PENDING PROBLEMS OF ASTRONOMY. 53 



then progress would be as by wings : we should fly, where now we 

 crawl. 



As to the physical problems presented by the moon, the questions 

 relating to the light and heat — the radiant energy — it sends us, and 

 to its temperature, are the most attractive at present, especially for the 

 reason that the results of the most recent investigators seem partially 

 to contradict those obtained by their predecessors some years ago. It 

 now looks as if we should have to admit that nearly all we receive 

 from the moon is simply reflected radiation, and that the temperature 

 of the lunar surface nowhere rises as high as the freezing-point of 

 water, or even of mercury. At the same time, some astronomers of 

 reputation are not disposed to admit such an upsetting of long-received 

 ideas ; and it is quite certain that, in the course of the next few years, 

 the subject will be carefully and variously investigated. Closely con- 

 nected with this is the problem of a lunar atmosphere — if, indeed, she 

 has any. 



Then there is the very interesting discussion concerning changes 

 upon the moon's surface. Considering the difference between our 

 modern telescopes and those employed fifty or a hundred years ago, 

 I think it still far from certain that the differences between the repre- 

 sentations of earlier and later observers necessarily imply any real 

 alterations. But they, no doubt, render it more or less probable that 

 such alterations have occurred, and are still in progress ; and they 

 justify a persistent, careful, minute, and thorough study of the details 

 of the lunar surface with powerful instruments : especially do they 

 inculcate the value of large-scale photographs, which can be preserved 

 for future comparison as unimpeachable witnesses. 



I will not leave the moon without a word in respect to the remark- 

 able speculations of Professor George Darwin concerning the tidal 

 evolution of our satellite. Without necessarily admitting all the nu- 

 merical results as to her age and her past and future history, one may 

 certainly say that he has given a most plausible and satisfactory ex- 

 planation of the manner in which the present state of things might 

 have come about through the operation of causes known and recog- 

 nized, has opened a new field of research, and shown the way to new 

 dominions. The introduction of the doctrine of the conservation of 

 energy, as a means of establishing the conditions of motion and con- 

 figuration in an astronomical system, is a very important step. 



In the Planetary system we meet, in the main, the same problems 

 as those that relate to the moon, with a few cases of special interest. 

 For the most part the accordance between theory and observation in 

 the motions of the larger planets is as close as could be expected. The 

 labors of Leverrier, Hill, Newcomb, and others, have so nearly cleared 

 the field that it seems likely that several decades will be needed to 

 develop discrepancies sufficient to furnish any important corrections 

 to our present tables. Leverrier himself, however, indicated one strik- 



