RADIATION IN THE SOLAR SYSTEM. 



189 



We see from this table that the temi)erature at the earth's distance 

 is remarkably near the axcrage temperature of the eartlrs surface, 

 -which is usually estimated as about U'>'^ C., or ()()° F. This can hardly 

 be regarded as a mere coincidence. The siii-facc of the earth receives, 

 we know, an amount of heat fi'oni tlie inside almost infinitesimal com- 

 pared with that Avliich it receives from the siiii. and on the sun, there- 

 fore, we depend for our temperature. The earth ac(|uires such a tem- 

 perature, in fact, that it radiates out what it I'cceives from the sun. 

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

 to play any serious ]>art in equalizing the temj^erature of ditlereut 

 regions. But the rotation about its axis seciii'es neai'ly uniform tem- 

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

 tend to equalize temperatures in ditl'erent latitudes. Hence we should 

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

 the small l)lack lw)dy at the same distance, slightly less because it 

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

 10" C. less. 



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

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

 fourth power laAv. 



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

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

 their conditions : 



'J'dhtc <>l IciHix'nil iircs of ((ntlilil,!' phiiicts. 



°C. 



Mercury l!»4 



Venus (it) 



Earth 17 



Mars — 38 



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

 nearl}' the same in length as ours. His axis is inclined to the ecliptic 

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

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

 can differ much from ■ — 38° C. His atinos})hei"e may be less pro- 

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

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



