Eclipses 



the moon varies between fifty-five and sixty-two 

 terrestrial radii; on the other hand, the length 

 of the shadow-cone thrown by the moon varies be- 

 tween fifty-seven and sixty terrestrial radii and 

 is not reduced by any refraction, as there is no 

 lunar atmosphere ; it follows that this cone may 

 or may not reach the earth, and that therefore an 

 eclipse need not necessarily be total. Let us 

 suppose the favourable conditions to be realised; 

 then if a giant wandering through the worlds 

 could observe what is happening he would see 

 the cone of shadow, sweeping behind the moon, 

 strike the earth and mark on its surface a circular 

 black spot, some 200 kilometres in diameter, 

 which travels with extreme rapidity in the 

 general direction from west to east. This spot, 

 which no sunlight penetrates, is itself surrounded 

 by a more extensive zone, in which the light 

 decreases progressively from the interior towards 

 the edges; this is the zone of penumbra, 

 illuminated by a variable portion of the solar 

 disc. As a matter of fact, an observer in a 

 balloon, or on a high mountain, sees quite clearly 

 the moving cone formed by the lunar shadow 

 as it advances like a dark and threatening 



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