LUNAR EMISSIVITY. 139 



sion curves of the water vapor (curve c, fig. 102) it will be seen that in the 

 region from 5 to 8 /j. nearly all the energy will be absorbed by the earth's 

 atmosphere. The reflecting power of the moon for visible rays, according 

 to Langley (loc. cit.), is only -g"oo 1 oo"ff ^ u ^ sunlight. Assuming that at 9 fi 

 the reflecting power of the silicates is, on an average, 10 times that at 0.5 

 to 4 ft, this value becomes 5^oo"o or 0.00002. (This seems a fair estimate 

 of ratio of the reflecting power of the silicates at 1 and 9 fi. In the 

 absence of better data this solution can be only a rough approximation.) 

 Using the above values for the temperature of the moon and of the 

 sun, from Planck's 1 formula for the distribution of the energy in the spec- 

 trum of a complete radiator (which, of course, the moon is not) 



E k = c l X-*(e c * nT i)- 1 



we can obtain the ratio of the intensities for the two temperatures T=t ) ^o 

 abs. and T 2 =$goo abs. from the formula 



Where c 2 = 14,500 and X=g p. The ratio is 0.00316. But the moon, not 

 being a perfect radiator, will have a smaller emissivity at 8 to 10 fi. If 

 its surface were iron oxide, 2 its emissivity would be only 0.3 that of a full 

 radiator, and, for the region at 9 /<, judging from the drop in the emission 

 curve (see Carnegie Publication No. 65, p. in) and the high reflecting 

 power of the silicates, the emissive power may be less than this, say 0.1. 

 This ratio of the emissive power of the moon to that of the sun will then 

 be 0.000316, which is 16 times (0.000316 -=-0.00002) the reflected energy 

 of the sun from the moon. If we had taken 300 as the temperature of 

 the moon, then this ratio would be (0.00014 ^0.00002) = 7 instead of 16. 

 Computations 3 like these, which require all sorts of assumptions, can be of 

 little value ultimately. Any computation can not be more than a rough 

 approximation, for the reflecting powers, observed up to 4 fi, will be too 

 high, due to internal reflection. In the region of 8 to 10 p. (for silicates) 

 there can be but little if any internal reflection. Hence, the ratios just 

 obtained are too low, but how much so is difficult to estimate because 

 of the lack of data. We know that Langley observed also direct radia- 

 tion from the sun, in this region of the spectrum, and from existing data 

 of the radiation from the moon in this region we do not know how much of 

 it is selectively reflected energy from the sun. The amount reflected must 

 be small, but, since the total amount emitted is also small, it is important 

 to establish the fact that there is selective reflection in this region. 



1 Planck: Verb. Deutsch. Phys. Ges., 2, p. 202, 1900. 



2 Kayser: Spectroscopy, vol. 2, p. 80. 



3 Very: Astrophys. Jour., 24, p. 353, 1906, computes a much greater difference between 

 the amount reflected and the amount emitted. 





a 



