124 Prof. N. S. Shaler 07i Earthlight on the Moon, 



not that it gives a chance to say something concerning the ap- 

 pearance of this dark surface at this state of ilhimination when 

 seen through a telescope of first-class light-gathering power. 

 With the 15-inch Mertz of the observatory of this University it 

 is possible, under favourable conditions^ to see all the principal 

 features of the topography on the dark region illuminated only 

 with this earthshine. In the course of some years of study upon 

 the geology^ if we may so call it^ of the moon^ I have had several 

 opportunities of seeing, under these conditions, all the great 

 features of the dark surface shine out with amazing distinctness. 

 The curious point, however, is that the eye is not enabled to re- 

 cognize the craters by light and shade, for the light is too feeble 

 for that, besides being too vertical for such a result ; but the 

 relief is solely due to the difference in the light-reflecting power 

 of the various features of the topography. Whatever becomes 

 very brilliant under the vertical illumination of the full moon 

 (the edges and floors of many craters, certain isolated hills, and 

 the radiating bands of light) shines out with a singular distinct- 

 ness when lit by our earth^s light. This is important, inas- 

 much as it shows pretty conclusively that the differences in 

 the brightness of various parts of the surface of the moon is 

 not due to the effects of the heating of the surface during the 

 long lunar day, but is dependent upon difference in the light- 

 reflecting power. There are several degrees of brightness ob- 

 servable in the different objects which shine out by the earth- 

 light. In this climate there are not over three or four nights in 

 the year when the moon can be caught in favourable condi- 

 tions for this observation. The moon should not be over twenty- 

 four hours old* (the newer the better), and the region near the 

 horizon should be reasonably clear. Under these conditions 

 I have twice been able to recognize nearly all the craters on 

 the dark part over fifteen miles in diameter, and probably one 

 half the bands which show" with a power of 100 when the 

 moon is full. 



That this partial illumination of the dark part of the moon 

 is in one way connected with the action of an atmosphere is 

 clearly shown by the fact that the light is evenly distributed over 

 the whole surface, and does not diminish as we go away from 

 the part which is lit by direct sunlight^ as it should do if the 

 supposition of Mr. Sharpe were well founded. 



It will be noticed that this fact probably explains the greater 

 part of the perplexing statements concerning the illumination 

 of certain craters before the terminator came to them. It cer- 

 tainly accounts for the volcanic activity which has so often 



* Sometimes when the moon is two days old, and the other conditions 

 are favourable, this illumination is faintlv visible. 



