THE SUN
From its comparative nearness, its brightness and size, and its
supreme importance to ourselves, the sun commands our attention; and
in the phenomena which it presents there is found a source of abundant
and constantly varying interest. Observation of these phenomena can
only be conducted, however, after due precautions have been taken. Few
people have any idea of the intense glow of the solar light and heat
when concentrated by the object-glass of even a small telescope, and
care must be exercised lest irreparable damage be done to the eye.
Galileo is said to have finally blinded himself altogether, and
Sir William Herschel to have seriously impaired his sight by solar
observation. No danger need be feared if one or other of the common
precautions be adopted, and some of these will be shortly described;
but before we consider these and the means of applying them, let us
gather together briefly the main facts about the sun itself.
Our sun, then, is a body of about 866,000 miles in diameter, and
situated at a distance of some 92,700,000 miles from us. In bulk
it equals 1,300,000 of our world, while it would take about 332,000
earths to weigh it down. Its density, as can be seen from these
figures, is very small indeed. Bulk for bulk, it is considerably
lighter than the earth; in fact, it is not very much denser than
water, and this has very considerable bearing upon our ideas of its
constitution.
Natural operations are carried on in this immense globe upon a scale
which it is almost impossible for us to realize. A few illustrations
gathered from Young's interesting volume, 'The Sun,' may help to
make clearer to us the scale of the ruling body of our system. Some
conception of the immensity of its distance from us may first be
gained from Professor Mendenhall's whimsical illustration. Sensation,
according to Helmholtz's experiments, travels at a rate of about 100
feet per second. If, then, an infant were born with an arm long enough
to reach to the sun, and if on his birthday he were to exercise this
amazing limb by putting his finger upon the solar surface, he would
die in blissful ignorance of the fact that he had been burned, for
the sensation of burning would take 150 years to travel along that
stupendous arm. Were the sun hollowed out like a gigantic indiarubber
ball and the earth placed at its centre, the enclosing shell would
appear like a far distant sky to us. Our moon would have room to
circle within this shell at its present distance of 240,000 miles, and
there would still be room for another satellite to move in an orbit
exterior to that of the moon at a further distance of more than
190,000 miles. The attractive power of this great body is no less
amazing than its bulk. It has been calculated that were the attractive
power which keeps our earth coursing in its orbit round the sun to
cease, and to be replaced by a material bond consisting of steel wires
of a thickness equal to that of the heaviest telegraph-wires, these
would require to cover the whole sunward side of our globe in the
proportion of nine to each square inch. The force of gravity at the
solar surface is such that a man who on the earth weighs 10 stone
would, if transported to the sun, weigh nearly 2 tons, and, if he
remained of the same strength as on earth, would be crushed by his own
weight.
[Illustration:
PLATE V.
The Sun, February 3, 1905. Royal Observatory, Greenwich.]
The first telescopic view of the sun is apt, it must be confessed,
to be a disappointment. The moon is certainly a much more attractive
subject for a casual glance. Its craters and mountain ranges catch
the eye at once, while the solar disc presents an appearance of almost
unbroken uniformity. Soon, however, it will become evident that the
uniformity is only apparent. Generally speaking, the surface will
quickly be seen to be broken up by one or more dark spots (Plate V.),
which present an apparently black centre and a sort of grey shading
round about this centre. The margin of the disc will be seen to be
notably less bright than its central portions; and near the margin,
and oftenest, though not invariably, in connection with one of the
dark spots, there will be markings of a brilliant white, and often of
a fantastically branched shape, which seem elevated above the general
surface; while as the eye becomes more used to its work it will be
found that even a small telescope brings out a kind of mottled or
curdled appearance over the whole disc. This last feature may often
be more readily seen by moving the telescope so as to cause the solar
image to sweep across the field of view, or by gently tapping the
tube so as to cause a slight vibration. Specks of dirt which may have
gathered upon the field lens of the eye-piece will also be seen; but
these may be distinguished from the spots by moving the telescope
a little, when they will shift their place relatively to the other
features; and their exhibition may serve to suggest the propriety of
keeping eye-pieces as clean as possible.
[Illustration: PLATE VI.
Photograph of Bridged Sunspot (Janssen). _Knowledge_, February, 1890.]
