* 8] The Celestial Sphere : its Poles 7 



true even if he moves to another place. A sphere has, 

 however, only one centre, and therefore if the sphere 

 remains fixed the observer cannot move about and yet 

 always remain at the centre. The old astronomers met 

 this difficulty by supposing that the celestial sphere was so 

 large that any possible motion of the observer would be 

 insignificant in comparison with the radius of the sphere and 

 could be neglected. It is often more convenient when 

 we are using the sphere as a mere geometrical device for 

 representing the position of the stars to regard the sphere 

 as moving with the observer, so that he always remains at 

 the centre. 



8. Although the stars all appear to move across the// 

 sky ( 5), and their rates of motion differ, yet the distance"^ 

 between any two stars remains unchanged, and they were \ 

 consequently regarded as being attached to the celestial 

 sphere. Moreover a little careful observation would have 

 shown that the motions of the stars in different parts of the 

 sky, though at first sight very different, were just such 

 as would have been produced by the celestial sphere with 

 the stars attached to it turning abo'ut an axis passing 

 through the centre and through a point in the northern 

 sky close to the familiar pole-star. This point is called 

 the pole, As, however, a straight line drawn through the 

 centre of a sphere meets it in two points, the axis of 

 the celestial sphere meets it again in a second point, 

 opposite the first, lying in a part of the celestial sphere 

 which is permanently below the horizon. This second 

 point is also called a pole; and if the two poles have to 

 be distinguished, the one mentioned first is called the 

 north pole, and the other the south pole. The direction 

 of the rotation of the celestial sphere about its axis is 

 such that stars near the north pole are seen to move round 

 it in circles in the direction opposite to that in which the 

 hands of a clock move; the motion is uniform, and a 

 complete revolution is performed in four minutes less than 

 twenty-four hours ; so that the position of any star in the 

 sky at twelve o'clock to-night is the same as its position at 

 four minutes to twelve to-morrow night. 



The moon, like the stars, shares this motion of the 

 celestial sphere, and so also does trie sun, though this 



