192 RADIATION IN THE SOLAR SYSTEM., 



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

 about -20 miles, and the pull would ecjual 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 oidy hold in those 

 which were lar<>er. 



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

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

 ture to one-twentieth, say 80(3° absolute, or the temperature of the 

 earth. Then the radiation Avould be reduced to the fourth power of 

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

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

 thousandth of 'iO 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 would neither attract 

 nor rep<d each other — their radiation pressure would l)alance the 

 gravitative pull — when their diameters were about 2.20 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 small 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 the^^ 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 radiaticm presses back against any 

 surface from wdiich 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 between the pressures. But suppose that it is 

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

 crowding it into a smaller spa(;e than if it were at rest, making it 

 more dense. Hence the pressui-e is slightly greater, and it can be 

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

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

 Avhich it i)ours out behind, thinning it out, as it were, and the pressure 

 at the back is slightly less than if the sphere were at rest. 



The net result is a force opposing the motion, a force like a viscous 

 friction, always tending to reduce the s|)eed. 



Thus calculation shows that there is a i-etarding 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 years it will only reduce the velocity 

 bv one in a million, and it will onlv have serious effects if the life of 



