18 RADIATION 
and 2.5 p, but also the energy for 0.29n < \ <0.34 yu 
and) > 2.5 pu (since energy below 0.29 » does not reach 
the ground). Hence, if to the area 2A under the 
spectral curve is added the energy « in the wave lengths 
which reach the ground but are not measured spectrally 
(namely 0.29 np < X\ <0.34pand X > 2.5 p) then the 
area under the curve, e + 2AX, will be proportional 
to the corresponding pyrheliometric measurement J. 
The area Jo, under the curve of Jo, versus (for 
0.344 < > < 2.5) can now be converted to energy 
units, since 
I _=hA\+e 
Teal) GEN 
It remains to add to Ja the energy « for \ < 0.34 uw 
and \ > 2.5 » in order to obtain Jp, the solar constant. 
This “long” method implies that a is constant dur- 
ing the period of measurements (2-3 hr), and also that 
I, the pyrheliometrically measured radiation, is known 
in absolute units (langleys per minute). In practice the 
corrections for all unmeasured spectral regions are 
applied together. 
Equation (4) requires that the atmospheric trans- 
mission @ be constant during a series of spectral meas- 
urements which are performed during a period of a few 
hours (from air mass 5.0 to 1.5). To the extent that a) 
is not constant, errors will be introduced in J), and 
hence in the solar constant. Partly for that reason, the 
Smithsonian Institution devised a “short” method for 
measuring the solar constant. In this method, a is de- 
termined by a measurement of the brightness of the sky 
in the vicinity of the sun and from an empirical rela- 
tion between the brightness and the transmission coef- 
ficient a. 
The Numerical Value of the Solar Constant. Let us 
consider 7. The standard of pyrheliometry adopted by 
the Smithsonian Institution in 1913, when applied to 
solar-constant measurements, leads to 1.94 ly min as 
the average value for J. This is the value most often 
used at present. The Smithsonian standard was known 
to be about 3.5 per cent higher than the Angstrém 
standard, but in 1932 and on subsequent occasions 
Abbot and Aldrich redetermined their standard and 
found that their 1913 standard was too high by 2.3 
per cent [12], thus decreasing the disagreement with the 
Angstrém scale. The new Smithsonian standard scale 
reduces the value of the average solar constant from 
1.94 to 1.90 ly min. 
Before accepting this new value we should examine 
the corrections applied to the solar constant for the 
unmeasured spectral radiation [8, Vol. 5, p. 103]. In 
practice a correction is made for the unmeasured radia- 
tion in the interval 0.34 » > A > 0.27 u, and for this 
spectral region about 3.4 per cent of the total measured 
radiation is added. Apparently no energy is added for 
radiation below 0.27 ». A composite curve of the 55- 
km rocket measurement and Smithsonian surface meas- 
urements indicates that the region from 0.34 pu to 0.27 u 
comprises 2.9 per cent of the area between 0.34 » and 
2.5 » and that about 0.5 per cent of the total radiation 
is included between 0.22 » and 0.27 » [56]; hence the 
(5) 
energy ordinarily added for \ < 0.34 p is about right. 
We have assumed here that the 55-km rocket observa- 
tion represents Io, for \ < 0.34 yu. 
Tn the infrared, 2 per cent is added for radiation be- 
yond 2.5 » during Jp determinations. For a black body 
at 6000K this should amount to about 3.1 per cent. 
Hence if the energy beyond 2.5 p is that of a black body 
at 6000K or more [5, 63], it would appear that a some- 
what greater amount than 3.1 per cent is the necessary 
correction. 
The area 2J,AX is adjusted to agree with J by esti- 
mating the unmeasured spectral energy. In this process 
of adjustment, errors in the estimated correction are 
offset somewhat by the adjustment mechanism itself. 
This adjustment process, however, does not account for 
errors in ¢€) in the spectral regions which cannot be ob- 
served at the ground. It is difficult therefore to say 
exactly what effect the substitution of new corrections 
would have on Jp. As a first approximation we might 
assume that the ultraviolet correction is about right 
and that the infrared correction is too low by 1 or 2 
per cent, so that the computed solar constant can tenta- 
tively be given by 1.90 < Jy) < 1.94, the range making 
some allowance for the uncertainty of the corrections. 
On the basis of the Smithsonian measurements there 
does not seem to be much justification for a solar con- 
stant of more than 2.0 ly min!# 
Variation of the Solar Constant. We turn now to a 
consideration of the following questions: (1) Does the 
“solar constant”? vary with time? and (2) Do the 
measurements of J) indicate the actual variation? The 
controversy regarding these questions has been raging 
for a long time, and a definite answer to the second 
question cannot yet be given. The state of the con- 
troversy is shown in a criticism by Paranjpe [67] and a 
reply by Abbot [1]; Waldmeir [77] has summarized 
some of the earlier arguments. Two main points have 
been at issue: (1) Do the measured values at two or 
more widely separated stations vary in the same way 
with time? and (2) Do the J) measurements or the 
transmission coefficients a, vary seasonally? If so, one 
would expect that the earth’s atmosphere is introducing 
the variations and that they are not true solar changes. 
Abbot has pointed out on several occasions that the 
data from the various stations do agree for monthly 
means, but that daily values show appreciable de- 
partures. Paranjpe [67] states, however, that the data 
from the stations undergo statistical adjustment im such 
a way as to make the data between stations comparable. 
From this view, of course, interstation correspondence 
would have no significance. Abbot [1] states definitely 
3. Karandikar [50] assumed a value of more than 2.0 ly 
min—1, At the Ionospheric Physics Conference held at State 
College, Pennsylvania, in July, 1950, Dr. M. O’Day announced 
that a measurement from a rocket indicated a value of more 
than 2.0 ly min. This measurement has apparently not yet 
been completely checked. See Discussion of the paper Physical 
Characteristics of the Upper Atmosphere” by T. R. Burnight 
in “Proceedings of the Conference on Ionospheric Physics 
(July 1950).”” Geophys. Res. Pap., Air Force Cambridge Research 
Laboratories, Cambridge, Mass. (1951) (in press). 
