Mayl, 1874.] lOD [Frazer. 



NOTE ON THE COLUR OF THE MOON. 



By Pkof. Persifer Frazer, Jr. 



{Bead before the American Philosophical Society, May, \st, 1874.) 



Ou tlie 19th of September, 1873, I presented to the Society certain 

 views which as it seemed to me offered a satisfactory explanation of the 

 change of color undergone by the moon during the jjassage of the 

 twilight circle over her disc. I stated at that time that since what light 

 we get from the moon is reflected solar light, which so far as we can 

 discover has suffered no change on the surface of the moo , it would be 

 natural to suppose that the color of the light would be the same as that 

 of the Sun's light. 



The Sun's light is well known to be orange, and the Moon's in the day 

 time white, while at night the latter exhibits the same color as the Sun, 

 though the light is vastly more feeble. « 



That this change of color in the Moon depends upon the position of 

 the observer relative to the Sun there can be no doubt, and it is equally 

 certain that the phenomenon is of atmospheric origin, for the moon still 

 remains white for some time after the Sun has set. 



If, as Tyndall supposes, the blue color of the sky be due to the scat- 

 tering of the smaller waves of light by the inlinitessimal particles or 

 motes of the upper atmosphere ; and if the paths pursued by these re- 

 flected blue waves be, as experiment proves, in all directions from all 

 parts of this attenuated matter, the change of color may be easily ex- 

 plained.* 



Thus the Sun appears to us orange or yellow, because, of the waves 

 constituting white light, which impinge upon our atmosphere, a greater 

 proportion of blue than of red and yellow waves are scattered. Of these 

 waves thus scattered, a large proportion is thrown out again into sj^ace, 

 while what remain are sent in all directions — even directly towards the 

 Sun. 



This is 07ie cause of the blueness of the sky, if not the only one. 



When the Moon is shining at night the same conditions are fulfilled. 

 A small fraction of the Sun's light is thrown unchanged into our atmos- 

 phere and suffers the same filtering which his beams in daylight undergo; 

 with this diffei'ence, that as the blue rays are very inferior to the yellow 

 in luminousness, the more the amount of light is diminished, the brighter 

 relatively to the whole amount will appear these scattered rays ; and 



*The objection that if the waves of light were thus sifted by tenuous matter, those 

 of least length (or the ultra violet) would impart their color to the sky is invalid be- 

 cause Tyndall has shown, and every one can demonstrate for himself, that the earliest 

 appearance of color in a medium in which inflnitessimally fine particles of matter are 

 suspended is blue. Vide " Blue color of sky," &c., Tyndall, 



