SECT. XH. EFFECTS OF NUTATION. 77 



tions. 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 diminished, ac- 

 cording 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 4he 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 longi- 

 tudes estimated from the equinox are augmented by 

 precession ; but as it effects all the bodies equally, it 

 makes no change in their relative positions. Both the 

 celestial latitudes and longitudes 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 , 

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

 12,934 years hence the star a Lyrae will come within 

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

 the northern hemisphere. 



SECTION XII. 



Mfean and Apparent Sidereal Time Mean and Apparent Solar Time 

 Equation of Time English and French Subdivisions of Time Leap 

 Year Christian Era Equinoctial Time Remarkable Eras depending 

 upon the Position of the Solar Perigee Inequality of the Lengths of 

 the Seasons in the two Hemispheres Application of Astronomy to Chro- 

 nology English and French Standards of Weights and Measures. 



ASTRONOMY has been of immediate and essential use 

 in affording invariable standards for measuring duration, 

 distance, magnitude, and velocity. The mean sidereal 

 day measured by the time elapsed between two consec- 

 utive transits of any star at the same meridian, and the 

 mean sidereal year, which is the time included between 

 two consecutive returns of the sun to the same star, 

 are immutable units with which all great periods of 

 02 



