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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.

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.

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.

FIG. 14.—SUN-SPOT, JUNE 18, 1889.

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 2½-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).

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.

FIG. 17.—ANOTHER PHASE OF SPOT (FIG. 16).

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 14½ 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.'

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 2½ 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 2½ 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.