52 LIGHT SCIENCE FOR LEISURE HOURS. 



and say: "At such and such a long-past epoch, a given 

 star or planet occupied such and such a position upon 

 the celestial sphere." But how are they to verify such 

 a statement ? It is clear that, in general, they cannot 

 do so. Those who are able to appreciate (or better, 

 to make use of) the predictions of astronomy, will, 

 indeed, very readily accord a full measure of confidence 

 to calculations of past events. They know that as- 

 tronomy is justly named the most exact of the sciences, 

 and they can see that there is nothing, in the nature 

 of things, to render retrospection more difficult than 

 prevision. But there are hundreds who have no such 

 experience of the exactness of modern astronomical 

 methods who have, on the contrary, a vague notion 

 that modern astronomy is merely the successor of 

 systems now exploded ; perhaps even that it may one 

 day have to make way in its turn for new methods. 

 And if all other men were willing to accept the cal- 

 culations of astronomers respecting long-past events, 

 astronomers themselves would be less easily satisfied. 

 Long experience has taught them that the detection of 

 error is the most fruitful source of knowledge ; there- 

 fore, wherever such a course is possible, they always 

 gladly submit their calculations to the test of obser- 

 vation. 



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 



