TEMPERATURE OF THE MOON. 115 



this, say o.i. The ratio of the emissive power of the moon to that of 

 the sun will then be 0.00016, which is four times (0.00016-1-0.00004) 

 the reflected energy of the sun from the moon. On this assumption, 

 the moon at o C. would radiate twice as much as it would reflect from 

 the sun. 



This shows that unless there is something radically wrong in the 

 assumptions made, the above coincidence is fortuitous. This, however, 

 does not settle the question, for Langley observed also direct radiation 

 from the sun in this region, and from existing data of the radiation 

 from the moon we do not know how much of it is selectively reflected 

 energy from the sun. Computations which require all sorts of assump- 

 tions will not settle the question; but a bolometric comparison of the 

 spectrum energy curves of the sun and of the moon, made at high alti- 

 tudes, will be of the greatest service in clearing up this matter. 



Since writing this appendix I have computed the fall of temperature of the 

 lunar surface, neglecting the conductivity from the interior (which simplified the 

 computation) and find that for an emissivity of 0.3 of a "black body" (iron oxide) 

 the temperature would fall from 300 to 273 abs. in 1.2 hours, while for an 

 emissivity of o.i, the time would be over 3 hours. By allowing for conductivity 

 these periods would be considerably increased, so that unless the moon's emissiv- 

 ity is considerably higher, the cooling curve will not be coincident with the eclipse 

 curve in fig. 89. Assuming that the sky does not radiate to the bolometer, then 

 the latter would give zero, and finally negative deflections as the temperature falls. 

 Langley does not mention negative deflections from the moon. He records 

 negative deflections for his sky-screen curves. 



