1G8 



LAPLACE. 



the orbit of each planet is iu a state of continual movement from the 

 disturbing action of the other planets. In some cases it makes the com- 

 plete tour of the heavens ; in others it merely oscillates around a mean 

 position. In the case of the earth's orbit, the perihelion is slowly ad- 

 vancing in the same direction as that in which all the planets are re- 

 volving around the sun. The alteration of its position with respect to 

 the stars amounts to about 11" in a year, but since the equinox is re- 

 gressing in the opposite direction at the rate of 50" in a year, the whole 

 annual variation of the longitude of the terrestrial perihelion amounts 

 to Gl". Laplace has considered two remarkable epochs in connection 

 with this fact, viz, the epoch at which the major axis of the earth's orbit 

 coincided with the line of the equinoxes, and the epoch at which it stood 

 perpendicular to that line. By calculation he found the former of these 

 epochs to be referable to the year 4107 B. C, and the latter to the 

 year 1215 A. D. He accordingly suggested that the latter should be 

 used as a universal epoch for the regulation of chronological occurrences. 



