PRESSURE OF LIGHT IN ASTRONOMY 75 



luminosity in the tail is due to electric action, the 

 nature of the action is only as yet a matter of 

 speculation. Perhaps we shall find some time that 

 both light-pressure and electric action come into 

 play. 



Leaving the formation of comets' tails as an 

 enigma yet unsolved, let us turn to another class 

 of bodies, large compared with the particles which 

 are repelled and yet minute compared with the 

 planets, a class of bodies which reveal their 

 abundant presence in the solar system when they 

 enter our atmosphere and perish as shooting-stars. 

 From the amount of light which they give out in 

 burning up they must as a rule be small, many of 

 them hardly more than specks of matter. 



We will suppose that one of these bodies is 

 travelling round the sun nearly in a circle and at 

 the distance of the earth. Then it is to all intents 

 and purposes a minute planet. Let it be black so 

 that it absorbs all the sunlight falling on it. If it 

 reflects some of the light, the first effect which we 

 are going to discuss will be rather greater than for 

 a black body. 



Let us suppose that it is I cm. in radius, and 

 of the density of the earth, viz. 5j. The push 

 of the sunlight against it will lessen the total pull 

 on it by about I in 74,000. This implies that it 

 does not need to move round with quite such a 

 high velocity as the earth to keep it from falling 



