V 



THE PRESSURE OF LIGHT IN ASTRONOMY. 

 SOME POSSIBLE CONSEQUENCES 



THE forces due to light pressure are so small, 

 and the disturbances due to the air are, in com- 

 parison so great, that here on the surface of the 

 Earth and immersed in its atmosphere, we can- 

 not expect to find any recognizable results of 

 the pressure except in carefully made laboratory 

 experiments. 



Out in the space, however, in which the planets 

 roll round the sun, where the rarity of any matter 

 present must be vastly greater than that in any 

 so-called vacuum which we can produce, light- 

 pressure may have undisturbed play, and may 

 produce very considerable results. 



We cannot expect to detect any effect on the 

 large bodies in our system. For instance, the 

 whole pressure of the sunlight falling on the Earth, 

 if it were entirely absorbed, would amount only to 

 about 74,000 tons weight 1 This appears a big 



1 I use here and elsewhere the energy of sunlight at the 

 distance of the Earth as 2*5 calories per minute per sq. 

 cm. 



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