IV. ANALYSIS OF SOLAR RECORDS 



A vast amount of solar heat passes out from the sun into space and we 

 ourselves exist by favor of that minutest fraction caught, in passing, by the 

 earth and employed in producing winds and rain and a thousand complex 

 processes, including vegetal growth and even human life itself. 



Nothing could be more natural in a discussion of climate and tree-ring 

 growth than a study of the variable activity of this great source of terrestrial 

 energy. Many of the basic facts about the sun were laid down in past 

 centuries; its distance, size, mass, temperature, radiation, and its spots and 

 their periodicity and latitude change. Since 1900 the attention of investi- 

 gators has gone largely to atomic and spectroscopic studies, the constitution 

 of matter, source of the sun's energy, internal motions of sunspots, and com- 

 position of different parts of the sun. More closely associated with our pres- 

 ent subject are the improved measures upon the rotation of the sun with its 

 equatorial acceleration (Adams), the persistence of spots in longitude (Nichol- 

 son and others), and, perhaps more fundamental, the magnetic polarity of 

 spots and their change at minima, and the polarity of the sun itself (by Hale, 

 carried on by Nicholson). 



The sun rotates on its axis in 25 to perhaps 32 days, as seen from the 

 stars (siderial rotation), or 27 to 35 days as seen from the earth (synodic 

 rotation). The equator takes the faster rate; and high latitudes, north and 

 south, the slower. The causes of this "equatorial acceleration" are not yet 

 fully understood although this phenomenon has been connected by some with 

 the planetesimal hypothesis. In order of time, a cycle of spots, from mini- 

 mum to maximum and back, taking about 11 years, begins at 25° north and 

 south latitude and works to within about 5° of the equator. Rarely one goes 

 beyond the equator. The maximum number of spots occurs at some inter- 

 mediate latitude. The best visualizing of that change is obtained from a 

 photograph arranged by Dr. W. S. Adams and Dr. S. B. Nicholson of the 

 Mount Wilson Solar Observatory of the Carnegie Institution of Washington, 

 reproduced here by permission, Plate 17. The spots are relatively inconspicu- 

 ous in small scale photographs of the sun and they have substituted photo- 

 graphs of the calcium clouds or flocculi that surround the spots and are very 

 conspicuous. After allowing for change in slant of the solar equator, one 

 can recognize in them the beginning of the spot cycle at high latitudes, the 

 increase in numbers at somewhat lower latitudes, then the disappearance on 

 close approach to the equator. When this is plotted, it forms what Maunder 

 called the "butterfly" diagram, as in figure 29. Dr. George E. Hale at the 

 Mount Wilson Observatory in 1908 found magnetic effects in the spots in- 

 volving systematic distribution of north and south magnetic poles in pairs 



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