$ 7 ] The Celestial Sphere 5 



can tell us about the position of such an object as a star 

 is its direction') its distance can only be ascertained by 

 indirect methods, if at all. If we draw a sphere, and 

 suppose the observer's eye placed at its centre o (fig. i), 

 and then draw a straight line from o to a star s, meeting 

 the surface of the sphere in the point s ; then the star 

 appears exactly in the same position as if it were at s, 

 nor would its apparent position be changed if it were 

 placed at any other point, such as s' or s", on this same 



Q 



FIG. I. The celestial sphere. 



line. When we speak, therefore, of a star as being at 

 a point s on the celestial sphere, all that we mean is that 

 it is in the same direction as the point s, or, in other 

 words, that it is situated somewhere on the straight line 

 through o and s. The advantages of this method of repre- 

 senting the position of a star become evident when we wish 

 to compare the positions of several stars. The difference 

 of direction of two stars is the angle between the lines 

 drawn from the eye to the stars ; e.g., if the stars are R, s, it 

 is the angle R o s. Similarly the difference of direction of 



