92 EFFECTS OF NUTATION. [SECT. xr. 



nutation in the earth's axis affects both the precession and 

 obliquity with small periodic variations. But in consequence 

 of the secular variation in the position of the terrestrial orbit, 

 which is chiefly owing to the disturbing energy of Jupiter 

 on the earth, the obliquity of the ecliptic is annually di- 

 minished, according to M. Bessel, by 0"'457. This variation 

 in the course of ages may amount to 10 or 11 degrees ; but 

 the obliquity of the ecliptic to the equator can never vary 

 more than 2 42* or 3, since the equator will follow in some 

 measure the motion of the ecliptic. 



It is evident that the places of all the celestial bodies are 

 affected by precession and nutation. Their longitudes esti- 

 mated from the equinox are augmented by precession ; but, 

 as it affects all the bodies equally, it makes no change in their 

 relative positions. Both the celestial latitudes and longi- 

 tudes are altered to a small degree by nutation ; hence all 

 observations must be corrected for these inequalities. In 

 consequence of this real motion in the earth's axis the pole 

 star, forming part of the constellation of the Little Bear, 

 which was formerly 12 from the celestial pole, is now within 

 1 24' of it, and will continue to approach it till it is within 

 j, after which it will retreat from the pole for ages ; and 

 12,934 years hence the star a Lyrse will come within 5 of 

 the celestial pole, and become the polar star of the northern 

 hemisphere. 



