June 20, 1913] 



SCIENCE 



951 



Although I have not heretofore attempted 

 to revise the memoir on " The Temperature of 

 the Moon," let me say here that the tempera- 

 ture for which I am responsible in that work 

 (namely, +45° C. on the average), and which 

 is given, indeed, but in such a guarded way 

 as to lose much of its force, is certainly too 

 low. In proof of this, reference may be made 

 to figures 11 and 12,' which show the position 

 of the lunar image on the very wide slit which 

 was necessary in order to obtain a readable 

 galvanometer deflection in the almost evan- 

 escent lunar spectrum. It will be seen that 

 at no time was the slit completely filled by 

 the lunar image. There were always corners 

 occupied by bits of sky, or by the unillumin- 

 ated part of the moon, while the blackened 

 screen containing boiling water always com- 

 pletely filled the slit aperture. Consequently, 

 the lunar heat was underestimated. More- 

 over, the lunar radiation was an average per- 

 taining to regions which include a wide range 

 of temperatures, and necessarily fell much 

 below the maximum radiation from the sub- 

 solar point. 



In the description of the instrumental ar- 

 rangements' we read: 



Care was taken that the lunar image formed by 

 the condensing mirror, and having a diameter of 

 6.4 mm., should fall accurately upon the central 

 portion of the slit, and thus only that portion is 

 illuminated. In reducing the observations to a 

 standard width of slit, the variation in this width 

 from night to night having been considerable, the 

 assumption is made that the amount of heat 

 passing through the slit varies simply as the 

 width, which is the same as to disregard the 

 curvature of the upper and lower limbs of the 

 lunar image, as well as the secondary effect of the 

 variation of this lunar semi-diameter. 



The inaccuracy of this disposition of lunar 

 image and slit is obvious. The only excuse 

 for neglecting it was that the condition of 

 steadiness of our galvanometer at that time 

 was not such as to call for any greater nicety 

 in the other adjustments. No attempt was 



'Op. cit., p. 12. 

 ^Op. cit., p. 121. 



made to assign a probable error to the nu- 

 merical results. They were quantitative, but 

 only roughly so. Hence it has seemed to me 

 a waste of time to attempt to revise these 

 measures. It would be better to repeat them 

 with improved facilities. 



Whether Secretary Langley ever accepted 

 the results of the later measures which assign 

 a temperature of 454° Abs. Cent.' to the lunar 

 subsolar point is not known; but as he had 

 already stepped up from — 225° C. to a tem- 

 perature " a little above zero," let us hope 

 that he may finally have been willing to go 

 the rest of the way. 



To those who find it diflicult to accept a 

 lunar surface temperature above that of boil- 

 ing water, because of the low temperature at 

 great elevations in the earth's atmosphere, 

 where the rarefaction is still much less than 

 on the moon, it may be pointed out that the 

 insolation temperature attained by a planetary 

 surface, after allowing for the variation in the 

 intensity of sunshine, depends mainly on two 

 factors : The duration of continuous insola- 

 tion, and the absorbent power of the planetary 

 atmosphere for return radiation from the 

 planet's surface. The diminution of solar 

 radiation in proportion to the inverse square 

 of the sun's distance determines the available 

 radiant energ-y, but the temperature acquired 

 through exposure to sunshine depends to a 

 still greater extent upon the nature of the 

 atmospheric trap by which heat is captured. 

 In this respect the greatest variety prevails 

 among the planets of the solar system. The 

 major planets possess denser and more highly 

 absorbent atmospheres, capable of trapping 

 greater and greater amounts of heat, as their 

 distances from the sun increase. The evi- 

 dences of heat, namely, strong aqueous ab- 

 sorption-bands in the spectrum, prevalence of 

 cloud, and a vigorous circulation in the at- 

 mospheres of planets at so great a distance 

 from the sun, may be explained on these prin- 

 ciples, as is shown in my paper on " The 

 Greenhouse Theory and Planetary Tempera- 



' Frank W. Very, "The Probable Eange of 

 Temperature on the Moon," II., Astrophysical 

 Journal, Vol. 8, p. 284, December, 1898. 



