THE SUN.
There was once a book published, the title of which was “The Sun, Ruler,
Fire, Light and Life of the Planetary System.” The title was by no means
a bad one, for without doubt the Sun may fairly be said to represent
practically all the ideas conveyed by the designations quoted.
There is certainly no one body in creation which is so emphatically
pre-eminent as the Sun. Whether or no there are stars which are
suns—centres of systems serving in their degree the purposes served by
our Sun, I need not now pause to enquire, though I think the idea is a
very probable one; but of those celestial objects with which our Earth
has a direct relationship, beyond doubt the Sun is unquestionably
entitled to the foremost place. It is, as it were, the pivot on which
the Earth and all the various bodies comprising the Solar System revolve
in their annual progress. It is our source of light and heat, and
therefore may be called the great agent by which an Almighty Providence
wills to sustain animal and vegetable life. The consideration of all the
complicated questions which arise out of these functions of the Sun
belongs to the domain of Physics rather than that of Astronomy; still
these matters are of such momentous interest that an allusion to them
must be made, for they ought not to be lost sight of by the student of
Astronomy. Half a century ago the actual state of our knowledge
respecting the Sun might without difficulty be brought within the
compass of a single chapter in any book on Astronomy, but so enormous
has been the development of knowledge respecting the Sun of late years,
that it is no longer a question of getting the materials properly into
one chapter, but it is a matter of a whole volume being devoted to the
Sun, or even, as in the case of Secchi, of two large octavo volumes of
500 pages each being required to cover the whole ground exhaustively.
The reader will therefore easily understand that in the space at my
disposal in this little work nothing but a passing glimpse can possibly
be obtained of this great subject. It is great not only in regard to the
vast array of purely astronomical facts which are at a writer’s command,
but also on account of the extensive ramifications which the subject has
into the domains of chemistry, photography, optics and cognate sciences.
I shall therefore endeavour to limit myself generally to what an amateur
can see for himself with a small telescope, and can readily understand,
rather than attempt to say a little something about everything, and fail
in the effort.
The mean distance of the Earth from the Sun may be taken to be about 93
millions of miles, and this distance is employed by Astronomers as the
unit by which most other long celestial distances are reckoned. The true
diameter of the Sun is about 866,000 miles. The surface area exceeds
that of the Earth 11,946 times, and the volume is 1,305,000 times
greater. The mass or weight of the Sun is 332,000 times that of the
Earth, or about 700 times that of all the planets put together. Bulk for
bulk the Sun is much lighter than the Earth: whilst a cubic foot of the
Earth on an average weighs rather more than 5 times as much as a cubic
foot of water, a cubic foot of Sun is only about 3½ times the weight of
the same bulk of water. This consideration of the comparative lightness
of the Sun (though in his day the Sun was thought to be lighter than it
is now supposed to be) led Sir J. Herschel to infer that an intense heat
prevails in its interior, independent it may be of its surface heat, so
to speak, of which alone we are directly cognizant by the evidence of
our senses.
The Sun is a sphere, and is surrounded by an extensive but attenuated
envelope, or rather series of envelopes, which taken together bear some
analogy to the atmosphere surrounding the Earth. These envelopes, which
we shall have to consider more in detail presently, throw out rays of
light and heat to the confines of the Solar System, though as to the
conditions and circumstances under which that light and heat are
generated we are entirely ignorant. Of the potency of the Sun’s rays we
can form but a feeble conception, for the amount received by the Earth
is, it has been calculated, but one 2300-millionth of the whole. Our
annual share would, it is supposed, be sufficient to melt a layer of ice
spread uniformly over the Earth to a depth of 100 feet, or to heat an
ocean of fresh water 60 feet deep from freezing point to boiling point.
The illuminating power of the Sun has to be expressed in language of
similar profundity. Thus it has been calculated to equal that which
would be afforded by 5563 wax candles concentrated at a distance of one
foot from the observer. Again, it has been concluded that no fewer than
half a million of full moons shining all at once would be required to
make up a mass of light equal to that of the Sun. I present all these
conclusions to the reader as they are furnished by various physicists
who have investigated such matters, but it is rather uncertain as to how
much reliance can safely be placed on such calculations in detail.
[Illustration: Fig. 5.—Ordinary Sun-spot, June 22, 1885.]
To an amateur possessed of a small telescope, the Sun offers (when the
weather is above the English average of recent years) a very great and
constant variety of matters for studious scrutiny in its so-called
“spots.” To the naked eye, or even on a hasty telescopic glance, the Sun
presents the appearance of a uniform disc of yellowish white colour,
though often a little attention will soon result in the discovery of a
few, or it may be many, little black, or blackish patches, scattered
here and there over the disc seemingly without order or method. We shall
presently find out, however, that this last-named suggestion is wholly
inaccurate. Though commonly called “spots,” these dark appearances are
not simple spots, as the word might imply, for around the rather black
patch which constitutes generally the main feature of the spot there is
almost invariably a fringe of paler tint; whilst within the confines of
the black patch which first catches the eye there is often a nucleus or
inner portion of far more intense depth of shade. The innermost and
darkest portion being termed the _nucleus_, the ordinary black portion
is known as _umbra_, whilst the encompassing fringe is the _penumbra_.
It is not always the case that each individual umbra has a penumbra all
to itself, for several spots are occasionally included in one common
penumbra. And it may further be remarked that cases of an umbra without
a penumbra and the contrary are on record, though these may be termed
exceptional, often having relation to material organic changes either
just commencing or just coming to a conclusion. A marked contrast
subsists in all cases between the luminosity of the penumbra and that of
the general surface of the Sun contiguous. Towards its exterior edge the
penumbra is usually darker than at its inner edge, where it comes in
contact with the umbra. The outline of the penumbra is usually very
irregular, but the umbra, especially in the larger spots, is often of
regular form (comparatively speaking of course) and the nucleus (or
nuclei) of the umbra still more noticeably partakes of a compactness of
outline.
Spots are for the most part confined to a zone extending 35° or so on
each side of the solar equator; and they are neither permanent in their
form nor stationary in their position. In their want of permanence, they
are subject, apparently, to no definite laws, for they frequently appear
and disappear with great suddenness.
