APPENDIX I. 



THE TEMPERATURE OF THE MOON. 



I. 



From his Mount Whitney observations 1 Langley concluded that the 

 soil of an airless planet at the moon's distance from the sun would have 

 a temperature not greatly above 225 C. His later observations on 

 the radiation (reflection) from the moon led him to conclude that the 

 temperature of the sunward surface of the moon is about o C. This 

 inference, he mentions, is contradictory to the one drawn from the 

 Mount Whitney observations, which in themselves he considered exact. 

 "The most reliable spectrum comparisons with a blackened screen show 

 an average effective temperature of +45 C., near the time of full 

 moon." 1 He could detect no radiation from the dark moon. For the 

 bright moon, his spectrum radiation curves rise from a zero value at 

 6 to 7 fj. to a maximum at 8.6 /j,, with a smaller maximum at lOju. 

 There are absorption bands, due to atmospheric water, at 6 /x and 9.6 /x. 

 He found also direct radiation from the sun in this region, but does 

 not consider it in arguing for a direct radiation from the moon. He 

 considered that the part of radiation from the moon, which was trans- 

 mitted by glass, is reflected energy from the sun, whereas the part 

 absorbed by glass is radiation from the moon. The same assumption 

 was also made by Lord Rosse, but it is untenable if we admit a selective 

 reflection of the moon. To the writer the line of reasoning seems false, 

 in the light of o.ur present knowledge of infra-red radiation, and a better 

 criterion for judging whether or not the surface of the moon becomes 

 appreciably warmer under the rays of the sun would seem to be to 

 reexamine the rapidity of the fall and rise in intensity of the radiation 

 from the moon during an eclipse. Langley (loc. cit.) made observa- 

 tions on the eclipse of September 23, 1885, which indicate a sudden and 

 very rapid falling off in the heat as the eclipse commenced, with some 

 indications of a rise nearly as rapid after its conclusion. In fig. 89 are 

 plotted his observed galvanometer deflections during the progress of the 

 eclipse. The ordinates are galvanometer deflections; the abscissae are 

 the predicted times in hours to mid-eclipse. The three curves are for 

 the center, the east, and the west limbs of the moon. The observations 

 were interrupted by the rapid formation of clouds just at the predicted 

 time for the moon to leave the umbra. The eye observations showed 



1 Mem. Nat. Acad. Sci., 4, p. 197, 1887. 

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