PRACTICAL ASTRONOMY 



139 



in comparison with it, the earth may be considered as a mere 

 dot. 



The earth's axis produced indefinitely is called the axis of the 

 celestial sphere. This axis intersects the celestial sphere in two 

 points, called the north Pole and the south pole of heavens. All 

 the great circles of the celestial sphere passing through this 

 axis are called hour circles. The circle in which the plane of the 

 equator intersects the celestial sphere is called the celestial 

 equator. The point on the equator that the sun in its apparent 

 motion over the celestial sphere crosses on March 21, as it 

 passes from the southern to the northern hemisphere, is called 

 the vernal equinox. 



REFERENCE CIRCLES 



The accompanying illustration, which represents the celestial 

 hemisphere, shows all the reference circles that are used for 

 determining the position of a heavenly body. O is the position 

 of the earth; OP, one-half of the axis of the celestial sphere, 



P being the north pole; VQV'L, part of the celestial equator; 

 X, the vernal equinox; and YXC, part of the sun's path. 

 PX is the hour circle passing through X, called the equinoctial 

 colure. S is any star, and PSA is the hour circle passing 

 through it. XA is the right ascension of the star, which is the 

 arc on the equator measured eastwards from the vernal equinox 



