44 LIGHT SCIENCE FOR LEISURE HOURS. 



course is possible, 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 

 have 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 instru- 

 mental means, have in general but little actual value. 

 It has been found easy to confirm them without any 

 special regard to accuracy of calculation. 



But there is one class of phenomena which no in- 

 accuracy 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 his- 

 torians should be calculated retrospectively. An un- 

 expected result rewarded the undertaking: it was 

 found that ancient eclipses could not be fairly ac- 

 counted for without assigning a slower motion to the 

 moon in long-past ages than she has at present I 



Here was a difficulty which long puzzled mathe- 



