80 CLIMATIC CYCLES AND TREE GROWTH 



somewhat over 31 days; 1918.4 to 1920.9 show a period close under 30 days; 

 1924 and 1925 do the same. In 1926 and 1927 the common 27-day period 

 seems to become 26 days ; this lengthens again to 27 days near the beginning 

 of 1928. Thence it continues for two years and in November and December 

 of 1929 seems to have a phase change of several days, which leads to the begin- 

 ning of the stable sequences of 27.0 days in the years 1930-35. Altogether we 

 have a very interesting pattern of rotation periods, here stated only in a pre- 

 liminary way. 



NAKED-EYE SUNSPOT RECORDS 



Naked-eye sunspots are not infrequent. They may occur at any part of 

 the sunspot cycle but are more likely to be seen near the maximum. Per- 

 sonal experience placed three conspicuous naked-eye spots, one at a maximum 

 (1917), one two years before a maximum (1882), and one two years before a 

 minimum (1921). Each was accompanied by northern lights, especially the 

 last. 



Historical accounts of naked-eye sunspots have come chiefly from Chinese 

 records and are quoted by Fritz in connection with his studies of northern 

 lights. His list gives us really only four groups that might supply or suggest 

 dates of maxima. Any well-established date of maximum before 1610 has 

 an important value. These groups are as follows (with observed cycles) : 



Intervals Cycles 

 Years 



1. A.D. 300-375 10.0 no strong location of maximum: 373 



possible. 

 370-400 13.8 no strong location of maximum. 



2. 800-880 11.1 maximum possible at 840. 



3. 1078-1206 8.3 no location of sunspot maximum: 



1120 is possible. 



4. 1369-1383 10. ± evident sunspot maximum 1370-72. 



The second is the weakest of the four but gives rather a clear 11+ year 

 cycle. 



RECORDS OF NORTHERN LIGHTS 



The other effect of solar activity visible to the unaided eye and forming 

 perhaps a guide to sunspot history is the aurora or northern lights. The 

 data here analyzed have been gathered together by Professor H. Fritz, Zurich, 

 1893, with a translation by W. W. Reed in the Monthly Weather Review, 

 October 1928. (See Bibliography.) Early records of this phenomenon are 

 necessarily fragmentary. Fritz has taken the descriptions from historical 

 references. These data have been transferred to three cycleplots and 

 analyzed, as follows: 