The spots when more closely examined will be found to present
endless irregularities in outline and size, as will be seen from the
accompanying plates and figures. On the whole, there is comparative
fidelity to two main features--a dark central nucleus, known as the
umbra, and a lighter border, the penumbra; but sometimes there are
umbræ which have no penumbra, and sometimes there are spots which can
scarcely be called more than penumbral shadings. The shape of the spot
is sometimes fairly symmetrical; at other times the most fantastic
forms appear. The umbra appears dark upon the bright disc, but is in
reality of dazzling lustre, sending to us, according to Langley, 54
per cent. of the amount of heat received from a corresponding area of
the brilliant unspotted surface. Within the umbra a yet darker deep,
if it be a deep, has been detected by various observers, but is
scarcely likely to be seen with the small optical means which we are
contemplating. The penumbra is very much lighter in colour than the
umbra, and invariably presents a streaked appearance, the lines all
running in towards the umbra, and resembling very much the edge of a
thatched roof. It will be seen to be very much lighter in colour on
the edge next the umbra, while it shades to a much darker tone on
that side which is next to the bright undisturbed part of the surface
(Figs. 14 and 15). Frequently a spot will be seen interrupted by a
bright projection from the luminous surface surrounding it which may
even extend from side to side of the spot, forming a bridge across
it (Plate VI., and Figs. 16, 17, and 18). These are the outstanding
features of the solar spots, and almost any telescope is competent to
reveal them. But these appearances have to be interpreted, so far as
that is possible, and to have some scale applied to them before their
significance can in the least be recognised. The observer will do
well to make some attempt at realizing the enormous actual size of the
seemingly trifling details which his instrument shows. For example,
the spot in Figs. 14 and 15 is identical with that measured by Mr.
Denning on the day between the dates of my rough sketches; and its
greatest diameter was then 27,143 miles. Spots such as those of 1858,
of February, 1892, and February, 1904, have approached or exceeded
140,000 miles in diameter, while others have been frequently recorded,
which, though not to be compared to these leviathans, have yet
measured from 40,000 to 50,000 miles in diameter, with umbræ of 25,000
to 30,000 miles. Of course, the accurate measurement of the spots
demands appliances which are not likely to be in a beginner's hands;
but there are various ways of arriving at an approximation which is
quite sufficient for the purpose in view--namely, a realization of
the scale of any spot as compared with that of the sun or of our own
earth.
[Illustration: FIG. 14.--SUN-SPOT, JUNE 18, 1889.]
[Illustration: FIG. 15.--SUN-SPOT, JUNE 20, 1889.]
Of these methods, the simplest on the whole seems to be that given
by Mr. W. F. Denning in his admirable volume, 'Telescopic Work for
Starlight Evenings.' Fasten on the diaphragm of an eye-piece (the
blackened brass disc with a central hole which lies between the field
and eye lenses of the eye-piece) a pair of fine wires at right angles
to one another. Bring the edge of the sun up to the vertical wire, the
eye-piece being so adjusted that the sun's motion is along the line
of the horizontal wire. This can easily be accomplished by turning the
eye-piece round until the solar motion follows the line of the wire.
Then note the number of seconds which the whole disc of the sun takes
to cross the vertical wire. Note, in the second place, the time which
the spot to be measured takes to cross the vertical wire; and, having
these two numbers, a simple rule of three sum enables the diameter of
the spot to be roughly ascertained. For the sun's diameter, 866,000
miles, is known, and the proportion which it bears to the number of
seconds which it takes to cross the wire will be the same as that
borne by the spot to its time of transit. Thus, to take Mr. Denning's
example, if the sun takes 133 seconds to cross the wire, and the spot
takes 6·5, then 133: 866,000::6·5:42,323, which latter number will be,
roughly speaking, the diameter of the spot in miles. This, method
is only a very rough approximation; still, it at least enables the
observer to form some conception of the scale of what is being seen.
It will answer best when the sun is almost south, and is, of course,
less and less accurate as the spot in question is removed from
the centre of the disc; for the sun being a sphere, and not a flat
surface, foreshortening comes largely and increasingly into play as
spots near the edge (or limb) of the disc.
Continued observation will speedily lead to the detection of the
exceedingly rapid changes which often affect the spots and their
neighbourhood. There are instances in which a spot passes across the
disc without any other apparent changes save those which are due to
perspective; and the same spot may even accomplish a complete rotation
and appear again with but little change. But, generally speaking, it
will be noticed that the average spot changes very considerably during
the course of a single rotation. Often, indeed, the changes are so
rapid as to be apparent within the course of a few hours. Figs. 14
and 15 represent a spot which was seen on June 18 and 20, 1889, and
sketched by means of a 2-1/2-inch refractor with a power of 80. A
certain proportion of the change noticeable is due to perspective, but
there are also changes of considerable importance in the structure
of the spot which are actual, and due to motion of its parts. Mr.
