THE PHENOMENA OF LIGHT 337 



the somewhat indefinite but distinctly circular outline of 

 this shadow he sees confirmation of his belief that the form 

 of the earth is spherical. A so-called " solar" eclipse is to 

 him the result of the moon coming between the observer 

 and the sun. The shadow of the moon overspreads him 

 as an observer, and it is really a case of an eclipse of the 

 observer. It would be so called by observers elsewhere than 

 on the earth itself. In those somewhat infrequent occur- 

 rences where the moon comes directly in line between sun 

 and earth, the astronomer is given a very few minutes of 

 time when the sun's disc may be completely covered by the 

 moon. He then has an opportunity to observe through the 

 telescope the phenomena of the sun's atmosphere as it extends 

 far out into regions not hidden by the moon. By aid of the 

 telescope it is possible to get photographs of these appear- 

 ances. The description of what is seen at these times of a 

 total eclipse of the sun, and the interpretation of this as 

 given in texts on astronomy, makes fascinating reading. 

 Some knowledge of the beliefs of astronomers concerning 

 the universe, and of the basis for such beliefs, should be 

 included in the schooling of all persons. 



It was from a study of the times of disappearance of 

 one of the moons of Jupiter which suffers eclipse every few 

 days, and from his interpretation of what these signified, 

 that Roemer calculated the velocity of light as about 

 186,000 miles per second. So exact are the calculations of 

 astronomers, and so regular are the movements of the heav- 

 enly bodies, that it is possible to state (from calculations) 

 the time of occurrence of any eclipse in the past or in the 

 future, its duration, and where visible. The astronomer 

 deals with distances and periods of time too great to be 

 comprehended readily. But so exact are his measurements, 

 and the calculations based upon them, that any slight depar- 

 ture of his observations from calculated results leads to 



