2 LIGHT SCIENCE FOR LEISURE HOURS. 



him wholly from our view, and considerably to overlap 

 his disc all round. 



But there is another circumstance besides proxi- 

 mity to the earth which affects the moon's apparent 

 dimensions. She appears to grow larger as she rises 

 above the horizon. I am not referring, of course, 

 to the appearance which she presents to the naked 

 eye. Judged in this way she seems to grow smaller 

 as she rises above the horizon. But when she is 

 measured by any trustworthy instrument the reverse 

 is found to be the case. The cause of the peculiarity 

 is not far to seek. We see the moon, not from the 

 centre of her orbit (that is, the earth's centre), but 

 from a point on the earth's surface a point, there- 

 fore, which is four thousand miles nearer to the moon's 

 orbit. Accordingly, if the moon were directly over- 

 head (which never happens in our latitudes), her dis- 

 tance from us would be diminished by four thousand 

 miles, and she would look proportionately larger. The 

 sun is not affected in this way, because four thousand 

 miles is a mere nothing in comparison with the enor- 

 mous distance at which the sun is removed from us. 

 Accordingly, other things being equal, the higher the 

 moon is at the time of a total eclipse, the greater is 

 the eclipse. 



In order, therefore, that an eclipse may be as 

 great as possible, the sun should be as far as possible 

 from the earth, which happens about the beginning of 

 July ; the moon should be as near as possible to the 

 earth, which happens (roughly speaking) once in every 



