OUTLINES OF NATURAL PHILOSOPHY. 



less changes the inclination of the ecliptic to 

 the equator, and also the line of their intersec- 

 tion. 



This change in the inclination, and in the position 

 of the line of the equinoxes, is easily deduced by 

 spherical trigonometry from the retrogradation of 

 the intersection of the two planes, and from the 

 constancy of their inclination. See LA LANDE, 

 Astron. 2751, &c. 



326. The variations in the obliquity of the 

 ecliptic, thus produced, are among the number 

 of the secular inequalities which have long pe- 

 riods, and, after reaching a maximum, return in 

 a contrary direction. 



As far back as observation goes, the obliquity of the 

 ecliptic has been diminishing, and is doing so at 

 present, by 52" in a century ; it will not, however, 

 always continue to diminish, but in the course of 

 ages will again increase, oscillating backwards and 

 forwards on each side of a mean, from which it 

 never can depart far. 



The secular variation of the obliquity was less in an- 

 cient times than it is at present ; it is now near its 

 maximumi, and will begin to decrease in the 22d 

 century of our era. 



LA GRANGE has shewn, that the total change of the 

 obliquity, reckoning from that in 1700, must be 

 less than 5 23'; Mem. Acad. de Berlin, 1782, 



p. 284. 



