut off--by the ball.
It may seem curious at first sight that the moon, which is very small
compared with the sun, should have power to cover the sun. But remember
the difference of the distance. The sun is very far, and the moon
is very near. Any small object very near will easily hide from your
sight a large object at a considerable distance. You may hold up a
shilling-piece at arm’s length, and make it cover from sight a man, or
even a house, if the latter be far enough away. The sun at a distance
of ninety-three millions of miles, and the moon at a distance of two
hundred and forty thousand miles, have to our vision the same seeming
size. So, when the moon glides between, her round face just about
covers the sun’s round face.
If the moon were traveling exactly in the same plane as the plane of
the earth’s orbit, an eclipse would be a very common affair indeed. But
the plane of the moon’s orbit being not quite the same as the plane of
the earth’s orbit, she passes sometimes a little above, and sometimes a
little below, the exact spot where she would hide the sun’s rays from
us. Now and then, at certain intervals, she goes just between. And so
well is the moon’s path in the heavens understood that astronomers can
tell us, long years beforehand, the day and the hour an eclipse will
take place.
An eclipse of the sun is sometimes partial, sometimes total, sometimes
annular. In a partial eclipse, the moon does indeed pass between, but
only so as to hide from us _part_ of the sun. She is a little too
low, or a little too high, to cover his face. In a total eclipse, the
moon covers the sun completely, so that for a few minutes the bright
photosphere seems blotted out from the heavens, a black round body,
surrounded by light, taking its place. In an annular eclipse, the moon
in like manner crosses the sun, but does not succeed in covering him
entirely, a rim of bright photosphere showing round the black moon. For
in an annular eclipse, the moon, being a little farther away from the
earth than at the time of a total eclipse, has too small a disk quite
to hide the sun’s disk.
The blackness of the moon during an eclipse is caused by the fact that
her bright side is turned towards the sun, and her dark side toward us.
An eclipse of the sun can take place only at new moon, never at full
moon. At her full, the moon is outside the earth’s orbit, away from the
sun, and can not by any possibility pass between.
Eclipses, like comets, have always been interpreted as the indication
of inevitable calamities. Human vanity sees the finger of God making
signs to us on the least pretext, as if we were the end and aim of
universal creation. Let us mention, for example, what passed even in
France, with reference to the announcement of an eclipse of the sun, on
August 21, 1560. For one, it presaged a great overthrow of States and
the ruin of Rome; for another, it implied another universal deluge;
for a third, nothing less would result than a conflagration of the
globe; finally, for the less excited, it would infect the air. The
belief in these terrible effects was so general that, by the express
order of the doctors, a multitude of frightened people shut themselves
up in very close cellars, well heated and perfumed, in order to shelter
themselves from these evil influences. Petit relates that the decisive
moment approached, that the consternation was at its height, and that
a parish priest of the country, being no longer able to confess his
parishioners, who believed their last hour had come, was obliged to
tell them in a sermon “not to be so much hurried, seeing that, on
account of the wealth of the penitents, the eclipse had been postponed
for a fortnight.” These good parishioners found no more difficulty in
believing in the postponement of the eclipse than they had in believing
in its unlucky influence.
History relates a crowd of memorable acts on which eclipses have
had the greatest influence. Alexander, before the battle of Arbela,
expected to see his army routed by the appearance of a phenomenon of
this kind. The death of the Athenian general Nicias and the ruin of
his army in Sicily, with which the decline of the Athenians commenced,
had for their cause an eclipse of the moon. We know how Christopher
Columbus, with his little army, threatened with death by famine at
Jamaica, found means of procuring provisions from the natives by
depriving them in the evening of the light of the moon. The eclipse
had scarcely commenced when they supplied him with food. This was the
eclipse of March 1, 1504. We need not relate other facts of this
nature, in which history abounds, and which are known to every one.
Eclipses no longer cause terror to any one, since we know that they
are a natural and inevitable consequence of the combined motions of
the three great celestial bodies--the sun, the earth, and the moon;
especially since we know that these motions are regular and permanent,
and that we can predict, by means of calculation, the eclipses which
will be produced in the future as well as recognize those which have
occurred in the past.
The following description of the total eclipse of 1860 will be found
interesting. Mr. Lowe, who observed it at Santander, Spain, thus writes:
“Before totality commenced, the colors in the sky and on the hills were
magnificent beyond all description. The clear sky in the north assumed
a deep indigo color, while in the west the horizon was first black
like night. In the east the clear sky was very pale blue, with orange
and red, like sunrise. On the shadow creeping across, the deep blue in
the north changed, like magic, to pale, sunrise tints of orange and
red, while the sunrise appearance in the east had changed to indigo.
The darkness was great; the countenances of men were of a livid pink.
The Spaniards lay down, and their children screamed with fear; fowls
hastened to roost, ducks clustered together, pigeons dashed against the
houses; flowers closed; many butterflies flew as if drunk, and at last
disappeared. The air became very humid, so much so that the grass felt
to one of the observers as if recently rained upon.”