LUNAR TEMPERATURE. 135 



temperature of about 6300 abs. One of the most refractory substances 

 farthest removed from a full radiator in its emissive power is bright plati- 

 num. For this substance the constant is 2630 instead of 2920. If the 

 radiation from the sun is purely thermal it must lie between the "black 

 body" and bright platinum in its radiating properties. 

 If the sun radiates like platinum, from the equation 



^max T = 2630, using i max = 0.46 n T = 5700 abs. 



The value of the sun's temperature, viz, 5900 , used in the present compu- 

 tations, is about the mean of the " observed " and the computed tempera- 

 tures. From the results given in the preceding chapter, it is evident that 

 the true temperature of the sun can not be determined by these methods. 



THE LIMITING TEMPERATURE OF THE SURFACE OF THE MOON. 



Poynting (loc. cit.) further shows that when there is no conduction 

 inwards from the surface, the highest temperature of a full radiator is 

 attained when its radiation is equal to the energy received. Equating the 

 energy to the solar constant, using 5=0.175 Xio 7 ergs 



5.35 Xio- 5 4 = 0.175 Xio 7 



whence a , Q , 



o =426 abs. 



which is the upper limit of the temperature of the moon, assuming that it 

 absorbs all the energy received from the sun. If part of the energy is 

 reflected and only a fraction x of that falling on it is absorbed, then the 

 effective lunar temperature is 426 -\/x abs. From Langley's 1 estimate that 

 the moon absorbs x=| the energy it receives, Poynting {loc. cit.) computes 

 the upper limit of temperature of the surface exposed to a zenith sun to be 



0=426X (|) i =4i2 abs. 



But this upper limit to the temperature of the hottest part of an airless 

 planet is never attained because the moon turns the same face to the 

 earth instead of to the sun. He shows that, if N is the normal stream of 

 radiation from a unit of surface of the moon immediately under the sun, 

 the normal stream from the equivalent flat disk is Nd=%N. 



" The effective temperature of the flat disk is therefore V tnat of the surface imme- 

 diately under the sun at the same distance from it. Then the effective average temperature 

 is 412 X -\/i = 371 abs. The upper limit then, to the average effective temperature of the 

 moon's disk, is just below that of boiling water." 



If we use 5 = 0.147 X io 7 instead of 0.175 X IC)7 > = 408 abs. and the 

 "effective average temperature" is 408 V I = 350 abs. or 82 C. for 

 the full moon. This assumes no conduction inwards. Evidently there 



1 Langley: Nat. Acad. Sci., vol. 4, part. 2, p. 197. 



