1903.] Radiation in the Solar System. 265 



* Eadiation in the Solar System : its Effect on Temperature and 

 its Pressure on Small Bodies." By J. H. POYNTING, Sc.D., 

 F.E.S., Professor of Physics in the University of Birmingham. 

 Received June 16, Read June 18, 1903. 



(Abstract.) 

 PART I. Temperature. 



We can calculate an upper limit to the temperatures of fully absorbing 

 or " black " surfaces receiving their heat from the sun, and on certain 

 assumptions we can find the temperatures of planetary surfaces, if we 

 accept the fourth power law of radiation, since we know approximately 

 the solar constant, that is, the rate of reception of heat from the sun, 

 and the radiation constant, that is, the energy radiated at 1 abs. by a 

 fully radiating surface.* 



The effective temperature of space calculated from the very uncertain 

 data at our command is of the order 10 abs. Bodies in interplanetary 

 space and at a much higher temperature may, therefore, be regarded as 

 being practically in a zero temperature enclosure except in so far as 

 they receive heat from the sun. 



The first case considered is that of an ideal earth, more or less 

 resembling the real earth, and it is shown that the temperature of its 

 surface is, on the average, 325, 302, or 290 abs. according as we 

 take for the solar constant Angstrom's value 4 cal./min., Langley's 

 value 3 cal./min. or a value deduced from Rosetti's work 2*5 cal./min. 

 The lowest value found, 290 abs., is very near the average temperature 

 of the earth's surface, which may be taken as 289 abs. As the earth's 

 effective temperature must, if anything, be below this, and cannot diner 

 much from that of the ideal planet, Rosetti's value for the solar 

 constant, 2'5 cal./min. or 0'175 x 10 7 ergs/sec., is probably nearest to 

 the true value and is therefore used in the following calculations. 



The preceding calculations may be turned the other way. It is 

 shown that, on certain assumptions, the effective temperature of the 

 sun is 21*5 times that of the ideal earth. If we consider that the real 

 earth with a temperature 289 abs. sufficiently resembles the ideal, we 

 get a solar temperature 21'5 x 289 = 6200 abs. 



The upper limit to the temperature of the surface of the moon is 

 determined and is shown to be 412 abs. when no heat is conducted 

 inwards. But Langley finds that the actual temperature is not much 



* W. Wien (' Cong. Int. de Physique,' 1900, vol. 2, p. 30) has pointed out that 

 Stefan's law enables us to calculate the temperatures of celestial bodies which 

 receive their light from the sun, by equating the energy which they radiate to the 

 nergy which they receive from the sun, and remarks that the temperature of 

 Neptune should be below -200 C. 



