NO. 7 ASTROPHYSICAL OBSERVATORY, I904-I953 — ABBOT 7 



tions whose individual mean values differ only through the small range 

 from 76.0 to 77.9 parts in 10,000 of the solar constant. 



As for the extreme limits of solar variation, I sent Dr. Roberts 

 of High Level Observatory a rough plot of all Montezuma solar 

 constant daily mean values, 1923 to 1952. He retains a copy of it. 

 From numerous of these values, some as low as 1.910, some as high 

 as 1.970, I believe it fair to set solar variation limits as above 2 per- 

 cent. Dr. Franz Baur of the University of Frankfurt, A.M., Germany, 

 has published 2.2 percent as his conclusion.^ 



On one occasion the solar constant appears to have gone as low as 

 1.870 calories in 1922 and 1923, for a considerable time. This unique 

 depression may be associated with a long-range period in solar varia- 

 tion that would require scores or centuries of years of observation to 

 verify. 



PERIODIC SOLAR VARIATION 



Number 8 — The 27-day period in solar variation. 



It has long been known that the sun's surface shows rotation vary- 

 ing in velocity from the sun's equator to its poles. At the equator the 

 sun rotates in about 25 days, and in about 35 days at 80° latitude. 

 A weighted value of the sun's rotation period, considering areas and 

 latitude, may be taken at 27 days. In Pub. 2499,* I showed strong 

 correlation in solar constant measures ranging continuously from 

 + 25 to —30 percent, with a period of 27 days. See figure 15, Pub. 

 4545.2 jj^jg ^i(je range was observed in Mount Wilson values in 1915, 

 but not in other years, 1912-1920. 



Having discovered the master period, 22 years, 9 months (273 

 months) in solar variation about 1940, it seemed to me probable that 

 the highly accurate solar constant measures made at Montezuma would 

 show a 27-day period strongly in the year 1937 (1915 + 22 years), 

 perhaps repeated several times. I have computed from records in 

 volume 6 of the Observatory's Annals for four recurrences of the 

 27-day period in daily solar constant values observed at Montezuma 

 from early April to late September of 1937. Table 1 and accompany- 

 ing Figure 1 show the detailed and mean results. A 27-day periodic 



^Met. Rundschau, 17 January, Jahrb. 1, Heft 1964, pp. 19-25. 



273 273 



^ Solar rotation and solar variation. Periods 27 days and ■j-.r^ and 



1250 2500 



months shown by correlation in 1915 and 1916, by C. G. Abbot. Smithsonian 

 Miscellaneous Collections, vol. 66, No. 6. 1918. 