Denning's drawing ('Telescopic Work,' p. 95) shows the spot on the day
between these two representations, and exhibits an intermediate stage
of the change. The late Professor Langley has stated that when he was
making the exquisite drawing of a typical sun-spot which has become so
familiar to all readers of astronomical text-books and periodicals,
a portion of the spot equal in area to the continent of South America
changed visibly during the time occupied in the execution of the
drawing; and this is only one out of many records of similar tenor.
Indeed, no one who has paid any attention to solar observation can
fail to have had frequent instances of change on a very large scale
brought under his notice; and when the reality of such change has been
actually witnessed, it brings home to the mind, as no amount of mere
statement can, the extraordinary mobility of the solar surface, and
the fact that we are here dealing with a body where the conditions are
radically different from those with which we are familiar on our own
globe. Changes which involve the complete alteration in appearance
of areas of many thousand square miles have to be taken into
consideration as things of common occurrence upon the sun, and must
vitally affect our ideas of his constitution and structure (Figs. 16,
17, 18).
[Illustration: FIG. 16.--SUN-SPOT SEEN IN 1870.]
Little more can be done by ordinary observation with regard to the
spots and the general surface. Common instruments are not likely to
have much chance with the curious structure into which the coarse
mottling of the disc breaks up when viewed under favourable
circumstances. This structure, compared by Nasmyth to willow-leaves,
and by others to rice-grains, is beautifully seen in a number of the
photographs taken by Janssen and others; but it is seldom that it can
be seen to full advantage.
[Illustration: FIG. 17.--ANOTHER PHASE OF SPOT (FIG. 16).]
[Illustration: FIG. 18.--PHASE OF SPOT (FIGS. 16 AND 17).]
On the other hand, the spots afford a ready means by which the
observer may for himself determine approximately the rotation period
of the sun. A spot will generally appear to travel across the solar
disc in about 13 days 14-1/2 hours, and to reappear at the eastern
limb after a similar lapse of time, thus making the apparent
rotation-period 27 days 5 hours. This has to be corrected, as the
earth's motion round the sun causes an apparent slackening in the rate
of the spots, and a deduction of about 2 days has to be made for this
reason, the resulting period being about 25 days 7 hours. It will
quickly be found that no single spot can be relied upon to give
anything like a precise determination, as many have motions of their
own independent of that due to the sun's rotation; and, in addition,
there has been shown to be a gradual lengthening of the period in high
latitudes. Thus, spots near the equator yield a period of 25·09 days,
those in latitude 15° N. or S. one of 25·44, and those in latitude 30°
one of 26·53.
This law of increase, first established by Carrington, has been
confirmed by the spectroscopic measures of Dunér at Upsala. His
periods, while uniformly in excess of those derived from ordinary
observations, show the same progression. For 0° his period is 25·46
days, for 15° 26·35, and for 30° 27·57. Continuing his researches up
to 15° from the solar pole, Dunér has found that at that point the
period of rotation is protracted to 38.5 days.
Reference has already been made to the bright and fantastically
branched features which diversify the solar surface, generally
appearing in connection with the spots, and best seen near the limb,
though existing over the whole disc. These 'faculæ,' as they are
called, will be readily seen with a small instrument--I have seen them
easily with a 2-inch finder and a power of 30. They suggest at once to
the eye the idea that they are elevations above the general surface,
and look almost like waves thrown up by the convulsions which
produce the spots. The rotation-period given by them has also been
ascertained, and the result is shorter than that given by the spots.
In latitude 0° it is 24·66 days, at 15° it is 25·26, at 30° 25·48.
These varieties of rotation show irresistibly that the sun cannot
in any sense of the term be called a rigid body. As Professor Holden
remarks: 'It is more like a vast whirlpool, where the velocities of
rotation depend on the situation of the rotating masses, not only as
to latitude, but also as to depth beneath the rotating surface.' Plate
VII., from a photograph of the sun taken by Mr. Hale, in which the
surface is portrayed by the light of one single calcium ray of the
solar spectrum, presents a view of the mottled appearance of the disc,
together with several bright forms which the author of the photograph
considers to be faculæ. M. Deslandres, of the Meudon Observatory, who
has also been very successful in this new branch of solar photography,
considers, however, that these forms are not faculæ, but distinct
phenomena, to which he proposes to assign the name 'faculides'; and
for various reasons his view appears to be the more probable. They
are, however, in any case, in close relation with the faculæ, and, as
Miss Clerke observes, 'symptoms of the same disturbance.'
