82 CLIMATIC CYCLES AND TREE-GROWTH. 



i 



From the description above it is easily seen that the sunspots are 

 not likely to be in themselves the fundamental solar activity, but rather 

 an index of something else, and possibly a very sensitive index, for the 

 percentage change in spot numbers is hundreds of times as great as the 

 percentage changes in measured radiation between sunspot maxima 

 and minima. 



Suggested causes of sunspots. The cause of sunspots is still a matter 

 of conjecture, and there is no generally accepted hypothesis to explain 

 them. There is analogy to our clouds in that both indicate decreased 

 temperature. In their limitation to certain latitudes they resemble the 

 belts of Jupiter. The belts of Jupiter are roughly the lines of division 

 between the powerful easterly equatorial current and the slower moving 

 zones on either hand; and indeed this has been suggested as an expla- 

 nation of the particular location in latitude of the sunspots, for there 

 is an increase in speed of rotation of the sun's surface as the equator 

 is approached. Their periodic character is very difficult to explain. 

 Fundamental periodic changes in the body of the sun have been sug- 

 gested and, in the absence of better explanations, some such statement 

 hazily indicating the direction in which explanation is to be sought, is 

 perhaps the best that we can do. Planetary influence, however, has 

 often been proposed as the cause. The near agreement between the 

 revolution period of Jupiter and the sunspot period has naturally 

 attracted attention. Stratton (1911-1912) has made a very interest- 

 ing study of the appearance, continuance, and disappearance of spots 

 on portions of the sun facing toward or away from Jupiter and Venxiis. 

 A few per cent more spots do originate and disappear on the " after- 

 noon" of the side facing Venus than on other longitudes, but he con- 

 siders the case of physical relationship not proven. 



Planetary influence is sought in a theory proposed by W. J. Spill- 

 man (1915). In this theory gravitation is assumed to be due to 

 pressure variations in the ether arising from electronic rotation in the 

 attracting body. The varying speed of a planet in its orbit between 

 perihelion and aphelion, involving varying quantities of energy, requires, 

 he says, an interchange between the kinetic energy of the plant and 

 the atomic energy of the central attracting body. This atomic energy 

 is in the vibrations of the electrons, but he thinks it is likely to affect 

 both the temperature and the electric activity of the central body. 

 The effect in this way of Jupiter and Saturn would exceed the sum of 

 all the other planets combined and is therefore the only one considered. 

 The effect of Jupiter with its substantial variations in distance between 

 perihelion and aphelion predominates, and we have a marked resem- 

 blance between the sunspot curves since 1770 and the differential 

 planetary effect. One notices that this interchange of energy would 

 presumably affect all parts of the sun alike and that therefore we could 

 not expect an excess of sunspots on the side facing Jupiter. 



