CHAPTER III. 



RADIATION FROM SELECTIVELY REFLECTING BODIES, WITH 

 SPECIAL REFERENCE TO THE MOON. 



In Carnegie Publication No. 65, p. no, on infra-red reflection spectra, 1 

 the writer showed that all the silicates examined have a region of strong 

 selective reflection in the region of 8 to 10 jj. and discussed the effect this 

 would have upon a reflecting surface like the earth, or the moon, which is 

 composed largely of silicates. Since this discussion has brought forth 

 comments favorable and unfavorable, in print and in private communica- 

 tions, the purpose of the present paper is to summarize, in a general way, 

 the present data bearing upon the subject, and to discuss certain points 

 not mentioned in the previous communication. The computed temper- 

 atures of the sun and of the moon derived in the present paper will be used 

 in the subsequent calculations, which will be found to be in slight disagree- 

 ment with the writer's previous results. As a whole, it will be shown how 

 easy it is to obtain estimated values which ultimately can have but little 

 meaning other than a guide in experimental work. 



THE EFFECTIVE TEMPERATURE OF THE SUN. 



It has been shown by Poynting 2 that, when a surface is a complete 



radiator and absorber, its temperature can be determined at once by the 



fourth power law, if we know the rate at which it is radiating energy. If 



it radiates what it receives from the sun, then a knowledge of the solar 



constant enables us to find the temperature, which will be the highest the 



surface can attain when it is receiving heat only from the sun. Knowing 



the solar constant and the radiation constant (cr=5.32Xio~ 5 ergs Kurl- 



baum) 3 Poynting computes the effective temperature of the sun. If 5 is 



the radius of the sun's surface, R is the radiation per square centimeter; 



then the total rate of emission is 47:s 2 R. This must equal the radiation 



passing through the sphere of radius r, at the distance of the earth, and 



with a surface 4xr 2 gives 



47rs 2 R = 47cr 2 S 



where S is the solar constant. Hence R = 46000 S. Using the solar 



1 Investigations of Infra-red Spectra, Parts III and IV, published by the Carnegie Insti- 

 tution of Washington, D. C, December, 1906. 



2 Phil. Trans. Roy. Soc. Lond., vol. 202 A, p. 525, 1903. 



3 Kurlbaum: Wied. Ann., 65, p. 748, 1898. 



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