36 THE POPULAR SCIENCE MONTHLY. 



LAPLACE'S PLAN FOR PERPETUAL MOONLIGHT. 



BY DANIEL KIRKWOOD. 



ONE of the questions considered by Laplace in the early part 

 of the century, and which he thought of sufficient interest 

 to have a place in his System of the World, has dropped almost 

 wholly out of view. I refer to the relation of the moon to the 

 earth what it is and what it might have been. The subject is 

 not even referred to in any recent text-book on astronomy. The 

 conclusion of Laplace, however, was not hastily reached, and it 

 remained in his hands, without modification, for a number of 

 years. The great name of the author probably prevented astrono- 

 mers of the day from undertaking any criticism of his conclu- 

 sions, and especially from the expression of any opinion on a 

 mathematical question different from that of the greatest astrono- 

 mer of the century. If Laplace himself ever saw his mistake he 

 never mentioned it, as in the case of a mathematical error pointed 

 out by Dr. Bowditch,the translator of the Me'canique Celeste. Dr. 

 Bowditch's letter informing him of the error was never acknowl- 

 edged. The mistake, however, was rectified in a new edition. But 

 I proceed with the subject. 



If moonlight, it has been said, be always pleasant and desira- 

 ble if it contribute to the convenience and enjoyment of life, and 

 if its perpetuity be not inconsistent with the laws by which the 

 world is governed why has its use been so largely denied us ? 

 Why has Nature, or the Author of Nature, left us so great a por- 

 tion of our time in almost total darkness ? Such questions have 

 doubtless occurred to thoughtful minds in all ages. The subject 

 is one of interest and curiosity. Let us briefly consider some of 

 the possible relations of a satellite to its primary, including a spe- 

 cial case proposed by Laplace. 



Sir Isaac Newton, who preceded Laplace by about a century, 

 had found evidence, as he claimed, that the material universe is 

 the work of an all-wise designer. The author of the Me'canique 

 Celeste, the greatest mathematical astronomer of his age, seldom 

 discussed questions of a moral nature ; but, not accepting New- 

 ton's views on the doctrine of final causes, or the doctrine of de- 

 sign in the material world, he took occasion to point out a so-called 

 failure of Nature in adapting means to ends. If the moon was 

 designed to give light by night, the purpose, he said, had largely 

 failed, and he (Laplace) could suggest a better plan. But the en- 

 tire passage is quoted as follows : 



" Some partisans of final causes have imagined that the moon 

 was given to the earth to afford it light during the night. But in 

 this case Nature would not have attained the end proposed, since 



