OUR CHIEF TIME-PIECE LOSING TIME. 37 



they always gladly submit their calculations to the test 

 of observation. 



Now, looking backward into the far past, it is only 

 here and there that we see records which afford means 

 of comparison with modern calculations. The planets 

 had swept on in their courses for ages with none to 

 note them. Gradually, observant men began to notice 

 and record the more remarkable phenomena. But such 

 records, made with very insufficient instrumental means, 

 had in general but little actual value: it has been 

 found easy to confirm them without any special regard 

 to accuracy of calculation. 



There is one class of phenomena, however, which 

 no inaccuracy of observation can very greatly affect. A 

 total eclipse of the sun is an occurrence so remarkable, 

 that (1) it can hardly take place without being recorded, 

 and (2) a very rough record will suffice to determine 

 the particular eclipse referred to. Long intervals elapse 

 between successive total eclipses visible at the same 

 place on the earth's surface, and even partial eclipses 

 of noteworthy extent occur but seldom at any assigned 

 place. Very early, therefore, in the history of modern 

 astronomy, the suggestion was made, that eclipses 

 recorded by ancient historians should be calculated 

 retrospectively. An unexpected result rewarded the 

 undertaking. It was found that ancient eclipses could 

 not be fairly accounted for without assigning a slower 

 motion to the moon in long-past ages than she has 

 at present ! 



Here was a difficulty which long puzzled mathe 



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