114 TEMPERATURE Ol- THE MOON. 



selectively reflecting surfaces, with a band of metallic reflection from 

 8.5 to 10 p.. Hence, in the region of the spectrum from 4 to 7 /A only 

 3 to 5 per cent of the radiation from the sun will be reflected from the 

 moon, under the best surface conditions, while in the region from 8 to 

 10 fji the reflecting power will rise to 50, or even 90 per cent. Under the 

 worst conditions the balance will still be in favor of the region of 

 8 to lOfji. 



The occurrence of the maxima of the reflection curves and Langley's 

 observed emission curve of the moon (that part which is transmitted in 

 the transparent region of the earth's atmosphere between 7 and lOju,) 

 in this region of the spectrum may, of course, be nothing more than a 

 mere coincidence, and the author calls attention to it merely as an inter- 

 esting side issue to the main research. The reflecting power of the 

 moon for visible rays is onh 5 ooVoo full sunlight. Assuming that at 9ju, 

 the reflecting power is, on an average, 20 times this, the value becomes 

 25oo7' or 0.00004 P er cent. Assuming the temperature 1 of the moon to 

 be 300 abs., we can get some idea of its emissive power at 9/x, as com- 

 pared with the sun at a temperature 2 of 5500. 



From Planck's formula 3 for the distribution of energy in the spec- 

 trum of a "black body" (which, of course, the moon is not) 



/ _L x 



-*f * r > 



I=c^ \e -l) 



we can obtain a ratio of the intensities for the two temperatures 

 ^i = 300 abs. and T 2 = 5500 from the formula 



where C 2 14,500, using ), (max.)=9 /*. 



This ratio is 0.0016. But the moon, not being a black body, will have 

 a smaller emissivity. 



If its surface were of iron oxide 4 its emissivity would be only 0.3 

 that of a black body at 300, and, judging from the rapid decadence of 

 the radiation curve (which may indicate a low conducting material) 

 during an eclipse (fig. 89). its emissive power may be even less than 



1 Very : Astrophys. Jour., 8, 199, 1898 ; also Langley, loc. cit. ; Poynting : Jahrb. 

 der Radivaktivitat und Elektronik, 2, 42, 1905. 



1 Warburg: Verh. Deutsch. Phys. Ges., i, 2, p. 50, 1899; Day and Van Ostrand: 

 Astrophys. Jour., 19, p. 1, 1904. 



'Planck: Vehr. d. Deutsch. Phys. Ges., 2, p. 202, 1900; Ber. d. k. Akad. d. Wiss., 

 Berlin, p. 544, 1901. 



4 Kayser : Spectroscopy, vol. II, p. 80. 



