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.'
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.