Their motions are evidently of a two-fold nature; the Sun itself rotates
on its axis, and the spots collectively participate in this movement of
rotation; but over and above this it has been conclusively proved that
sometimes a spot has a proper motion of translation of its own
independently of the motion which it has in consequence of the Sun’s
axial rotation. Curiously enough, spots are very rare immediately under
the Sun’s equator. It is in the zone extending from 8° to 20° North or
South, as the case may be, that they are most abundant; or, to be more
precise still, their favourite latitude seems to be 17° or 18°. They are
often more numerous and of a greater general size in the northern
hemisphere, to which it may be added that the zone between 11° and 15°
North is particularly noted for large and enduring spots. A gregarious
tendency is often very obvious, and where the groups are very straggling
an imaginary line joining the extreme ends of the group will generally
be found more or less parallel to the solar equator; and not only so,
but extending a long way, or sometimes almost entirely, across the whole
of the visible disc. With respect to the foregoing matters Sir John
Herschel remarked:—“These circumstances ... point evidently to physical
peculiarities in certain parts of the Sun’s body more favourable than in
others to the production of the spots, on the one hand; and on the
other, to a general influence of its rotation on its axis as a
determining cause in their distribution and arrangement, and would
appear indicative of a system of movements in the fluids which
constitute its luminous surface; bearing no remote analogy to our
trade-winds—from whatever cause arising.” More often than not when a
main spot has a train of minor spots as followers that train will be
found extending eastwards from the east side of the spot, rather than in
any other direction.
Spots remain visible for very diverse lengths of time, from the extreme
of a few minutes up to a few months; but a few days up to, say, one
month, may, in a general way, be suggested as their ordinary limits of
endurance. As the Sun rotates on its axis in 25¼ days, and as the spots
may be said to be, practically speaking, fixed or nearly so with respect
to the Sun’s body, no spot can remain continuously visible for more than
about 12½ days, being half the duration of the Sun’s axial rotation.
With regard to their size, spots vary as much as they do in their
duration. The majority of them are telescopic, that is, are only visible
with the aid of a telescope; but instances are not uncommon of spots
sufficiently large to be visible to the naked eye. The ancients knew
nothing about the physical constitution of the Sun, and their few
allusions to the subject were mere guesses of the wildest character.
They were, however, able to notice now and then that when the Sun was
near the horizon certain black spots could sometimes be distinguished
with the naked eye, but they took these for planets in conjunction with
the Sun, or phenomena of unknown origin. Earliest in point of date of
those who have left on record accounts of naked eye sun-spots are
undoubtedly the Chinese. In a species of Cyclopædia ascribed to a
certain Ma-touan-lin (whose records of comets have been of the greatest
possible use to astronomers), we find an account of 45 sun-spots seen
during a period of 904 years, from 301 A. D. to 1205 A. D. In order to
convey an idea of the relative size of the spots, the observers compared
them to eggs, dates, plums, etc., as the case might be. The observations
often extended over several days; some indeed to as many as ten
consecutive days, and there seem no grounds for doubting the
authenticity of the observations thus handed down to us. A few stray
observations of sun-spots were recorded in Europe before the invention
of the telescope. Adelmus, a Benedictine monk, makes mention of a black
spot on the Sun on March 17, 807. It is also stated that such a spot was
seen by Averröes in 1161. Kepler himself seems to have unconsciously
once seen a spot on the Sun with the naked eye, though he supposed he
was looking at a transit of the planet Mercury. None of these early
observers have told us the way in which they made their observations,
but the smallest of boys who has any claim to scientific knowledge is
aware of the fact, that by the use of so simple an expedient as a piece
of glass blackened with smoke, spots which are of sufficient size can be
seen with the naked eye. Before telescopes came into use it was
customary to receive the solar rays in a dark chamber through a little
circular hole cut in a shutter. It was thus that J. Fabricius succeeded
in December 1610 in seeing a considerable spot and following its
movement sufficiently well to enable him to determine roughly the period
of the Sun’s rotation.
The spots may often be easily observed with telescopes of small
dimensions, taking care, however, to place in front of the eye-piece a
piece of strongly-coloured glass. For this purpose glasses of various
colours are used, but none so good as dark green or dark neutral tint.
It is not altogether easy to say positively how large a spot must be for
it to be visible with the naked eye, or an opera glass, but probably it
may be taken generally that no spot of lesser diameter than 1′ of arc
can be so seen. This measurement must be deemed to apply to that central
portion of a normal spot already mentioned as being what is called the
nucleus, because penumbræ may be more than 1′ in diameter without being
visible to the naked eye, for the reason that their shading is so much
less pronounced than the shading of umbræ. Very large and conspicuous
spots are comparatively rare, though during the years 1893 and 1894
there were an unusual number of such spots. It often happens that a
conspicuous group is the result of the merging or joining up of several
smaller groups. In such cases a group may extend over an area on the Sun
3′ or 4′ of arc in length by 2′ or 3′ in breadth. The largest spot on
record seems to have been one seen on September 30, 1858, the length of
which in one direction amounted to more than 140,000 miles.
The observation of spots on the Sun by projecting them on to a white
paper screen with the aid of a telescope is a method so convenient and
so exact as to deserve a detailed description, the more so as it is so
little used. Let there be made in the shutter of a darkened room a hole
so much larger than the diameter of the telescope to be used as will
allow a certain amount of play to the telescope tube, backwards and
forwards, up and down, and from right to left. Direct the telescope to
the Sun and draw out the eye-piece to such a distance from the
object-glass as that the image projected on a white screen held behind
may be sharply defined at its edges. If there are any spots on the Sun
at the time they will then be seen clearly exhibited on the screen. An
image obtained in this way is reversed as compared with the image seen
by looking at the Sun through a telescope directly. If therefore the
telescope is armed with the ordinary astronomical eye-piece, which
inverts, then the projection will be direct, that is to say, on the
screen the N. S. E. and W. points will correspond with the same
terrestrial points. Under such circumstances the spots will be seen to
enter the Sun’s disc on the E. side and to go off on the W. side. The
contrary condition of things would arise if a Galilean telescope or a
terrestrial telescope of any kind were made use of. These instruments
erect the image, and therefore will give by projection a reversed image,
in which we shall see the spots moving apparently in a direction
contrary to their true direction.
