GRAVITATION IN THE SOLAR SYSTEM^ 



By Ernest W. Brown 

 Yale Universitj/ 



[With 4 plates] 



We are apt to regard the sun as important for life on the earth 

 chiefly because it is our main source of heat and light. But it is 

 equally important for the continued existence of life as we know 

 it that the earth shall continue to receive the proper amounts of 

 heat and light, and to do so it is necessary that our planet shall 

 remain at nearly the same distance from the sun at all times. The 

 amount of either that we receive varies inversely as the square of 

 the distance between the two bodies; should this distance vary too 

 greatly or should the earth leave the neighborhood of the sun al- 

 together, existence as we know it would cease. This distance is main- 

 tained by the operation of the laws of motion and of gravitation. 

 In this lecture, which is a rapid survey of the problem of the re- 

 conciliation of theory with observation, I shall adopt the familiar 

 Newtonian laws as the theoretical basis. 



According to the Newtonian view, gravitation is a force which al- 

 ways tends to draw two particles of matter together. This tend- 

 ency is balanced by the inertial property of matter. The effect of 

 this property is that when deviation from constant velocity in a 

 straight line is caused by such a force as gravitation, the amount of 

 the deviation in a given interval of time, properly measured, is 

 proportional to the force. In the case of the force of gravitation this 

 balancing effect results in the two bodies circling about one another, 

 never exceeding a certain distance apart or api^roaching nearer than 

 another definite distance, unless they happen to be started moving 

 exactly toward or away from each other, or unless the relative start- 

 ing velocity is too great. Newton showed that under the usual 

 conditions the curved motion is that of a closed oval — an ellipse — or, 

 if the starting conditions are just right, a circle. In the last case, 

 the balancing effect of the motion is often called the " centrifugal 

 force ", but it is better named the "kinetic reaction ", since it is not 



* The seconfl Arthur lecture, delivered at the Smitlisoniau Institution Jan. 25, 1033. 



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