OUR CHIEF TIME-PIECE LOSING TIME. 39 



* 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 unchanging, it is clear that 

 the actual extent of the orbit is slowly increasing. 

 Thus, the moon is slightly released from the sun's in- 

 fluence 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 accelera- 

 tion, 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 followed 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 