[Illustration:
PLATE VII.
Solar Surface with Faculæ. Yerkes Observatory.]
The question of the nature of the sun spots is one that at once
suggests itself; but it must be confessed that no very satisfactory
answer can yet be given to it. None of the many theories put forward
have covered all the observed facts, and an adequate solution seems
almost as far off as ever. No one can fail to be struck with the
resemblance which the spots present to cavities in the solar surface.
Instinctively the mind seems to regard the umbra of the spot as being
the centre of a great hollow of which the penumbra represents the
sloping sides; and for long it was generally held that Wilson's
theory, which assumed this appearance to correspond to an actual fact,
was correct. Wilson found by observation of certain spots that when
the spot was nearest to one limb the penumbra disappeared, either
altogether or in part, on the side towards the centre, and that this
process was reversed as the spot approached the opposite limb, the
portion of the penumbra nearest the centre of the disc being always
the narrowest.
This is the order of appearances which would naturally follow if the
spot in question were a cavity; and if it were invariable there could
scarcely be any doubt as to its significance. But while the Wilsonian
theory has been recognised in all the text-books for many years,
there has always been a suspicion that it was by no means adequately
established, and that it was too wide an inference from the number of
cases observed; and of late years it has been falling more and more
into discredit. Howlett, for example, an observer of great experience,
has asserted that the appearances on which the theory is based are not
the rule, but the exception, and that therefore it must be given up.
Numbers of spots seem to present the appearance of elevations rather
than of depressions, and altogether it seems as though no category has
yet been attained which will embrace all the varieties of spot-form.
On this point further observation is very much needed, and the
work that has to be done is well within the reach of even moderate
instruments.
The fact that sun-spots wax and wane in numbers in a certain definite
period was first ascertained by the amateur observer Schwabe of
Dessau, whose work is a notable example of what may be accomplished by
steadfast devotion to one particular branch of research. Without any
great instrumental equipment, Schwabe effected the discovery of
this most important fact--a discovery second to none made in the
astronomical field during the last century--simply by the patient
recording of the state of the sun's face for a period of over thirty
years, during which he succeeded in securing an observation, on the
average, on about 300 days out of every year. The period now accepted
differs slightly from that assigned by him, and amounts to 11·11
years. Beginning with a minimum, when few spots or none may be visible
for some time, the spots will be found to increase gradually in
number, until, about four and a half years from the minimum, a maximum
is reached; and from this point diminution sets in, and results,
in about 6·6 years, in a second minimum. The period is not one
of absolute regularity--a maximum or a minimum may sometimes
lag considerably behind its proper time, owing to causes as yet
unexplained. Still, on the whole, the agreement is satisfactory.
This variation is also accompanied by a variation in the latitude of
the spots. Generally they follow certain definite zones, mostly lying
between 10° and 35° on either side of the solar equator. As a minimum
approaches, they tend to appear nearer to the equator than usual; and
when the minimum has passed the reappearance of the spots takes the
opposite course, beginning in high latitudes.
It has further been ascertained that a close connection exists between
the activity which results in the formation of sun-spots, and the
electrical phenomena of our earth. Instances of this connection have
been so repeatedly observed as to leave no doubt of its reality,
though the explanation of it has still to be found. It has been
suggested by Young that there may be immediate and direct action in
this respect between the sun and the earth, an action perhaps kindred
with that solar repulsive force which seems to drive off the material
of a comet's tail. As yet not satisfactorily accounted for is the fact
that it does not always follow that the appearance of a great sun-spot
is answered by a magnetic storm on the earth. On the average the
connection is established; but there are many individual instances
of sun-spots occurring without any answering magnetic thrill from the
earth. To meet this difficulty, Mr. E. W. Maunder has proposed a view
of the sun's electrical influence upon our earth, which, whether it
be proved or disproved in the future, seems at present the most living
attempt to account for the observed facts. Briefly, he considers it
indubitably proved--
1. That our magnetic disturbances are connected with the sun.
2. That the sun's action, of whatever nature, is not from the sun as
a whole, but from restricted areas.
3. That the sun's action is not radiated, but restricted in
direction.
On his view, the great coronal rays or streamers seen in total
eclipses (Plate VIII.) are lines of force, and similarly the magnetic
influence which the sun exerts upon the earth acts along definite and
restricted lines. Thus a disturbance of great magnitude upon the sun
would only be followed by a corresponding disturbance on the earth
if the latter happened to be at or near the point where it would fall
within the sweep of the line of magnetic force emanating from the
sun. In proportion as the line of magnetic force approached to falling
perpendicularly on the earth, the magnetic disturbance would be large:
in proportion as it departed from the perpendicular it would diminish
until it vanished finally altogether. The suggestion seems an inviting
one, and has at least revived very considerably the interest in these
phenomena.
