168 
The Solar Constant of Radiation, Its 
Imperfect Constancy, and Its Relationship 
to Meteorology 
The main research of the Astro- 
physical Observatory from 1902 to the 
present time concerns what was 
beautifully and emphatically expressed 
by Langley in his report of the Mount 
Whitney expedition ° as follows: 
If the observation of the heat the sun sends 
the earth is among the most important and 
difficult in astronomical physics, it may 
also be termed the fundamental problem of 
meteorology, nearly all whose phenomena 
would become predictable if we knew both 
the original quantity and kind of this heat; 
how much of it reaches the soil; how, through 
the aid of the TEED USK: it maintains the 
surface temperature of the planet; and how, 
in diminished quantity and altered kind, 
it is finally returned to outer space. 
In the same report, Langley also 
said: ‘‘The difficulty of measuring sun 
rays accurately at the ground is indeed 
very great, but the difficulty of measur- 
ing their loss in the atmosphere is 
almost insuperable.” 
It was a great satisfaction, in view 
of these true statements of Langley, 
based on his long experience, when 
Turner, of Oxford, in a review of 
volume 2 of the Annals of the Astro- 
physical Observatory, wrote in 1908: 
“Mr. Abbot has shown that he is 
measuring something definite, for he 
has detected an annual diminution 
of 3}4 percent from August to October, 
due to our greater distance from the 
sun.” 
Since then great improvements in 
accuracy, both in ground measures 
and in estimates of atmospheric losses, 
have been made. In a recent paper * 
I was able to show that a periodic 
change in the sun’s output of radia- 
tion, averaging but 0.13 percent, can 
3 Langley, S. P., A report of the Mount 
Whitney expedition. Prof. Pap. Signal 
Service, No. 15, p. 11, 1884. 
4 The sun’s short regular variation and its 
large effect on terrestrial temperatures. 
Smithsonian Misc. Coll., vol. 107, No. 4, 
Apr. 4, 1947. 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 
1948 
be recognized in our measurements of 
the last 25 years, and the form of its 
curve of solar variation can be clearly 
shown, though its whole average range 
is only one twenty-seventh of that 
which Turner referred to in 1908. 
The steps taken which have led up to 
the present condition of our research 
will now be mentioned. 
After an interlude of observing the 
total solar eclipses of May 28, 1900, 
and May 18, 1901, we returned to 
Langley’s first interest, measurements 
of the intensity in different parts of 
the solar spectrum, as observed on the 
earth’s surface at low and high eleva- 
tions above sea level, and as it would 
be observed at mean solar distance 
outside the atmosphere altogether. 
When we began these new studies, 
the 1900 edition of Hann’s Lehrbuch 
of meteorology gave without preference 
values of the solar constant ® of radia- 
tion, ranging from Pouillet’s 1.76 
calories per, square centimeter per 
minute to Angstrém’s suggestion of 
4.0 calories. Various types of instru- 
ments for measuring the total in- 
tensity of solar heating were then in 
use which disagreed widely. Only 
one of them, the Angstrém electrical 
compensation pyrheliometer,® has sur- 
vived to the present time, and that 
has been decidedly improved. But 
even as of the year 1900, Angstr6m’s 
pyrheliometer was within 3 percent 
of the truth. 
However, being unfamiliar with the 
merit of Angstrém’s pyrheliometer, 
we developed our instrument from 
Pouillet’s. After several years of im- 
provements, we perfected the Abbot 
silver-disk pyrheliometer about 1910. 
Nearly a hundred copies of this in- 
5 The “solar constant” is the number of 
calories of radiant energy from the sun falling 
each minute on 1 square centimeter of surface 
at the earth’s mean distance from the sun. 
6 The pyrheliometer is an instrument for 
measuring fairly intense beams of radiation. 
It is better adapted to absolute measures 
than the bolometer. 
