THE ORIGIN OF THE EARTH. 65 



an evolution in the direction of circularity and symmetry, in the course 

 of the aggregation of the scattered matter. The method will be pres- 

 ently indicated. 



The part played by ellipticity of orbit. — The ellipticity of the orbits 

 is the very factor that furnishes the assigned agency of aggregation, 

 and of this progressive evolution towards circularity and symmetry. 

 In the initial stages, the ellipses of the nuclei and of the innumerable 

 planetesimals were, by reason of their common origin, rudely concentric. 

 They were, to be sure, more or less discordant in form and in attitude 

 from the effects of unequal projection, of differential expansion of the 

 solar matter when set free by projection, and of the collisions of the 

 constituent planetesimals; but all of this was subordinate to a general 

 concentric arrangement of the elliptical paths. Under the laws of 

 celestial mechanics, these paths must have been constantly modified 

 by the different attractions of the different portions of the nebula. 

 The axes of the orbits must have shifted, the attitudes of their orbital 

 planes must have varied, and their eccentricities must have been 

 modified. It will suffice to consider the shifting of the major axes of 

 the orbits, technically "the motion of the line of apsides," as that is 

 the most vital factor in the process of aggregation. 



So long as the major axes of the orbits were essentially parallel 

 to one another, as A and B in Fig. 24, the bodies would remain apart 

 and aggregation be prevented; but when they became shifted differ- 

 entially to the positions A' and B', the orbits would touch, and con- 

 junction be possible, if the orbital planes were appropriately related 

 to one another. 



The shifting of the lines of apsides is in constant progress in the 

 present system, and must of necessity take place in any such system, 

 as the laws of celestial mechanics require it. The shifting is differ- 

 ential and subject to various perturbations, involving alternate move- 

 ment forward and backward, but the average result is an advance. 1 

 At present the line of apsides of quickest revolution is that of Saturn, 

 which completes its circuit in 67,000 years, roundly speaking, while 

 that of Neptune requires 540,000 years, and that of the earth a little 

 more than 100,000 years. 2 In the course of time, the major axis of 

 each orbit is thrown athwart that of its neighbors, and whenever the 



1 Celestial Mechanics, Moulton, p. 245. 



2 Young's General Astronomy, p. 313. 



