160 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



year from 1826 to 1923 the percentage of the days of observation 

 upon which the sun appeared to be free from spots. It shows clearly 

 that all years are not equally prolific as to sunspots. Confining 

 ourselves to the years represented, it appears that there were eight 

 cycles completed when there was a most marked absence of spots ; a 

 ninth cycle being now nearly completed. The dates of such absences 

 are very clearly defined and are marked by the actual year inserted 

 just above the apices. In the ninety-eight years represented there 

 is in no case any doubt as to the year of the greatest solar quiescence, 

 and a simple examination of the curve demonstrates that the average 

 length of the interval, from one year of greatest quiescence to the 

 next, was nearly 11^ years, and that the range in length of that in- 

 terval was from 10 to 13 years. 



— i ii 1 1 1 1 1 1 1 1 1 1 1 j 1 1 1 1 1 1 1 1 i i 1 1 1 1 1 i ti 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ii it 1 1 1 1 1 1 1 i 1 1 1 i 1 1 1 1 1 i 1 1 1 ; 1 1 1 1 1 1 1 1 1 mrnr 

 » * dw ' wfeo ' wto ' »k> isto wb * rata wto * isko _ 



Fig. 1. — Annual percentages of spotless days, 1826-1923 



This interval is often spoken of as " the sun-spot period." The use 

 of the word " period " has been unfortunate, for it has given rise to 

 an impression which is not in accord with the facts of the case. 

 Let us speak of it in future as "the sun-spot cycle." A series of 

 events may be, and often is, "cyclical" without being in the usual 

 mathematical sense " periodic " ; but we are not, at present, justified 

 in applying the term "period," in its strictest mathematical sense, 

 to the interval that occurs between one sun-spot minimum and the 

 next. 



The second illustration (fig. 2) gives a somewhat fuller study, 

 derived from the measures of the areas of sunspots made at Green- 

 wich Observatory on the photographs of the sun, taken during the 

 years 1874-1923 ; it is an extension of a similar diagram given in the 

 Monthly Notices, 64, page 748. It exhibits the mean daily spotted 



