1 4 POPULAR LECTURES AND ADDRESSES. 



the north celestial pole has moved among the stars 

 for thousands of years, and may be expected to 

 move for hundreds of thousands of years to come. 

 It represents a small circle among the stars on which 

 the earth's celestial pole travels at the rate of once 

 round in 25,868 years, or at the rate of i of great 

 circle per 180 years. The diameter of this circle 

 is about 47 (more nearly 4655'), and its centre 

 is in the direction perpendicular to the plane of 

 the earth's orbit round the sun. 1 To understand 

 this motion, and its effect on the line of the 

 equinoxes, as the line of intersection of the 

 earth's equator and the ecliptic is called, look at 

 the model before you which shows the rotation 

 of the earth round an axis constantly changing 

 position in space. The line of the equinoxes 

 travels once completely round the ecliptic in 

 25,868 years, or at the rate of i per 71-85 years 

 in the direction contrary to the earth's rotation. 

 This motion is called the precession of the 



1 Or is what is called the " pole of the ecliptic," the ecliptic being 

 a name given by the Greeks to the plane of the earth's orbit, because 

 the moon must be nearly, if not exactly, in this plane to produce or 

 experience an eclipse. 



