162 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



The third diagram (fig. 3) is an extension of that appearing in 

 the Monthly Notices, 74, Plate 3, between pages 114 and 115, and 

 brings out another relation. The solar dichotomy is not illustrated 

 here; we are dealing with the whole of the sun's disk as we see it. 

 The points set forth are the variations in the area covered by sun- 

 spots, and the variations in the mean distance from the equator of 

 the sun of all spots. Area and latitude are the two factors brought 

 into notice. The period covered by the diagram is 1854-1923. The 

 statistics for the first two cycles are derived from the work of the 

 late Professor Spoerer; for the last four, from the Greenwich 

 Photoheliographic Results. 



The continuous line shows the variation in the total daily spotted 

 area of the sun during the progress of each cycle; the dotted line 



1 1 1 II 1 1 1 1 1 1 1 1 1 ii n 1 1 1 1 1 1 1 ii 1 1 1 1 1 ii 1 1 1 1 ii 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 : 1 1 1 1 1 1 1 1 ii 1 1 1 1 1 1 1 1 1 1 1 



, 1880 4 1870 * 1880 ' 1890 1900 * 1910 * 1920 ,«• 



Fig. 3. — Mean latitude and mean spotted area, 1854—1923 



shows the changing distance from the solar equator of the center of 

 gravity of the sun-spot zones. In this latter curve, we have the 

 fact that the solar cycle is one, and one only, brought out in a most 

 unmistakable fashion; each cycle begins with an activity in high 

 latitude ; each cycle ends with the last remnants of activity transferred 

 to a low one; but, as the new cycle begins before the old cycle has 

 completed its course, the two overlap for a short time. This is what 

 is known as " Spoerer's law of zones." Broadly speaking, we may 

 say that the approach toward the equator is continuous from the 

 beginning of the cycle to its end. The distinction between the forms 

 of the two curves is emphatic. The area curve is continuous; it 

 begins practically at zero ; it increases up to a certain point ; it then 

 diminishes again to the next zero ; and so on. Flow and ebb, flow and 

 ebb, follow each other continuously. But the latitude curve is dis- 



