OUTLINES OF NATURAL PHILOSOPHY. 



Since the diameter of the earth is very small in re- 

 spect of the distance of the stars, we may suppose 

 the centre of the sphere in which these stars appear 

 to be placed, to be the centre of the earth, and not 

 the eye of the observer. 



6. The straight line which passes through the 

 pole of the Northern Hemisphere, and through 

 the centre of the earth, is called the Axis of the 

 Heavens, and is the line about which the hea- 

 vens appear to revolve. The point in which this 

 line seems to meet the superficies of the South- 

 ern Hemisphere, is called the South Pole of the 

 Heavens, as the other is called the North Pole. 



The Poles are also distinguished by the names of Arc- 

 tic and Antarctic. 



6. As it is necessary, in order to ascertain the 

 position of any point on the surface of a sphere, 

 to refer it to two planes given in position ; so, 

 in order to determine the places of the stars for 

 any moment, and thereby to ascertain the laws 

 of their motion, it is necessary to suppose cer- 

 tain planes to pass through the centre of the ce- 

 lestial sphere, to which these places may be re- 

 ferred. 



This gives rise to what is called the Doctrine of the 

 Sphere, and the application of spherical trigono- 

 metry to astronomy. 



7. A 



