RADIATION IN THK SOLAR SYSTEM. 



189 



We soo from this table that the temperature at the earth's distance 

 is remai-kahly near the average temi)eratiire of the earth's surface, 

 whieh is usually estimated as about IC" C, or ()0° F. This can hardly 

 be regarded as a mere coincidence. Tlie sui'face of the earth receives, 

 Ave know, an amount of heat from the inside almost infinitesimal com- 

 pared with that which it receives from the sun, and on the sun, there- 

 fore, we depend for our temperature. The earth acquires such a tem- 

 perature, in fact, that it radiates out what it receives from the sun. 

 The earth is far too great for the distribution of heat by conduction 

 to play any serious part in equalizing the temperature of different 

 regions. But the rotation about its axis secures nearly uniform tem- 

 perature in a given latitude, and the movements of the atmosphere 

 tend to equalize temperatures in different latitudes. Hence we should 

 expect the earth to have, on the average, nearly the temperature of 

 the small black body at the same distance, slightly less because it 

 reflects some of the solar radiation, and we find that it is, in fact, some 

 10° C. less. 



Professor Wien was the iirst to point out that the temperature of 

 the earth has nearly the value which we should expect from the 

 fourth power law. 



Here is a table showing the average temi)eratures of the surfaces of 

 the first four planets on the supposition that they are earth-like in all 

 their conditions: 



Tabic of tiiiipcnttun's <if ((irtli-likc phniclK. 



°C. 



Mercury ^-'^ 



Venus '»•> 



Earth 1" 



Mars — 3S 



The most interesting case is that of Mars. He has, we know, a day 

 nearly the same in length as ours. His axis is inclined to the ecliptic 

 only a little more than ours, and he has some kind of atmosphere. It 

 is exceedingly difficult to suppose, then, that his average temperature 

 can differ much from — 38° C. His atmosphere may be less pro- 

 tective, so that his day temperature may be higher, but then, to com- 

 pensate, his night temperature will be lower. Even his highest equa- 