If the reader has grasped the broad general outlines now given
respecting the Sun and its spots he will perhaps be interested to learn
a few further details, but these must be presented in a somewhat
disjointed fashion, because the multitude of facts on record concerning
sun-spots are so great as to render a methodical treatment of them
extremely difficult within the limits here imposed on me. These matters
have been gone into in a very exhaustive way by Secchi in his great
treatise on the Sun, and in what follows I have made much use of his
observations.
Let us look a little further into the laws regulating the movement of
the spots. If it is not a question of seeing a spot spring into view,
but of watching one already in existence, we shall, in general, see such
a spot appear on the Eastern limb of the Sun just after having turned
the corner, so to speak. The spots traverse the Sun’s disc in lines
which are apparently oblique with reference to the diurnal movement and
the plane of the ecliptic, and after about 13 days they will disappear
at the Western limb if they have not done so before by reason of
physical changes in their condition. It is not uncommon for a spot after
remaining invisible for 13 days on the other side of the Sun, so to
speak, to reappear on the Eastern limb and make a second passage across
the Sun; sometimes a third, and indeed sometimes even a fourth, passage
may be observed, but more generally they change their form and vanish
altogether either before passing off the visible disc, or whilst they
are on the opposite side as viewed from the Earth.
[Illustration: Fig. 6.—Change of Form in Sun-spots owing to the Sun’s
rotation.]
When several spots appear simultaneously, they describe in the same
period of time similar paths which are sensibly parallel to one another
although they may be in very different latitudes. The conclusion from
this is inevitable, that spots are not bodies independent of the Sun, as
satellites would be, but that they are connected with the Sun’s surface,
and are affected by its movement of rotation. If we make every day for a
few days in succession a drawing of the Sun’s disc with any spots that
are visible duly marked thereon, we shall see that their apparent
progress is rapid near the centre of the Sun, but slow near either limb.
These differences, however, are apparent and not real, for their
movement appears to us to take place along a plane surface, whilst in
reality it takes place along a circle parallel to the solar equator. The
spots in approaching the Sun’s W. limb, if they happen to seem somewhat
circular in form when near the centre, first become oval, and then seem
to contract almost into mere lines. These changes are simple effects of
perspective, and are to be explained in the same manner as the apparent
decrease in the size of many of the spots is often explicable. But this
condition of things proves, however, that the spots belong to the actual
surface of the Sun, for, on a contrary supposition, we should have to
regard them as circular bodies greatly flattened like lozenges, and this
would be contrary to all we know of the forms affected by the heavenly
bodies. Of course besides the apparent changes of form just alluded to
as the effect of perspective, it is abundantly certain that solar spots
often undergo very real changes of form, not only from day to day, but
in the course of a few hours. Several spots will often become
amalgamated into one, and it was ephemeral changes of this character
which hindered generally the early observers from determining with
precision the duration of the Sun’s rotation.
The apparent movements of the spots vary also from month to month during
the year according to the season. In March their paths are very
elongated ellipses with the convexity towards the N., the longer axis of
the ellipse being almost parallel to the ecliptic. After that epoch the
curvature of the ellipse diminishes gradually, at the same time that the
major axis becomes inclined to the ecliptic, so that by June the
flattening of the ellipse has proceeded so far that the path has become
a straight line. Between June and September the elliptical form
reappears but in a reversed position; then, following these reversed
phases, the ellipticity decreases, and for the second time there is an
epoch of straight lines. This happens in December, but the straight
lines are inclined in a converse direction to that which was the case in
June. It must again be impressed on the reader that all these seemingly
different forms of path pursued by the spots are merely effects of
perspective, for in reality, the spots in crossing the Sun’s disc
describe lines which are virtually parallel to the solar equator. These
projections really depend of course on the position of the observer on
the Earth, and vary as his position varies during the Earth’s annual
circuit round the Sun. The number of the spots varies through wide
limits. Sometimes they are so numerous that a single observation will
enable us to recognise the position of the zones of maximum frequency.
Sometimes, on the other hand, they are so scarce, that many weeks may
pass away without hardly one being seen. A remarkable regularity is now
recognised in the succession of these periods of abundance and scarcity,
as we shall see later on.
It is both useful and interesting in studying the spots to record
methodically their number and their size, but it is not easy to teach
observers how to do this so systematically that observations by one
person can be brought into comparison with those of another. Photography
and hand-drawing on a screen alone furnish a trustworthy basis of
operations. Spots in general may naturally be classified into (1)
isolated spots or points, and (2) groups of spots; but often one
observer will describe as a small spot an object which another observer
would regard as a mere point; and one observer will record several
groups where another observer will see but one. A very few days’
experience with a telescope will bring home to the observer’s mind the
difficulty of dealing with the spots where it is a question of
systematic methodical observation of them.
Let us now take a brief survey of some of the theories which have been
put forth regarding the nature of the spots on the Sun. In the early
days of the telescope, that is to say, during the 17th century, two
general ideas were current. Some thought the spots to be shapeless
satellites revolving round the Sun; others that they were clouds, or
aggregations of smoke, floating about in a solar atmosphere. Scheiner,
the author of the first theory, abandoned it towards the close of his
life, having arrived at the conclusion that the spots were situated
below the general level of the Sun’s surface. Another idea, but of later
date, was that the Sun is a liquid and incandescent mass of matter, and
the spots immense fragments of _Scoriæ_, or clinkers, floating upon an
ocean of fire.
Somewhat more than a century after the spots had been generally studied
with the aid of a telescope a Scotchman named Wilson made a memorable
discovery. He showed by the clearest evidence that they are cavities,
and he propounded the first intelligible idea of the true physical
constitution of the Sun, when he compared to a strongly illuminated
cloud the luminous layer of solar material which we now term the
“photosphere.” On November 22, 1769, he observed on the Sun’s disc a
fine round spot encompassed by a penumbra, also circular, and concentric
with the nucleus. He watched that spot up to the time that it
disappeared, and he soon remarked that the penumbra ceased to be
symmetrical: the part turned towards the centre of the Sun became
smaller and smaller, and eventually disappeared altogether; whilst the
part on the opposite side preserved its fulness and dimensions almost
unchanged. Let us suppose we chanced to turn a telescope on to the Sun
on a given day, and were fortunate enough to discover a spot in the
centre of the disc, with a penumbra concentric with the nucleus. When
such a spot arrives about midway towards the limb, it will exhibit a
penumbra narrower on the left side than on the right; later on the
penumbra will disappear almost or quite completely on the left side:
then the nucleus itself will seem to be encroached upon. Finally, very
near the limb, there will remain only a slender thread of penumbra, and
the nucleus will have ceased to be directly visible. Such were the
phases of transformation observed by Wilson and often studied since.
