THE POINTERS THE POLE STAR. 



Although the proper names of the principal stars of Ursa 

 major are not now in general use, they ought not on that account 

 to be altogether overlooked or neglected, since they are often the 

 means of identifying these objects with those indicated in ancient 

 historical records. 



Close to the star Mizar, in the tail of the Great Bear, is a small 

 star called Alcor, which Humboldt says the Arabs called " saidak," 

 which signifies trial or test, since they used it as a test of the 

 sharpness of the sight of the observer. 



18. If a straight line be imagined to be drawn from the star 

 to a, and continued beyond o to a distance equal to five times the 

 distance between o and 0, or, what is nearly the same, to the 

 whole distance between o and r?, it will arrive at the principal 

 star of a smaller constellation called Ursa minor or the lesser 

 Bear. This is the star already mentioned as being within a 

 degree and a half of the pole, and which, being generally adopted 

 as the easiest practical means of marking that important point, is 

 called the Pole star. The other stars of the constellation of Ursa 

 minor have nearly the same configuration as those of Ursa major ; 

 but the position of the figure is reversed, the tail, at the tip of 

 which the pole star is placed, corresponding with the head of Ursa 

 major. 



The important service thus performed by the stars o and 13 Urss& 

 majoris, in indicating by their direction the position of the pole 

 star, has given them the name of the Pointers ; they are also 

 sometimes called the Guards. 



This method of ascertaining the position of the principal star 

 and the constellation generally of Ursa minor, by means of the 

 more conspicuous and better known constellation of Ursa major,, 

 has been generalised with the greatest benefit to astronomical 

 students and amateurs by extending the method of pointers, so 

 as to trace one constellation from another throughout the entire 

 firmament, as will presently appear. 



19. The constellation of Ursa minor being so placed that the 

 principal star, at the tip of the bear's tail, is close to the pole, 

 the diurnal motion of the sphere causes the figure of the bear to 

 swing round the pole feet foremost, as if its tail were nailed to 

 that point. The four successive positions of the constellation at 

 intervals of six hours, are shown in fig. 3, p. 161. 



The star j8 of this constellation, situate on the head of the bear, 

 and therefore more distant from the pole, is easily seen to revolve 

 round the pole as a centre, so that this constellation was regarded 

 as a great .celestial clock, and before the advancement of science 

 furnished mariners with other and better means, it was of great 

 use in navigation. 



155 



