varialtlc. I,;iiiil»<la lauii. iii.i\ lie iiilCncil ulicn I stale llial tin- 

 (liametci of the primary is \i\acvi\ at 4.250.000 inilos. or al)()iit five 

 times that of our own sun. while that of the companion is .^.200.000 

 miles,- or three and a half times that of our sun. Accordingly the 

 volume of the primary is 110 times greater than that of our sun, and 

 that of the primary is 47 times greater. 



Hut after all these vast worlds are only magnificent balls of gas, 

 for the mass of the primary is only ec|ual to two and a half times that 

 of our sun. while its companion is barely ecjual to our sun in mass. 



VARIABILITY OF SUN'S RADIATION. 



Mr. C. G. Abbott of the Smithsonian Astrophysical Observato-y 

 gives an account of the results of investigations on the variability of 

 the sun's radiation in a paper published in the Proceedings of the 

 National Academy of Sciences for February, 1920. It is found that 

 the investigations of the Smithsonian Astrophysical Observatory con- 

 ducted at Washington. Mt. Wilson. Mt. Whitney, Bassour (Algeria), 

 and now the investigations supp'orted by the Smithsonian Institution 

 from its private funds in North Carolina and Chile have all united 

 in giving the impression that the solar radiation is not constant, but 

 \ aries from day to day through a range of certainly live, and possibly 

 at times ten per cent. The conclusion that the sun is a variable star 

 is confirmed in several w'ays, but most notably by the results of meas- 

 urements made by the Smithsonian Astrophysical Observatory at 

 Mt. Wilson. California, on the distribution of energy along the diam- 

 eter of the solar image. These measurements indicate, as well known 

 before, that the edge of the sun's disc is less bright than the center, 

 and that the contrast of brightness between the center and the edge 

 varies according to the w'ave-length of light, being greater for short 

 v^ave-lengths, less for long. 



But the measurements of recent years have shown that not only 

 is there a variation of contrast by wave-length, but also a variation 

 of contrast with the time. The contrast in each wave length is dif- 

 ferent for different days of observation and, on the average, for dif- 

 ferent years of observation. The changes of contrast have been 

 compared with the changes of total radiation of the sun determined 

 by the aid of the Pyrheliometer and spectrobolometer, and it is found 

 there is a moderate degree of correlation between them. The cor- 

 relation is of two kinds. For variations of long periods of years, high 

 values of the solar constant are found associated with the high 

 values of contrast betw^een the center and edge of the sun. On the 

 contrary, for the short period variations of the solar radiation, occu- 

 pying a few^ days, weeks or m.onths. it is found that high values of the 

 solar radiation are associated with diminished values of the solar 

 contrast. 



The cause of this two-fold variation is reasonably explained. 

 When the sun grows hotter and thus increases its output of radiation 



52 



