CHRISTIAN HUYGHENS 55 



in a somewhat different way from his in order more easily to ex- 

 hibit all the consequences. 



Suppose S to be the position of the sun, E A part of the orbit of 

 the earth, S E M a straight line intersecting in M, the orbit of the 

 moon, represented by the circle A M. 



Now if light requires time — say an hour — to move the distance be- 

 tween the earth and the moon, then [at the time of an eclipse] it 

 follows that when the earth has come to E its shadow, or the stop- 

 page of the light of the sun, will not yet have reached M [the moon], 

 and will not for an hour. Counting from the instant the earth 

 reaches E, it will be an hour before it will reach M if it is to be 

 obscured there. This eclipse will not be seen from the earth for yet 

 another hour. Suppose that during these two hours the earth has 

 moved to X, the moon appearing eclipsed at M, the sun still being 

 seen at S. For I assume as does Copernicus that the sun is fixed 

 and since light moves in straight lines, is always seen in its true 

 position. 



But as a matter of fact, we are assured that the eclipsed moon 

 always appears directly opposite the sun; while on the above sup- 

 position [that light takes an hour in passing between the moon and 

 the earth], its position ought to be back of the straight line by the 

 angle Y X M, the supplement of the angle S X M. But this is not 

 the case, for this angle Y X M would be very easily noticed, it being 

 about 33 degrees. For by our analysis (found in the essay on the 

 causes of the phenomena of Saturn), the distance from the sun to 

 the earth, S E, is about 12,000 times the diameter of the earth, and 

 hence 400 times the distance of the moon, which is 30 diameters. 



