THE SUN AND THE ATMOSPHERE — STETSON" 173 



ward to break out at high latitudes, characteristic of the next cycle. 

 He assumes it would take some 11 years for one family of tubular 

 vortices to descend from the high latitudes to the equator while the 

 second subsurface vortices with inverse polarity move northward for 

 the start of the new cycle. 



While Bjerknes thus accounts for many of the well-observed pecu- 

 liarities of sunspots by this theory, he gives no satisfactory explana- 

 tion of the origin of these circulating tubes of gas nor is any attempt 

 made in his theory to account for the 11-year periodicity together 

 with the slight variation in this 1 1-year interval. 



The question of what causes sunspots and why they wax and wane 

 in cycles is not yet answered. Most astrophysicists would dismiss 

 any attempt at predicting the cycle on the grounds that the chief 

 cause of sunspots lies within the sun itself and perhaps is inevitably 

 irregular in its action. 



Many investigators, on the other hand, have held to the idea that 

 the tidal forces of the planets acting on the solar atmosphere may be 

 the ultimate cause of the cyclical disturbance. Attempts to predict 

 in advance sunspot maxima, however, on the basis of planetary 

 periods has not met with much success. There is one element, per- 

 haps, in any planetary theory of sunspots that so far as I am aware 

 has not been previously considered, and that is the part played by 

 the period of the sun's rotation itself. 



On the basis of any equilibrium tidal theory, we should expect that 

 the tide-producing force from any planet, no matter how feeble, should 

 produce two tidal bulges directly opposite. A given region in the 

 solar atmosphere, therefore, will be subject to two periods of tidal 

 stress during each revolution of the surface with respect to a given 

 planet. In the case of a slow-moving planet such as Jupiter, the 

 interval between these tidal stresses would be about 12.5 days at the 

 solar equator. If, now, there were a free period in the solar atmos- 

 phere very nearly equal to this forced period of 12.5 days, an oscilla- 

 tion might be set up which would ultimately reach a maximum and 

 then subside again unless the two periods were exactly commensurate. 

 The length of time between the times of maximum oscillation would 

 therefore depend upon the ratio of the free period of the solar atmos- 

 phere to that of the rotation of the sun with respect to the planet in 

 question. 



It is conceivable that such an oscillation might reach a maximum 

 in the case of Jupiter in an interval of possibly as long as 11 years. 

 The fact that this interval might nearly equal the time of revolution 

 of Jupiter about the sun — 11.8 years — would be merely a matter of 

 coincidence. 



In the case of a more swiftly moving planet such as the Earth, 

 Venus, or Mercury, the rotation with respect to these nearer planets 



