14 RADIATION 
mates the spectral-energy distribution of a black body 
(Fig. 1). It is therefore not unusual to speak of the 
black-body temperature of the sun as 6000K, although 
this numerical value may vary by several hundred 
degrees depending upon the method used to calculate 
it [4]. For example, the total energy emitted by a black 
body per unit area per unit time is given by o7"*, where 
L is its temperature in degrees absolute and co is the 
Stefan-Boltzmann constant. The solar constant is com- 
monly taken as 1.94 ly min (where ly = langley = 
g cal em”), and considering the distance from the 
earth to the sun, we can calculate the total energy 
emitted by the sun. Then, by imtroducing the sun’s 
INTENSITY 
RELATIVE 
1910 
Lenoir sits QUARTZ mesuirsil 
1922 
=> BLACK BODY 6000 kK 
PROPOSED STANDARD CURVE 
SMITHSONIAN 
INSTITUTION 
0.4 0.5 0.7 1.0 Ke 20 sk) 
WAVELENGTH (MICRONS) 
Fig. 1.—Spectral intensity distribution of solar radiation 
outside the earth’s atmosphere. (After Moon [63].) 
area, we calculate the energy radiated per square centi- 
meter of solar surface. Placing that energy equal to oJ, 
we get 7 = 5770K. 
Another temperature estimate results from Wien’s 
displacement law for black bodies: Amax 7’ = const, 
where \max 1s the wave length of maximum intensity. 
Abetti [4] takes \max = 4740 A and finds 7’ = 6080K. 
It should be emphasized that at wave lengths out- 
side the region 0.45 » to 2.0 solar energy departs 
markedly from that of a black body at 6000K. Moon 
[63] has combined the results of several authors and ob- 
taimed Fig. 1, which shows the relative intensity Jo 
of the energy as a function of wave length. 
Ultraviolet Radiation. The energy in the ultraviolet 
spectrum is well below that of a black body at 6000K; 
this has been established by several investigators [35, 
37, 72]. Hulburt’s curve [47] in Fig. 2 contains the 
Naval Research Laboratory rocket measurement at 55 
km in which no ozone could be detected and was one 
of a series of spectra observed at levels up to 88 km. 
Other measurements from rockets up to 155 km [20] 
show results similar to those at 55 km and indicate that 
the energy is less than that of a black body at 6000K. 
A recent curve by Gotz and Schénmann [85], based 
on five observations in the region from 3300 A to 5000 
A, lies far below (by a factor of 2 or more at 3400 A) 
Moon’s curve of Fig. 1; this is also true of Hess’s data 
[55, p. 324]. Perhaps these lower values are due to the 
fact that the Smithsonian observations were troubled 
by scattered light at short wave lengths. If the rocket 
data (Fig. 2) had been joimed to the data of Gotz and 
Schénmann, the resultant spectral curve would ob- 
RELATIVE INTENSITY 
2200 
2400 2600 2800 3000 3200 3400 
WAVELENGTH (ANGSTROMS) 
Fie. 2.—Spectral intensity distribution of ultraviolet solar 
radiation measured from a rocket. (After Hulburt [47].) 
viously have been much farther below the 6000K curve. 
Future measurements will decide the true state of af- 
fairs. 
Infrared Radiation. The near infrared has been ob- 
served many times by the Smithsonian Institution. 
Abbot and others [2] considered their 1920-22 curve as 
the best estimate. Their data exceed the black-body 
curve (Fig. 1) in the region near 2 », but Moon, also 
accepting the earlier data, adopted the 6000K black- 
body curve as the true one beyond 1.25 p. Beyond 2.5 p, 
Adel [5] has observed the spectrum up to about 24 u. 
He finds that the solar temperature is about 7000K in 
the far infrared. 
Short-Wave Radio Radiation. Measurements up to 
24 uw have been made by optical means. Recently the 
observations have been extended by radio detection 
instruments into wave lengths of the order of a few 
centimeters to a few meters [39, 62]. These observations 
indicate black-body temperatures of the order of 10°K 
