176 ANISTJAL REPORT SMITHSONIAN INSTITUTION, 1942 



clockwise in the southern. The direction of the vortical rotation is 

 associated with the direction of the magnetic field of a spot; two spots 

 for which vortical motion is opposite have oppositely directed mag- 

 netic fields. 



For routine observation of magnetic fields and sunspots a special 

 analyzer is used by which one determines the magnetic polarity of a 

 sunspot, observing merely whether the positive (red) or the negative 

 (violet) component of a line is transmitted. 



Sunspots usually appear in elongated groups, and spots at opposite 

 ends of these groups have opposite polarities. Although it was soon 

 realized that the number of sunspots varied with time, it was not 

 until 1843 that Schwabe showed that they occurred in cycles. From 

 the results since then of many years of observation by devoted observers 

 in all parts of the world, the spottedness of the sun has been found 

 to vary in a somewhat irregular fashion with an average period of 

 about 11 years. This 11-year cycle does not progress smoothly but 

 in short-period pulsations which vary in length from about 15 months 

 to periods as short as 5 or 6 months. The polarities of spots in a new 

 cycle are opposite to those after the minimum of the preceding cycle. 

 The spots at the first of a new cycle are in zones about 30° from the 

 equator; they gradually approach the solar equator at the end of the 

 cycle. Thus solar activity apparently has a fundamental cycle double 

 that of the 11-year cycle. The fact that spots have been more 

 numerous in alternate 11-year cycles also indicates this. From the 

 viewpoint of efi^ects on the earth, the 11-year period is probably more 

 important. All spots appear to move across the sun's disk from east 

 to west. Many single spots and groups of spots, which have passed 

 from sight around the sun's western edge, have been recognized on 

 their reappearance, after about 2 weeks' time, at the eastern edge. 



It has been concluded from the recurrences of such groups that the 

 period of the sun's rotation is about 27 days. To be more precise, 

 the interval elapsing between two successive passages of a spot across 

 the sun's central line is a little less than 27 days for spots on the sun's 

 equator and this interval increases to more than 28 days for those 

 midway between the equator and the poles. Spots are rarely observed 

 any nearer to the sun's poles. The outlines of sunspots are variable ; 

 some do not reappear at all and others exist through several rotations, 

 but rarely persist for longer than half a year. Tliey do not endure 

 as do the surface irregularities on the earth and moon. 



Observations by the spectroscopic metliod have shown that the sun 

 also has a general magnetic field somewhat like that of the earth with 

 opposite poles of unchanging polarity in the northern and southern 

 hemispheres. However, the field intensity is small and unusual care 

 is required to detect it. The measurements made at the Mount Wil- 

 son Observatory indicate that the north magnetic pole of the sun is 



