674 
ASTRONOMY: C. G, ABBOT 
Proc. N. a. S". 
NEW OBSERVATIONS ON THE VARIABILITY OF THE SUN 
By C. G. Abbot 
ASTROPHYSICAL LABORATORY, SMITHSONIAN INSTITUTION, WASHINGTON 
Read before the Academy, April 26, 1920 
Photometric Observations of the Planets. — (a) Simultaneous spectro- 
bolometric observations of the solar constant of radiation in California 
and Algeria in 1911 and. 1912, and in California and Chile in 1918, (6) 
comparisons of the distribution of radiation over the sun's disk with 
simultaneous measurements of the intensity of total solar radiation, {c) 
comparisons of the temperature of the earth with the radiation of the sun, 
and {d) several other minor evidences, have all indicated a short irregular 
periodicity in the sun's emission. In other words, the sun appears to be a 
variable star ranging through about 0.10 stellar magnitude between 
extremes and often changing 0.03 magnitude within a few days. 
If this is so it must follow that the planets are also variable because they 
reflect solar light. Guthnick and Prager have made accurate photo- 
electric photometric comparisons of Saturn and Jupiter with available 
stars. Their published results led them to state a conclusion adverse 
to the solar variability (Verdffentlichungen K. Sternwarte Berlin-Bahels- 
herg, 2, Heft III, 1918 (126)). I wrote to Dr. Guthnick pointing out that 
this conclusion was premature, because the number of published photo- 
nletric observations . was not large, and they might have fallen on dates 
when the sun's emission was nearly the same. No direct solar observa- 
tions were available on those dates. Dr. Guthnick has kindly reopened 
his investigation and sent me results for January, February, March, 
April and May, 1920. 
We have available for comparison on almost all these days observations 
of the sun by Smithsonian observers at Calama, Chile. But it is 
not obvious that a comparison between the brightness of the planets 
and that of the sun should involve identical dates. Two hypotheses 
of solar variation may be made. First, the sun's emission may vary in 
all directions proportionally and simultaneously. Second, the sun may 
be surrounded by a ragged absorbing or radiating envelope so that his 
emission is unequal in different directions. Under this second hypothesis 
the rotation of the sun would carry with it shafts of unequal radiation, 
which would encounter the planets successively according to their helio- 
centric longitudes. This second hypothesis is plausible in view of the 
dissymmetry of the solar corona. It has the advantage, too, of not re- 
quiring rapid changes of the sun's emission, which would be hard to ac- 
count for in view of the immensity of the sun. 
Dr. Guthnick states that Jupiter varied irregularly and widely in sur- 
face conditions during the period of observation so that its fluctuations 
are not available for solar constant comparisons. Various well-known 