Such, then, are the solar features which offer themselves to direct
observation by means of a small telescope. The spots, apart from their
own intrinsic interest, are seen to furnish a fairly accurate method
by which the observer can determine for himself the sun's rotation
period. Their size may be approximately measured, thus conveying to
the mind some idea of the enormous magnitude of the convulsions which
take place upon this vast globe. The spot zones may be noted, together
with the gradual shift in latitude as the period approaches or recedes
from minimum; while observations of individual spots may be conducted
with a view to gathering evidence which shall help either to confirm
or to confute the Wilsonian theory. In this latter department of
observation the main requisite is that the work should be done
systematically. Irregular observation is of little or no value; but
steady work may yield results of high importance. While, however,
systematic observation is desirable, it is not everyone who has the
time or the opportunity to give this; and to many of us daily solar
observation may represent an unattainable ideal. Even if this be the
case, there still remains an inexhaustible fund of beauty and interest
in the sun-spots. It does not take regular observation to enable one
to be interested in the most wonderful intricacy and beauty of the
solar detail, in its constant changes, and in the ideas which even
casual work cannot fail to suggest as to the nature and mystery of
that great orb which is of such infinite importance to ourselves.
A small instrument, used in the infrequent intervals which may be all
that can be snatched from the claims of other work, will give the
user a far more intelligent interest in the sun, and a far better
appreciation of its features, than can be gained by the most careful
study of books. In this, and in all other departments of astronomy,
there is nothing like a little practical work to give life to the
subject.
In the conduct of observation, however, regard must be paid to the
caution given at the beginning of this chapter. Various methods have
been adopted for minimizing the intense glare and heat. For small
telescopes--up to 2-1/2 inches or so--the common device of the
interposition of a coloured glass between the eye-piece and the eye
will generally be found sufficient on the score of safety, though
other arrangements may be found preferable. Such glasses are usually
supplied with small instruments, mounted in brass caps which screw or
slide on to the ends of the various eye-pieces. Neutral tint is
the best, though a combination of green and red also does well.
Red transmits too much heat for comfort. Should dark glasses not be
supplied, it is easy to make them by smoking a piece of glass to the
required depth, protecting it from rubbing by fastening over it a
covering glass which rests at each end on a narrow strip of cardboard.
With anything larger than 2-1/2 inches, dark glass is never quite
safe. A 3-inch refractor will be found quite capable of cracking and
destroying even a fairly thick glass if observation be long continued.
The contrivance known as a polarizing eye-piece was formerly pretty
much beyond the reach of the average amateur by reason of its
costliness. Such eye-pieces are now becoming much cheaper, and
certainly afford a most safe and pleasant way of viewing the sun. They
are so arranged that the amount of light and heat transmitted can be
reduced at will, so as to render the use of a dark glass unnecessary,
thus enabling the observer to see all details in their natural
colouring. The ordinary solar diagonal, in which the bulk of the rays
is rejected, leaving only a small portion to reach the eye, is cheaper
and satisfactory, though a light screen-glass is still required with
it. But unquestionably the best general method of observing, and also
the least costly, is that of projecting the sun's image through the
telescope upon a prepared white surface, which may be of paper, or
anything else that may be found suitable.
To accomplish this a light framework may be constructed in the shape
of a truncated cone. At its narrow end it slips or screws on to the
eye-end of the telescope, and it may be made of any length required,
in proportion to the size of solar disc which it is desired to obtain.
It should be covered with black cloth, and its base may be a board
with white paper stretched on it to receive the image, which is viewed
through a small door in the side. In place of the board with white
paper, other expedients may be tried. Noble recommends a surface of
plaster of Paris, smoothed while wet on plate glass, and this is very
good if you can get the plaster smooth enough. I have found white
paint, laid pretty thickly on glass and then rubbed down to a smooth
matt surface by means of cuttle-fish bone, give very satisfactory
results. Should it be desired to exhibit the sun's image to several
people at once, this can easily be done by projecting it upon a sheet
of paper fastened on a drawing-board, and supported at right angles to
the telescope by an easel. The framework, or whatever takes its place,
being in position, the telescope is pointed at the sun by means of
its shadow; when this is perfectly round, or when the shadow of the
framework perfectly corresponds to the shape of its larger end, the
sun's image should be in the field of view.