162 ON THE OKKHN OF THE PLANETARY SYSTEM. 



planetary disturbances. By disturbances of the plane- 

 tary motion astronomers understand, as I have already 

 mentioned, those deviations from the purely elliptical 

 motion which are due to the attraction of various planets 

 and satellites upon each other. The attraction of the 

 sun, as by far the largest body of our system, is indeed 

 the chief and preponderating force which produces the 

 motion of the planets. If it alone were operative, 

 each of the planets would move continuously in a con- 

 stant ellipse whose axes would retain the same direc- 

 tion and the same magnitude, making the revolutions 

 always in the same length of time. But, in point of 

 fact, in addition to the attraction of the sun there are 

 the attractions of all other planets, which, though 

 small, yet, in long periods of time, do effect slow 

 changes in the plane, the direction, and the magnitude 

 of the axes of its elliptical orbit. It has been asked 

 whether these attractions in the orbit of the planet 

 could go so far as to cause two adjacent planets to 

 encounter each other, so that individual ones fall into 

 the sun. Laplace was able to reply that this could not 

 be the case ; that all alterations in the planetary orbits 

 produced by this kind of disturbance must periodically 

 increase and decrease, and again revert to a mean 

 condition. But it must not be forgotten that this 

 result of Laplace's investigations only applies to dis- 

 turbances due to the reciprocal attraction of planets 



