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THE POPULAR SCIENCE MONTHLY 



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Map of the Pyramids of Jbezeh. 



Besides this precession another or, more jjroperly speaking, several 

 others are produced indirectly by the action of the several planets upon 

 the plane of the ecliptic. As these bodies are out of the plane themselves 

 their action pulls the earth out of her orbital plane, causing a change in 

 the obliquity of the ecliptic. There is in consequence a slow alteration 

 in the place of the pole of the ecliptic and this results in a movement of 

 the equator other than what the sun and moon produce. It is known as 

 the planetary precession. Chiefly by such round-about process do the 

 planets make themselves precessionally felt. For all of them, even 

 Jupiter, are relatively too small and too distant to have their direct pull 

 upon the earth's equatorial ring of any account. The direct precession 

 due to Jupiter is only about 0".0001 a year and that caused by any 

 other planet inuch less. 



As all the planets are concerned in the work, and as they are all pull- 

 ing difEerent ways, the combined effect is not regular in either extent or 

 duration, although there is a general SAving to and fro of the ecliptic as 

 the result. Its periods are much longer than the luni-solar precession. 

 Thus, according to Leverrier, the one now going on had its maximum 

 effect about 40,000 years ago and the next will be 35,000 years hence. This 

 may be taken as about the mean swing ; which shows that it is thrice that 

 of the precession of the equinoxes properly so called. It is so combined 

 with the latter, however, that though quite extensive in itself, amount- 

 ing to more than 4°, it can never change the tilt of the ecliptic to the 

 equator by more than 1°, one third on either side, so that our seasons 



