Eclipses 



infrequence and short duration of eclipses. If 

 the moon circled round the earth in the plane 

 described by the latter in its yearly rotation 

 round the sun, every full moon would be accom- 

 panied by an eclipse of the moon and every new 

 moon by an eclipse of the sun. 



This is nearly the case, since the plane of the 

 lunar orbit is inclined only five degrees to the 

 plane of the ecliptic; but this slight inclination 

 is sufficient to upset everything, because it 

 makes the possibility of an eclipse depend on a 

 somewhat rare coincidence; the eclipse is only 

 possible if at full or new moon our satellite is 

 crossing the plane of the terrestrial orbit, which 

 then justifies its name of ecliptic. 



And this condition, necessary because it 

 places the three bodies in a straight line, is not 

 even sufficient. As a matter of fact, if, in the 

 case of lunar eclipses, we calculate from the 

 distance between the earth and the sun and 

 the diameter of these two globes the length of 

 the cone of shadow thrown, by the earth, we 

 find that it is at least 210 terrestrial radii long; 

 as the moon is only about sixty terrestrial radii 

 distant from our globe she should necessarily 



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