46 LIGHT SCIENCE FOR LEISURE HOURS. 



the moon's c annual equation;' but Laplace was the 

 first to point out that the variation is itself slowly 

 varying. The earth's orbit is slowly changing in 

 shape becoming more and more nearly circular year 

 by year. As the greater axis of her orbit is un- 

 changing, it is clear that the actual extent of the orbit 

 is slowly increasing. Thus, the moon is slightly 

 released from the sun's influence year by year, and so 

 brought more and more under the earth's influence. 

 She travels, therefore, continually faster and faster; 

 though the change is indeed but a very minute one 

 only to be detected in long intervals of time. Also 

 the moon's acceleration, as the change is termed, is 

 only temporary, and will in due time be replaced by 

 an equally gradual retardation. 



When Laplace had calculated the extent of the 

 change due to the cause he had detected, and when it 

 was found that ancient eclipses were now satisfactorily 

 accounted for, it may well be believed that there was 

 triumph in the mathematical camp. But this was not 

 all. Other mathematicians attacked the same problem, 

 and their results agreed so closely that all were con- 

 vinced that the difficulty was thoroughly vanquished. 



A very noteworthy result flowed from Laplace's 

 calculations. Amongst other solutions which had been 

 suggested, was the supposition (supported by no less 

 an authority than Sir Isaac Newton, who lived to see 

 the commencement of the long conflict maintained by 

 mathematicians with this difficulty), that it is not the 

 moon travelling more quickly, but our earth rotating 



