THE SUN 233 



half the size. Why, then, is he so distended? Doctor 

 See has estimated the superincumbent pressure at a 

 depth of one-tenth the sun's radius at 21,000,000 atmos- 

 pheres, and the pressure at his center should be at least 

 100,000,000 atmospheres. What, then, is the physical 

 state of his central substances ! 



One of the most peculiar things about the sun is the 

 manner of his rotation on his axis. Carrington and 

 Spoerer, noted observers of the sun showed : 



1. That the sun rotates around an axis inclined 

 about 7 to the plane of the ecliptic, and so that the axis 

 points midway between the polar star and Vega to a posi- 

 tion in right ascension 18 h. 44 m. and decimation 64. 



2. At the solar equator the rotation occurs in about 

 25 days. 



3. The period of one complete rotation increases on 

 either side of the equator equally, and is about 27-yi days 

 at 45 north or south solar latitude. 



As to the high degree of the roundness of the solar 

 disc this quotation from Doctor Abbot will show (The 

 Sun, p. 26) : 



Poor has lately maintained that observations indicate that 

 the sun's equatorial and polar diameters vary relatively as much 

 as o.i" during a sun-spot cycle of eleven years. According to 

 him, the equatorial diameter is the larger at sun-spot maximum, 

 and the polar diameter the larger at sun-spot minimum. Am- 

 bronn, however, denies that this is supported by the observations, 

 and Moulton opposes so large a variation on theoretical grounds. 



That such phenomena as sun-spots exist has been 

 known for more than four centuries. These spots are 

 not always visible, but come and go in cyclical periods, 

 increasing from a state of almost complete absence for 

 (according to Newcomb) 4.62 years and then decreasing 

 for a further space of 6.51 years making up a total 

 period of 11.13 years. It must be noted, however, that 

 11.13 years is only the mean interval, the individual 

 periods ranging between 7.3 and 17.1 years. Young has 

 described the formation and life history of sun-spots in 

 these words : 



