l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



RESULTS 



The results summarized in the tables are graphically shown in 

 figures I, 2, and 3. Figure 1 shows the relation between sky bright- 

 ness and hour angle, figure 2 the total sun and sky brightness and 

 hour angle, and figure 3 the intensity of solar radiation and air mass 

 (secant of the zenith distance). The intensity of sky brightness on 

 June 8 was nearly double that of June 9 due to the streaks of cirrus 

 cloud which prevailed on the former day. On June 9 the sky was 

 clear and blue during the afternoon up to an hour angle of about 

 five hours. At this time, cirri similar to June 8 spread over the whole 

 sky. This explains the deviation of the last two points on the curve 

 of June 9 (fig. 1). The great deviations in sky brightness early on 

 the afternoon of June 8 arise from the presence of cumulus clouds 

 scattered over the sky. By the time the eclipse began the cumuli had 

 practically disappeared and the values for the remainder of the day 

 yield a surprisingly smooth curve. 



It is apparent from these data that the total brightness of the sky 

 during totality was less than that of the twilight one hour after 

 sunset of the same day. From first contact to second during the 

 eclipse the decrease in sky brightness was almost linear. The curves 

 of figures 1 and 2, showing the relationship of sky and total sun and 

 sky brightness to hour angle, are in their general form in agreement 

 with those computed from theoretical considerations by King. 1 How- 

 ever, the ratio of sky brightness at large zenith distances of the sun 

 to that at high sun is much smaller in the observed values than the 

 ratio obtained from King's computed curves. In other words, the 

 falling off" of sky brightness as the sun approaches the horizon is 

 considerably more rapid in both the Mt. Wilson and Lakin curves 

 than would be anticipated from his theoretical considerations. It is 

 probable that the presence of clouds low in the west on June 8 behind 

 which the sun set made the after-sunset sky brightness values of that 

 day lower than a clear sky would have shown. 



Of the two values of the outgoing radiation to space obtained dur- 

 ing totality, the second is smaller than the first, due to the rapid cool- 

 ing off of the instrument and surrounding air and consequent decrease 

 in temperature difference between the instrument and the space to 

 which it is radiating. This agrees with the values obtained by 

 Angstrom" at the total eclipse at Aviken, Sweden, in 1914. Both 



1 Phil. Trans. Roy. Soc. London, Series A, 212, p. 429. 



2 " Radiation of the Atmosphere," A. Angstrom, Smithsonian Misc. Coll. 

 Vol. 65, No. 3, p. 74. 