Wilson suspected that he had come upon some great law that was ripe for
disclosure, and in order not to be misled he waited for the return of
the same spot, which indeed reappeared on the Sun’s W. limb after about
14 days. Then he found himself face to face with the same phases
reproduced, but in the reverse order: the penumbra contracted on one
side and full on the other, widening out on the contracted side as the
spot came up to the Sun’s centre. Henceforth doubt was no longer
possible; the spot had sensibly preserved the same shape during its
passage, and the alterations noticed were only apparent, and resulted
from an effect of perspective which was easy to be understood. The
different phases presented by such a spot as that just spoken of will be
so much the more sensible according as the depth of the cavity is
greater; but if the depth is inconsiderable the bottom of the cavity
will only disappear when a very oblique angle is attained, and this
cannot happen except when the spot is very near to the limb. By
observations carefully made under such circumstances it will be possible
to determine the depth of the cavity, and Wilson found that the depth of
a spot often amounted to about one-third of the Earth’s radius. Wilson’s
theory was not accepted without dispute; it was contested by several
astronomers, and in particular by Lalande. It was however taken up by
Sir W. Herschel, and as modified by him has met with general acceptance
down to the present time; though now and again challenged, perhaps most
recently and most vehemently by Howlett, a sun spot observer of great
experience. Wilson’s discovery was the point of departure for the grand
labours of Sir W. Herschel in the field of Solar Physics. Man of genius
that Herschel was, he was above all things an observer who took his own
line in what he did. He saw so many phenomena with the powerful
instruments constructed by himself, he described so minutely the marvels
which were revealed to him, that he left comparatively little for his
successors to do so far as regards mere telescopic observation.
Herschel’s main idea as to the Sun was based on Wilson’s discovery. He
remarked with reason, as that astronomer had done, that if the spots are
cavities the luminous matter could neither be properly called liquid nor
gaseous; for then it would precipitate itself with frightful rapidity to
fill up the void, and that would render it impossible that the spots
should endure as we often see they do during several revolutions of the
Sun. Moreover, the proper movements of the spots prove that the
photosphere is not solid. We can therefore only liken it to fogs or
clouds, and it must be suspended in an atmosphere similar to ours. Such
is, according to Herschel, the only hypothesis which can explain the
rapid changes which we witness. We shall see a little later on that
these phenomena do admit of another explanation.
In a second memoir Herschel followed up this inquiry with an acuteness
worthy of his genius. Unfortunately he allowed himself to be carried
away with the idea that the Sun was inhabited in order to sustain this
theory. He needed a solid kernel upon which his imaginary inhabitants
could dwell; and also a means whereby he could protect them from the
radiations of the photosphere. With this idea in view he conjectured the
existence above the Sun’s solid body of a layer of clouds always
contiguous to the photosphere which enveloped it, and which always being
rent when the photosphere was rent, thus enabled us to see the solid
body of the Sun lying behind. These notions can only be described as
very arbitrary, as unsupported by observation, and as involving
explanations quite out of harmony with the principles of modern physics.
However, the labours of Herschel resulted in so many positive
discoveries of visible facts, and in so many just conclusions, that they
contributed greatly to the growth of our present knowledge of the true
constitution of the Sun.
Since Wilson’s time, as Secchi pointedly remarks, astronomers generally
have verified his observations with good instruments, and by an
investigation of a great number of spots. De La Rue, discussing the Kew
observations, found that of 89 regular spots 72 gave results which
conformed to Wilson’s ideas, whilst the remaining 17 were opposed
thereto. There is nothing surprising in the existence of a contrarient
minority when we consider the great changes which in reality often occur
in the forms of the spots. De La Rue suggested a very simple expedient
for showing that the spots are cavities. Take two photographs of the Sun
made at an interval of one day: during that time every point on the
Sun’s surface will have been displaced, so far as the telescope is
concerned, by about 15°. Place these photographs in a stereoscope, and
we shall readily see the interior cavity, the edges of which will appear
raised above the photosphere. It is impossible therefore to entertain
the least doubt as to the truth of the theory that the spots are
excavations in the luminous stratum which envelopes the whole of the
solar globe.
If it be true that a spot is a cavity, it follows that when it reaches
the margin of the solar disc we ought to detect a hollow place; and this
will be so much the more easy to observe according as the cavity is
larger and deeper. As a matter of fact, numerous observations of this
sort have been recorded from the time of Cassini down to the present
time under the designation of “notches” on the Sun’s limb. On July 8,
1873, Secchi observed such a notch 8″, or 3600 miles deep.
Faye and some other astronomers are disposed to support a theory
according to which the spots are nothing else than aërial cyclones, but
this does not seem admissible. If the fundamental principle of a spot is
that it arises from a whirling movement, the rays (so to speak) which
compose the penumbræ must always be crooked, or the theory falls to the
ground. It is quite true that indications of cyclonic action do
sometimes appear, but they are at any rate very rare, for only a small
percentage exhibit in a distinct manner a spiral structure. Moreover,
when such a structure is seen it does not endure for the whole lifetime
of the spot but only for a day or two: the spot may last a long time
after it has lost its spiral features, if it ever had any. Sometimes
even the whirling movement, after having slackened, begins again, but in
the contrary direction. Under these circumstances, though this
occasional spiral structure is very curious and interesting, we are not
justified in taking it as the basis of a theory which has any
pretensions to explain the general nature of sun-spots.
[Illustration: Fig. 7.—Sun-spot seen as a Notch.]
