298 PHYSICAL SCIENCE 



ture of the radiating layer of the sun, which is 

 thus estimated to be from 6200 to 7000^ 

 absolute. Professor Poynting prefers the lower 

 value, which means about 6000 C. 



A small body, isolated in space, will, when a 

 steady state is reached, radiate as much heat 

 as it absorbs. If it be shielded from the sun, it 

 will attain a temperature which may be con- 

 sidered to be the effective temperature of space. 

 From estimates of the amount of heat received 

 from the stars, as compared with that received 

 from the sun, Poynting calculates the effective 

 temperature of space to be 10' absolute, or 

 263'' C. below the freezing-point of water. 



Similar principles give a basis for a determina- 

 tion of the temperatures of planets at any given 

 distance from the sun. Assuming that all the 

 heat absorbed is eventually radiated out again, 

 and that about one-tenth of the incident heat is 

 reflected, and making certain simplifying assump- 

 tions, the mean temperature of the surface of 

 the earth is calculated as 290' absolute, or 17° C. 

 The average temperature of the earth's surface 

 is known to be about 60' F., or 16C. The 

 calculation is made on the assumption that the 

 effective temperature of the sun is 6200^ absolute, 

 and its concordance with observation is the ground 

 given by Poynting for preferring that value for 

 the solar temperature. 



This success in calculating the effective 

 temperature of the earth lends weight to the 

 values given by the same method for the 

 temperatures of the other planets. Mercury 

 and Venus, with orbits inside that of the 

 earth, possess temperatures of 194' and 69° C. 



