8 A Short History of Astronomy [Cn. i. 



is more difficult to recognise owing to the fact that the sun 

 and stars are not seen together. 



As other motions of the celestial bodies have to be dealt 

 with, the general motion just described may be conveniently 

 referred to as the daily motion or daily rotation of the 

 celestial sphere. 



9. A further study of the daily motion would lead to the 

 recognition of certain important circles of the celestial sphere. 



Each star describes in its daily motion a circle, the size 

 of which depends on its distance from the poles. Fig. 2 

 shews the paths described by a number of stars near the 

 pole, recorded photographically, during part of a night. 

 The pole-star describes so small a circle that its motion can 

 only with difficulty be detected with the naked eye, stars a 

 little farther off the pole describe larger circles, and so on, 

 until we come to stars half-way between the two poles, which 

 describe the largest circle which can be drawn on the 

 celestial sphere. The circle on which these stars lie and 

 which is described by any one of them daily is called the 

 equator. By looking at a diagram such as fig. 3, or, better 

 still, by looking at an actual globe, it can easily be seen 

 that half the equator (E Q w) lies above and half (the 

 dotted part, w R E) below the horizon, and that in conse- 

 quence a star, such as s, lying on the equator, is in its daily 

 motion as long a time above the horizon as below. If- 

 a star, such as s, lies on the north side of the equator, i.e. 

 on the side on which the north pole P lies, more than half 

 of its daily path lies above the horizon and less than half 

 (as shewn by the dotted line) lies below; and if a star 

 is near enough to the north pole (more precisely, if it is 

 nearer to the north pole than the nearest point, K, of the 

 horizon), as o-, it never sets, but remains continually above 

 the horizon. Such a star is called a (northern) circumpolar 

 star. On the other hand, less than half of the daily path of 

 a star on the south side of the equator, as s', is above the 

 horizon, and a star, such as o-', the distance of which from 

 the north pole is greater than the distance of the farthest 

 point, H, of the horizon, or which is nearer than H to the 

 south pole, remains continually below the horizon. 



10. A slight familiarity with the stars is enough to shew 

 any one that the same stars are not always visible at ihc 



f 