When we examine the Sun with instruments of large aperture and high
magnifying power, we notice that its surface is far from being as smooth
and uniform as it appears in a small telescope. On the contrary, it
presents an irregular undulating appearance like a pond or other sheet
of water agitated by the wind. Careful scrutiny with a powerful
eye-piece reveals the fact that the Sun’s surface is marked by a
multitude of wrinkles and irregularities which it is well-nigh
impossible to describe in words. More or less everywhere there is a
general mottling visible; it is more distinct in some places than
others, and especially so towards the centre of the disc. This peculiar
appearance varies very much from time to time, and its distinctness
seems to depend a great deal on the state of the Earth’s atmosphere, for
it becomes invisible when the air is disturbed; but these variations
depend also on real variations of the photosphere—a fact which
observations made in very calm weather are thought clearly to indicate.
It is often said that the Sun exhibits a granulated structure. If we
wish to realise in the most precise manner what is meant by the word
“granulation” as applied to the structure of the Sun, we must abandon
the method of projection and examine the Sun directly with a powerful
eyepiece, taking advantage of a moment when the atmosphere is perfectly
calm, and before the eyepiece has had time to get hot. It may then be
seen that the Sun’s surface is covered with a multitude of little
grains, nearly all of about the same size, but of different shape,
though for the most part more or less oval. The small interstices which
separate these grains form a network which is dark without being
positively black. Secchi considered it difficult to name any known
object which exactly answers in appearance to this structure, but he
thought that we can find something resembling it in examining with a
microscope milk which has been a little dried up, and the globules of
which have lost their regular form. Exceptionally good atmospheric
conditions are under all circumstances indispensable for the study of
these details.
In point of fact, there is a mysterious uncertainty about the normal
condition of the Sun’s surface, in a visual sense, which a few years ago
engendered a very vehement controversy, and led to the use of such
expressions as “willow leaves,” “rice grains,” “sea beach,” and “straw
thatching,” to indicate what was seen. All these words are too precise
to be quite suitable to be taken literally, but perhaps, on the whole,
“rice grains” is not altogether a bad expression to recall what
certainly seems to be the granular surface of the Sun as we see it.
By making use of moderate magnifying powers, what we see will often
convey the impression of a multitude of white points on a black network.
This is very apparent during the first few moments that the telescope is
brought to bear on the Sun, but its clearness quickly passes away
because the eye gets fatigued, and the lenses becoming warm the air in
the telescope tube gets disturbed because also warmed. Sometimes the
appearance is a little different from that just described, and along
with the white and brilliant points little black holes are intermixed.
Oftentimes the grains appear as if suspended in a black network and
heaped together in knots more or less shaded and more or less broad.
Sometimes the grains exhibit a very elongated form, especially in the
neighbourhood of the spots. It is these elongated forms to which Nasmyth
applied the term “willow leaf,” whilst Huggins thought “rice grains” a
very suitable expression.
This granular or leaf-like structure—call it what we will—cannot be made
out except with considerable optical assistance, for the grains being
intrinsically very small, diffraction in enlarging them and causing them
to encroach on one another necessarily produces a general confusion of
image. The real dimensions of these grains cannot therefore readily be
determined by direct measurement, but by comparing them with the wires
used in micrometer eye-pieces it has been thought that their diameters
may usually be regarded as equal to ¼ or ⅓ of a second—say from 120 to
150 miles. The granules seem to be possessed of sensible movement, but
presumably it is not always or even generally a movement of translation
from place to place; only an undulatory movement like that of still
water when a stone is cast into it. Nevertheless, probably in certain
cases the granules actually are affected by a motion of translation, for
in the vicinity of spots they may sometimes be seen flowing over the
edges of the penumbræ. In order to explain the existence of the granules
the strangest theories have been broached. Sir William Herschel having
observed the granulations, applied to them the term “corrugations” or
“furrows”—words somewhat inexact, perhaps, but by which, as his
descriptions clearly show, he meant to designate the features which I am
now treating of. He even noticed the dark network which separates the
grains, and he applied to it the word “indentations.”
These granulations are without doubt prominences, probably of hydrogen
gas, which rise above the general surface, for this structure is much
more sharp and distinct at the centre of the sun’s disc than at the
limbs; that is to say, near the limbs of the Sun they partially overlap
one another, as indeed Herschel remarked. The idea of flames would
satisfy these appearances: and as the spectroscope suggests to us that
the Sun is habitually covered over with a multitude of little jets of
flame, the observations which have been made compel the opinion that the
grains are the summits of those prominences which exist all over the
Sun’s surface.
The surface is sometimes so thickly covered over with these
granulations—the network is so conspicuous—that we can readily imagine
that we see everywhere pores and the beginnings of spots, but this
aspect is not permanent, and seems to depend to some extent on
atmospheric causes combined also with actual changes in the Sun’s
surface itself. There seems however no doubt that the joints, so to
speak, of the dark network already referred to do sometimes burst
asunder and develope into spots.
The circumstances which accompany the formation of a spot cannot readily
be specified with certainty. It is impossible to say that there exists
any law as to this matter. Whilst some spots develope very slowly by the
expansion of certain pores, others spring into existence quite suddenly.
Yet it cannot be said that the formation of a spot is ever completely
instantaneous however rapid it may be. The phenomenon is often announced
some days in advance: we may perceive in the photosphere a great
agitation which often manifests itself by some very brilliant _faculæ_
(to be described presently) giving birth to one or more pores. Very
often we next notice some groups of little black spots, as if the
luminous stratum was becoming thinner in such a way as to disappear
little by little and leave a large black nucleus uncovered. At the
commencement of the business there is usually no clearly defined
penumbra. This developes itself gradually and acquires a regular
outline, just as the spot itself often takes a somewhat circular form.
This tranquil and peaceable formation of a spot only happens at a time
when calm seems to reign in the solar atmosphere: in general the
development is more tumultuous and the stages more complicated.
As a rule a spot passes through three stages of existence:—(1) the
Period of birth; (2) a Period of calm; and (3) the Period of
dissolution. When a spot is on the point of closing up, the flow of the
luminous matter which it, as it were, attracts, is not directed
uniformly towards the centre; it seems that the photospheric masses, no
longer meeting with resistance, are precipitated promiscuously anywhere
so as to fill up the hole. It is impossible to describe in detail the
phases which irregular spots go through, but two things may always be
remarked: that their structure is characterized by the existence of
luminous filaments, and that these filaments converge towards one or
several centres.
