528 PROFESSOR J. H. POYNTING ON RADIATION IN THE SOLAR SYSTEM; 
used above in finding Rosetti’s solar constant, for no doubt the transmission varies 
widely with time and place, and we have no reason to assume that 177 calories 
per minute, obtained by Langley, would have been received from the zenith at the 
time and in the place where Wilson was making bis determination. 
The Effective Temperature of Space. 
In determining the steady temperature of any body as conditioned by the radiation 
received from the sun, we have to consider whether it is necessary to take into account 
the radiation from the rest of the sky. If it receives S from the sun, p from the rest 
of the sky, and if its own radiation is II, then in the steady state 
R = 8 fi- p or R — p = 8. 
It behaves therefore as if it were receiving 8 from the sun, but as if it were placed 
in a fully radiating enclosure of sucli temperature that the radiation is p. This 
temperature is the “ effective temperature of space.” 
The temperature may perhaps be more definitely described as that of a small full 
absorber placed at a distance from any planet and screened from the sun. Various 
well-known attempts have been made to estimate this temperature, but the data are 
very uncertain. The fourtli-power law however shows that it is not very much 
above the absolute zero, if we can assume that the quality of starlight is not very 
ilitferent from that of sunlight. 
According to l’Hekmite ^ starlight is one-tenth full moonlight. Full moonlight is 
variously estimated in terms of full sunlight. Langley t takes it as 400 ^ 000 ’ These 
two values combined give sunlight as 4 X 10® starlight. But starlight comes from 
the whole hemisphere, while the sun only occupies a small part of it. In comparing 
temperatures ^ve have to use the Ijiightness of sunlight as if the whole hemisphere 
were paved vfith suns. 
If B is the illumination of a surface at O, fig. I, lighted by 
the sun in the zenith at 8, and if ttS' is the area of the sun’s 
diametral plane, then B/Vs" is the illumination at O due to 
each square centimetre. If the hemisphere were all of the 
same lirightness as the sun, the illumination at 0 due to the 
ring of sky between 6 and 6 d- dO would lie 
277 ?'' sin 6 cos OdB, 
TTS'- 
where r is the distance of the sun. 
* ‘ L’Astronomie,’ vol. 5, p. 406. 
t “ First Memoir on the Temperature of the Sinface of the Moon.” ‘ National Academy of Sciences 
vol. 3. 
S 
