192 RADIATION IN THE SOLAR SY8TEM. 



eter to one fort3^-two-thoiisandth of its orignal value, that is, to 

 about 20 miles, and the pull would equal the push. 



In other words, a sun as hot as ours and 20 miles in diameter would 

 repel bodies less than 1 cm. in diameter, and could only hold in those 

 which were larger. 



But it is, of course, absurd to think of such a small sun as this hav- 

 ing so high a temperature as 6,000°. Let us then reduce the tempera- 

 ture to one-twentieth, say 306° absolute, or the temperature of the 

 earth. Then the radiation would be reduced to the fourth poAver of 

 one-twentieth, or one one-hundred-and-sixty-thousandth, and the 

 diameter would have to be reduced to one one-hundred-and-sixty- 

 thousandth of 20 miles, or about 20 cm., say 8 inches, when again 

 radiation would balance gravitation. 



It is not very difficult to show that if we had two equal spheres each 

 of the density and temperature of the earth they Avould neither attract 

 nor repel each other — their radiation pressure would balance the 

 gravitative pull — when their diameters were about 2.26 cm., when, in 

 fact, they were about the size of large marbles. 



It must be remembered that this is only true for spheres out in 

 space receiving no appreciable radiation from the surrounding region. 



It would appear that we have arrived at a result of some impor- 

 tance in considering the aggregation of small meteorites. Imagine a 

 thinly scattered stream of snuill meteorites at the distance of the earth 

 from the sun. Then, even if they be as large as marbles, they may 

 have no tendency to move together. If they are smaller they may 

 even tend to move apart and scatter. 



In conclusion, let me mention one more effect of this radiation pres- 

 sure. You will remember that radiation ]:)resses back against any 

 surface from which it issues. If, then, a sphere at rest in space is 

 radiating equally on all sides it is pressed equally on all sides, and the 

 net result is a balance l)etween the pressures. But suppose that it is 

 moving. It is following up the energy which it pours forth in L'ront, 

 crowding it into a smaller space than if it were at rest, making it 

 more dense. Hence the ])ressure is slightly greater, and it can be 

 shown that it is greater the greater the velocity and the liigiier the 

 temperature. On the other hand, it is draAving away from the energy 

 which it pours out behind, thinning it out, as it were, and the pressure 

 at the back is sliglitly less tlian if the sphere were at rest. 



The net result is a force ()p|)<)sing th(> motion, a force like a viscous 

 friction, always tendinu" to re(hice tlie sj)eed. 



Thus cak'ulation shows tliat there is a retarthng force on the earth 

 as it moves along its orbit amounting in all to about 2,500 kgm. Not 

 very serious, for in billions of yeai"s it will only reduce the velocity 

 by one in a million, and it will only have serious efi'ects if the life of 