Secchi thus sums up certain conclusions which he arrived at relating to
spots generally:—(1) It is not on the surface of any solid body that the
solar spots are manifested; they are produced in a fluid mass, the
fluidity of which is represented by a gas, so that the constitution of
this medium may be likened to that of flames or clouds; (2) the known
details respecting the constitution of the penumbra and the phenomena
exhibited prove that the penumbra is not a mass of obscure matter which
floats across luminous matter, but that it is on the contrary a case of
luminous matter invading and floating about over darker materials and so
producing a half tint.
All the available evidence which we possess may be said to show that the
spots are not merely superficial appearances, but that they have their
origin deep in the interior of the Sun, and are produced by the
operation of causes still unknown to us which affect and disturb the
Sun’s mass to an extent which is sometimes very considerable. The spots
then are only the results of a great agitation in the materials of which
the Sun is composed, and this agitation extends far down below the
limits of the visible dark nucleus whatever that may consist of.
Besides the spots, streaks of light may frequently be remarked upon the
surface of the Sun towards the margin of the disc. These are termed
_faculæ_ (torches), and they are often found near the spots, or where
spots have previously existed or have afterwards appeared. When quite
near the Sun’s limb these faculæ are usually more or less parallel to
the limb. They are of irregular form and may be likened to certain kinds
of coral. They generally appear to be more luminous than the solar
surface immediately adjacent to them, but it is not improbable that this
is an optical illusion depending upon the fact that the edges of the Sun
always appear much more luminous than the centre. This last-named fact
may be readily recognised by the employment of a high magnifying power,
and moving the telescope rapidly from the limb to the centre of the
disc. If the Sun be projected on a screen, as already mentioned, this
degradation of the Sun’s light from centre to circumference becomes
particularly manifest.
After having studied the structure and the movement of the spots, one is
naturally led to ask if their apparitions at different periods are
subject to any general law. This question is one which has much engaged
the attention of modern astronomers. The older observers noticed that
the number of the spots visible differed in different years. There were
said to have been periods when months and even years passed away without
any spots being observed. Even allowing that this statement, so far as
“years” are concerned, might be exaggerated, and that the absence of
spots was due to the want of sufficient care in making the observations,
and especially to the want of efficient instruments, it is none the less
true that the number of the spots is extremely variable, and that there
have been epochs when they were very scarce.
Sir W. Herschel was the first who devoted himself to the question of
seeking to establish a relation between the variation of the spots and
terrestrial meteorology. For the want of any better object, he compared
the annual number of the spots with the price of wheat; but it is easy
to see that nothing could result from such a comparison. Without doubt
the meteorological phenomena of the globe must depend to some extent on
solar changes: but the term of comparison selected by Herschel had no
direct bearing on the state of the Sun.
In our time this question has been investigated to its very foundation
by Wolf, Director for many years at the Observatory of Zurich. It is to
his zeal that we owe a very interesting assemblage of old observations
which were buried in archives and chronicles. It was he who endeavoured
to reduce them into a systematic form, so as to supply as far as
possible the numerous gaps which exist in the different series.
The two most attentive observers at the period when the spots were
discovered were Marriott at Oxford and Scheiner at Ingoldstadt, but
Scheiner himself has informed us that he did not note down all the spots
which he saw; he only recorded those which were likely to assist him in
his special task of determining the period of the Sun’s rotation.
Several observers after him made isolated series of observations; but
some of these have been lost and the others show important gaps. J. G.
Staudacher, at Nuremburg, observed the Sun with great perseverance
during fifty years from 1749 to 1799. Before him the Cassinis, Maraldi,
and others were engaged in the same sort of work, but only in an
indirect way: that is to say, they contented themselves, whilst making
meridional observations of the Sun, with noting anything in the way of
spots which they deemed important. Zucconi and Flaugergues also left
behind them a good collection of observations which Wolf utilised,
rendering them comparable one with another by applying suitable
corrections. The great difficulty herein arises from the fact that the
observers were not provided with instruments of equal power; one man,
armed with a better telescope than his contemporaries, naturally
observed and recorded spots which would escape the others. The numbers
entered in their registers are therefore not comparable _inter se_. Wolf
endeavoured to replace these numbers by others which would represent the
spots which might have been seen if the observers had all employed
telescopes of a given kind and power. The result of his efforts in this
direction is an almost continuous series of Sun-spot records from an
epoch sufficiently remote, up to the time when this branch of science
was taken up with the vigour of modern scientific methods.
The observer who most assiduously devoted himself to this subject in
modern times was Schwabe of Dessau. From 1826 to 1868 he never failed to
make daily observations when the weather permitted him. His series of
records is specially valuable, for Carrington’s fits in with it, and
with that in turn Spörer’s is comparable, and the chain is complete by
the later photographic and other observations. All these Sun-spot
records, though differing in their details, may easily be used together
when it is a question of working out relative annual fluctuations.
At the present time there are many Astronomers who are engaged in
observing the spots with care; but just as formerly there are few who
possess sufficient perseverance. The photographic method is excellent,
but it takes much time and is costly. Some have decried, in a very
unreasonable manner, a drawing made by hand: such a drawing, of
sufficient size, and executed by projection by a skilful draughtsman
with a telescope driven by clockwork, may stand comparison with a
photograph, and this method has a better chance of being persevered in.
The Rev. F. Howlett’s name must be mentioned in this connection as a
draughtsman who has accomplished much by hand drawing. Though the once
famous Kew observations have been discontinued, they have been replaced
by a new series at Greenwich with similar appliances; whilst Janssen at
Meudon has also been carrying on for a number of years a splendid course
of photographic records.
Schwabe, when he had collected a considerable number of observations,
recognised clear indications of periodicity. Very definite epochs of
maxima and minima succeeded one another at intervals of 10 or 11 years.
