136 ESfFRA-RED EMISSION SPECTRA. 



is some heat conducted inwards, for the results of Langley Qoc. cit.) show 1 

 that the surface of the full moon is about 300 abs. 



FALL OF TEMPERATURE OF THE MOON DURING ECLIPSE. 



As already mentioned in Carnegie Publication No. 65, page no, 

 Langley made observations on the eclipse of the sun on September 23, 

 1885, which indicate a sudden and very rapid fall of energy received from 

 the moon at the beginning of the eclipse, with some indications of a rise 

 nearly as rapid after its conclusion. In Carnegie Publication No. 65, 

 page 113, are plotted his observed galvanometer deflections during the 

 progress of the eclipse. The observations were interrupted by the forma- 

 tion of clouds just at the predicted time for the moon to leave the umbra. 

 The curve shows that in the short time of about 1.5 hours, in passing from 

 the penumbra to the umbra, the radiation from the west limb has fallen 

 from a maximum to a zero value. In other words, the fall of temperature 

 is practically coincident with the change in illumination, and at first ap- 

 peared to the writer 2 to indicate that the greater part of the observed 

 energy is due to reflection. That the moon at mid-eclipse is still as warm 

 as the earth is shown by the fact that the galvanometer gave zero (or only 

 small positive) deflections. If the moon had been cooler than the earth, 

 then the deflections would have been negative. Subsequent search of the 

 literature on the subject shows that Very 3 found appreciable radiation from 

 the moon during totality. The following computations show that this is 

 to be expected. After about n days of insolation the temperature of 

 the sun-lit surface of the moon will be fairly constant. From the surface 

 inwards there will be a layer which may be considered at a uniform tem- 

 perature for the period of 1.5 hours, as compared with n days. If, 

 then, the moon were suddenly eclipsed, the fall of temperature of the 

 surface with time (x=o } 0=f(t)\ assuming at time t = o, that #0 = 300 abs.) 

 is found from the equation 



ax 2 at 



where k = conductivity, d = density, and 5 = specific heat. The first term 

 in the equation represents the energy lost by conduction (it is assumed 



1 Very, Astrophys. Jour., 8, pp. 273 and 274, 1898, however, records "inferred effective 

 temperatures" as high as 455 abs. for limited regions of the moon, and (loc. cit., p. 286) a 

 mean surface temperature of +97 C. The fact that the moon absorbs energy from the sun 

 (the maximum of which is of short wave-lengths) and emits energy of wave-lengths from 8 to 

 10 m, where, on account of the probable high reflecting power, the emissivity is very low, would 

 explain why Mr. Very has found a temperature which is much higher than that of a complete 

 radiator under similar conditions. 



2 Phys. Rev., 23, p. 247, 1906. 



3 Very. Prize Essay on the Distribution of the Moon's Pleat, p. 40. Professor Very has 

 called my attention to an error in Carnegie Publication No. 65, p. 112, third line from the 

 top: viz, "Harrison (not Langley) reminds the reader ..." 



