The Evolution of the Sciences 



enter the shadow-cone whenever the above 

 conditions are realised. 



However, what happens is less simple, owing 

 to the refraction caused by our atmosphere, 

 which acts on the solar rays as a regular con- 

 vergent lens. The summit of the shadow-cone 

 is thus thrown back to forty-two radii, instead 

 of 210; consequently a complete eclipse of any 

 part of the moon is impossible. All that can 

 happen is a decrease of brightness, because the 

 sun is never hidden in its entirety from any 

 point of our satellite. An observer placed on 

 the moon, and looking at the phenomenon 

 which constitutes for us an eclipse of the 

 moon, but for him an eclipse of the sun, 

 would still see, at the most favourable moment, 

 three-quarters of the sun's disc. On the other 

 hand, as the terrestrial atmosphere absorbs 

 chiefly the blue and violet rays, our observer 

 would receive only red light, such as the sun 

 sends us at his setting; this is the reason why 

 the moon, which only shines with borrowed 

 light, appears red during eclipses. 



In the case of eclipses of the sun the conditions 



are different; the distance from the earth to 



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