MOTION OF HEAVENLY BODIES. 



495 



the earth's axis, and by this latter we can assign to the equator its proper 

 place. For if pp> be the earth's axis, aq is the greatest circle drawn round 

 the earth, equally distant from both poles, and the plane of which cuts the 

 earth's axis at right angles. 



Furthermore, let us suppose the plane of the equator to be extended 

 till it reach the celestial concave ; we thus find the place of the celestial 

 equator, A Q, or equinoctial, as it is generally termed in opposition to the 

 equator, which always means the terrestrial equator. The equinoctial 

 divides the heavens into the northern and southern hemispheres. We can- 

 not actually describe the equinoctial and make it visible, but we can imagine 

 its line of direction by observing those stars through which it passes. 



We are now in a condition to assign to an observer different stations 

 in relation to the earth's axis on the earth's surface, which will essentially 

 modify the aspects under which celestial phenomena are represented. One 

 of these stations may be supposed to be at one of the two poles, for 

 example, at /, or at any one point of the equator, as at a, or, finally, on 

 any portion of the surface of the earth which lies between the pole and the 

 equator, as, for example, o. 



Fig 543- Great Nebulae in Orion. 