It is true that in following out such a study the observations are
certain to be in a sense a little defective. At first it was not
possible to observe the Sun every day, and the gaps which resulted from
bad weather necessarily added to the number of days which had to be set
down as being without spots. Moreover, every method of numbering the
spots must be a little arbitrary: there are often groups which, in
consequence of their sub-divisions, may be counted in different ways:
but in a mass of observations so considerable as those of Schwabe’s,
such uncertainties will compensate for one another and will disappear in
the final result. In fact the law is so striking that it suffices to
cast one’s eye over his table[2] to see that.
That table is both interesting and instructive at the same time. The
numbers exhibited in it speak for themselves, and it is sufficient to
examine them with even a small amount of attention to realise the
certainty of the conclusions which have been drawn.
It is therefore now to be deemed an ascertained fact that there are
periodical maxima and minima in the display of spots, and that the
extent of the period is between 10 and 12 years. In order to determine
this value with the utmost exactness, some astronomers have had recourse
to early observations. Wolf of Zurich made this the subject of some very
interesting inquiries. He was able to establish the chronology of the
phases which the Sun has passed through from the time of the first
discovery of the spots to the present day—more than 2½ centuries. His
calculations led him to a period of 11-1/9 years. Lamont fixed upon
10.43 years, but this number does not represent the more recent
observations with sufficient precision.
In order to exhibit this law in the plainest possible manner the dates
of maxima and minima should be laid down on ruled paper in proper
mathematical form, the _abscissæ_ of the curve representing the years,
and the _ordinates_ the number of spots observed.
An examination of a curve thus plotted shows two things:—(1) That the
period is clearly an eleven-year one, as has been already stated; (2)
that it is not however quite as simple in its form as it was at first
thought to be; for in reality there are two periods superposed, the one
rather more than half a century long, and the other extending over the
11 years already spoken of. We do not possess early observations
sufficiently numerous and sufficiently good to enable us to draw any
unimpeachable conclusions as to the nature of the long period; we can
only be certain that it exists. The later labours of Wolf, however,
fixed that period at 55½ years. It is a result of this that, according
to Loomis, a period of comparative calm on the Sun existed between 1810
and 1825.
Each maximum lies nearer to the minimum which precedes it than to the
minimum which follows it, for the spots increase during 3.7 years, and
then diminish during 7.4 years. According to De La Rue the increase
occupies 3.52 years, and diminution 7.55 years. This concurrence between
De La Rue and Wolf is surprising considering the diversity of the
methods which led to results almost identical, the one set being based
on the number of the spots, and the other on the superficial extent of
the spots. The different periods in succession are not absolutely
identical: but it has been remarked that if during any one period the
decrease is retarded or accelerated, then the increase next following
will be lengthened or contracted to a corresponding extent. In
consequence of this we are sometimes able to predict with fair accuracy
when the next ensuing maximum or minimum will take place.
The most striking feature of such a curve as that just alluded to is the
very sensible secondary augmentation which happens very soon after the
principal maximum.
A very curious circumstance has come to light in connection with the
epochs of maxima and minima. In arranging the spots according to their
latitude and longitude on a diagram sufficiently contracted, Carrington
found that their latitude decreases gradually as the period of minimum
draws near; then when their number begins to increase they begin to
appear again at a higher latitude. This seems to be a definite law. At
any rate Carrington’s conclusion has been found to hold good by the
observations of Spörer and Secchi.
The variations of the spots which we now recognise naturally recall
those obscurations of the Sun which are recorded in history; but it is
necessary to accept many of these with caution. A great number of these
phenomena which attracted the attention of people in early times are
only eclipses badly observed and still more badly described. In other
instances the obscuration has been produced by very protracted dry fogs.
It is probably to this last-named cause that we must ascribe the
obscuration which, according to Kepler and Gemma Frisius, took place in
1547.
It was in some such way as this that, according to Virgil (Georg. i,
630), who has echoed a tradition which he found in history, the Sun was
obscured at the death of Cæsar:—
Ille etiam extincto miseratus Cæsare Romam
Quum caput obscura nitidum ferrugine texit,
Impiaque æternam timuerunt sæcula noctem.
In the year 553 A.D., and again in the year 626 A.D. the Sun remained
obscured for several months; but these facts (if facts they are) besides
being ill-observed, and clothed, no doubt, in extremely exaggerated
language, are brought to our notice as having occurred at epochs which
are quite independent of one another, whilst the variations in the
markings on the Sun, which we have just been talking about, present an
almost mathematical regularity of sequence.
We must now institute some inquiries as to the causes of the periodicity
of the spots. A periodicity so well established would naturally invite
astronomers to seek the causes which produced it. The presence of spots
only in the Zodiacal regions led Galileo to suspect the existence of
some relation between the spots and the position of the planets; but
there is in this a mere surmise, which, when it was made, had nothing to
justify it, and it is still impossible for us to say anything for
certain on the point. The determining cause of the periodicity may exist
in the interior of the Sun, and may depend on circumstances which will
for ever remain unknown to us. Or it may be something external: it may
be due after all to the influence of the planets. It remains for us,
therefore, to search and see if any such influence can be traced.
According to Wolf, the attraction of the planets, or of some of them, is
the real cause of the periodicity which we are dealing with; that
attraction producing on the surface of the solar globe true tides, which
give birth to the spots, these tides themselves experiencing periodic
variations owing to the periodic changes of position of the celestial
bodies which cause them. It has even been thought safe to assert that
the fact of the principal period coinciding with the revolution of
Jupiter is of momentous significance; but this coincidence seems purely
accidental, and no certain conclusion can be drawn as to this matter.
The influence of Mercury and Venus would perhaps be much more potent,
for their distance from the Sun is not very great, and this should
render their influence more sensible. On the other hand, their masses
appear to be too small to be capable of producing any sufficient effect.
De La Rue, Balfour Stewart, and Löwy most perseveringly studied this
point of solar physics. They seem to have arrived at the conclusion that
the conjunctions of Venus and Jupiter do exercise a certain amount of
influence on the number of the spots and on their latitude; and that
this influence is less considerable when Venus is situated in the plane
of the solar equator. At any rate it is a fact, that a great number of
the visible inequalities in a duly plotted curve of the spots do really
correspond to special positions of these two planets.
