It may be taken for granted, however, that there is an analogy between the sun spots and terrestrial cyclonic storms, though as yet we are not very well able to understand its nature.
Then next we come to one of the most interesting discoveries ever made respecting the sun—the discovery that the spots increase and diminish in frequency in a periodic manner. We owe this discovery to the laborious and systematic observations made by Herr Schwabe of Dessau.
Schwabe found, in the course of about ten and a half years, the solar spots pass through a complete cycle of changes. They become gradually more and more numerous up to a certain maximum, and then as gradually diminish. At length the sun’s face becomes not only clear of spots, but a certain well-marked darkening around the border of his disk disappears altogether for a brief season. At this time the sun presents a perfectly uniform disk. Then gradually the spots return, become more and more numerous, and so the cycle of changes is run through again.
The astronomers who have watched the sun from the Kew Observatory have found that the process of change by which the spots sweep in a sort of “wave of increase” over the solar disk is marked by several minor variations. As the surface of a great sea wave will be traversed by small ripples, so the gradual increase and diminution in the number of the solar spots are characterized by minor gradations of change, which are sufficiently well marked to be distinctly cognizable.
Fig. 31.—Ptolemaic System
There seems every reason for believing that the periodic changes thus noticed are due to the influence of the planets upon the solar photosphere, though in what way that influence is exerted is not at present perfectly clear. Some have thought that the mere attraction of the planets tends to produce tides of some sort in the solar envelopes. Then, since the height of a tide so produced varies as the cube or third power of the distance, it has been thought that a planet when in perihelion would generate a much larger solar tide than when in aphelion. So that, as Jupiter has a period nearly equal to the sun-spot period, it has been supposed that the attractions of this planet are sufficient to account for the great spot period. Venus, Mercury, the Earth, and Saturn have, in a similar manner, been rendered accountable for the shorter and less distinctly marked periods.
Without denying that the planets may be, and probably are, the bodies to whose influence the solar-spot periods are to be ascribed, I yet venture to express very strong doubts whether the attraction of Jupiter is so much greater in perihelion than in aphelion as to account for the fact that, whereas at one season the face of the sun shows many spots, at another it is wholly free from them.[23]
However, we are not at present concerned so much with the explanation of facts as with the facts themselves. We have to consider rather what the sun is and what he does for the Solar System than why these things are so.
Let us note, before passing to other circumstances of interest connected with the sun, that the variable condition of his photosphere must cause him to change in brilliancy as seen from vast distances. If Herr Schwabe, for instance, instead of observing the sun’s spots from his watch-tower at Dessau, could have removed himself to a distance so enormous that the sun’s disk would have been reduced, even in the most powerful telescope, to a mere point of light, there can be no doubt that the only effect which he would have been able to perceive would have been a gradual increase and diminution of brightness, having a period of about ten and a half years.