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XII, Radiation in the Solar System: its Effect on Temperature and its 
Pressure on Small Bodies. 
By J. H, PoYNTiNG, Sc.D., F.R.S., Professor of Physics in the 
University of Birmingham. 
Received June 16,—Read June 18, 1903. 
PART 1. 
Temperature. 
When a surface is a full radiator and absorber* its temperature can be determined at 
once by the fourth-power law if we know the rate at which it is radiating energy. If 
it is radiating what it receives from the .sun, then a knowledge of the solar constant 
enables us to find the temperature. We can thus make estimates of the highest 
temperature which a surface can reach when it is only receiving heat from the sun. 
We can also make more or less approximate estimates of the temperatures of the 
planetary surfaces by assuming conditions under which the radiation takes place,! 
and we can determine, fairly exactly, the temperatures of very small bodies in 
interplanetary space. 
Ihese determinations require a knowledge of the constant of radiation and of either 
the solar constant or the effective temperature of the sun, either of which, as is well 
known, can be found from the other by means of the radiation constant. It will be 
convenient to give here the values of these quantities before proceeding to apply the}n 
to our special problems. 
* A surface which absorbs, and therefore emits, every hind of radiation, is usually described as “ black,” 
a description which is obviously bad when the surface is luminous, ft is much better described as “ a full 
absorber” or “a full radiator.” 
t This was pointed out by W. WiEN in his report on “ Les Lois Theoriques du Rayonnement ” 
(‘Congres International de Physique,’ vol. ii., p. ,30). He remarks that Stefan’s law enables us to 
■calculate the temperatures of celestial bodies which receive their light from the sun, by ecjuating the 
energy which they radiate to the energy which they receive from the sun, and states that for the earth 
we obtain nearly the mean temperature, using the reflecting power of Mars, while the temperature of 
ISTeptune should be below - 200° C. 
VOL. ccji.— A 357. 
18.12.03 