In order to determine with more precision these coincidences and the
importance which attaches to them, De La Rue extended his inquiries. He
separately analysed many different groups of spots, selecting for his
purpose more particularly those of which the observations happened to
have been specially continuous and complete, giving a preference
moreover to those which had been observed in the central portions of the
Sun’s disc. From an investigation of 794 groups De La Rue arrived at the
following conclusions:—(1) If we take a meridian passing through the
middle of the disc and represented by a diameter perpendicular to the
equator, we find that the mean size of the spots is not the same with
regard to that meridian. It appears certain that the correction required
for perspective does not suffice to explain this difference; and that
another element must be introduced in order to secure that the apparent
dimensions of the spots may be the same on both sides. We do not yet
possess a very clear explanation of this fact; but the most probable is
this:—the spots are surrounded by a projecting bank, which seems to
disappear in part during their transit across the Sun. This bank is more
elevated on the preceding than on the following side; accordingly, the
spots ought to seem smaller when they are in the eastern half of the
disc; larger when they are in the western half; for in the first
position the observer’s eye meets an elevated obstacle, which hides a
portion of the spot itself. (2) De La Rue specially studied the spots
observed at the times when the planets Venus and Mars were at a
heliocentric distance from the Earth equal to 0, 90, 180, and 270
degrees, and arrived at this result; the spots are larger in the part of
the Sun which is away from Venus and Mars, and they are smaller on the
side on which these planets happen to be. The same result was obtained,
whether Carrington’s figures or the Kew photographs were employed. (3)
Meanwhile it does not appear that Jupiter emits any similar influence.
This influence should be easily perceived, for if we calculate the
action of the planets in the way that we calculate the tides, treating
it as directly proportional to the masses and inversely proportional to
the cubes of the distances, the influence of Jupiter should greatly
outweigh that of Venus.
Wolf thought that he had noticed traces of some influence being exerted
by Saturn; but this remains altogether without confirmation.
De La Rue noticed that large spots are generally situated at extremities
of the same diameter. This law also often applies to the development of
large prominences. The coincidence agrees well with the theory that
there exists on the Sun some action resembling that of our tides.
Whatever may be the amount of probability which attaches to these
explanations we ought not to forget that we are still far off from
possessing the power of giving a vigorous demonstration of them. If we
consider with attention the periodical variations of the spots we shall
not be long in coming to the conclusion that it is impossible to connect
them directly with any one astronomical function in particular, for the
spots appear in a sudden and irregular manner which contrasts in a
striking degree with the continuous and progressive action of the
ordinary perturbations which we meet with in the study of Celestial
Mechanics. There is but one reply possible to this objection. The spots
and their changes must be visible manifestations of the periodical
activity of the Sun—an activity which itself depends (as assumed) on the
action of the planets and on their relative positions. The cause, thus
defined, of the Sun’s activity may be very regular; the activity itself
may vary in a continuous manner without the resulting phenomena
possessing the same continuity and the same regularity. We see this in
the periodical succession of the Seasons on the Earth. The position of
the Sun, and consequently its manner of acting upon our globe, varies
with a remarkable uniformity, but nevertheless the meteorological
phenomena which result are irregular and capricious. Thus it comes about
that physicists are more and more inclined to believe that the spots are
only secondary effects produced by causes more important and more
fundamental.
Whatever may be our ignorance as to the causes which produce variations
in the Sun’s activity we may at least draw one conclusion from the
preceding remarks: it is, that the Sun is a very long way from having
arrived at a state of tranquillity and freedom from internal commotion.
On the contrary, it is the seat of great movements. Its activity is
subject to numberless periodical changes which ought in their turn to
influence the intensity of the heat and light given out by the Sun; and
so re-act on the planets which receive their heat, light, and life from
the Sun.
No account of the periodicity of the spots on the Sun can be deemed
complete which does not include information respecting certain other
periodical phenomena which have been found to exhibit features of
alternation closely resembling in their sequence and character the
periodical changes which take place in regard to the spots on the Sun.
There is evidently a deep mystery lying hid under the curious fact
(which is clearly established) that the 11-year period of the spots
coincides in a manner as unexpected as it is certain with the period of
the variation of terrestrial magnetism. The magnetic needle is subject
to a diurnal variation which reaches its extreme amount every 11 years,
and not only so, but the epoch of maximum variation corresponds with the
epoch of the maximum prevalence of Sun spots. And similarly years in
which the needle is least disturbed are also years in which the Sun
spots are fewest. Two other very curious discoveries have also been made
which are in evident close connection with the foregoing. The
manifestation of the Aurora Borealis and of those strange currents of
electricity known as magnetic earth currents (which travel below the
Earth’s surface and frequently interfere with telegraphic operations),
likewise exhibit periodical changes which take 11 years to go through
all their stages. This fact alone would be sufficiently curious, but
when we come to find that the curve which exhibits the changes these two
manifestations of force go through, also shows that their maxima and
minima are contemporaneous with the maxima and minima of the Sun spots
and magnetic needle variations, we cannot doubt that (to use Balfour
Stewart’s words) “a bond of union exists between these four phenomena.
The question next arises, what is the nature of this bond? Now, with
respect to that which connects Sun spots with magnetic disturbances we
can as yet form no conjecture.” To cut a long story short, it may be
said generally that whilst without doubt electricity is the common basis
of the three last-named of the four phenomena just mentioned, it seems
scarcely too great a stretch of the imagination to go one step further
and suggest that electricity has in some or other occult manner
something to do with all these things and therefore with the spots on
the Sun.
[Illustration: Fig. 8.—The Sun totally eclipsed, July 18, 1860
(Feilitzsch).]
The reader who has followed me thus far will by this time be in a
position to appreciate a remark made in an earlier part of this chapter,
that the multitude of facts known to us in connection with the Sun and
its spots is so great, as to render it impossible to exhibit in a single
chapter anything more than the barest outline of them. The numerous
observations of recent eclipses of the Sun, especially since that of
1860, and the extensive application of the spectroscope to the Sun both
in connection with these eclipses, and generally, may be said to have
completely revolutionised our knowledge of solar phenomena during the
present generation; or perhaps it might be more correct to say have
enormously increased our knowledge of the facts of the case and have
revolutionised in no small degree the conclusions deduced from the
facts.