332 HOW TO OBSERVE THE HEAVENS. 



i he verified by ocular observation merely, and still less can the position of that 

 common centre be thus determined. 



Having recourse, however, to instruments, by which the exact elevations 

 of the stars may from time to time be observed, and their exact bearings noiet!, 

 data are obtained by which it is demonstrated that this first impression is 

 rigorously correct; that the objects which glitter on the firmament do, in fiet. . 

 appear to revolve round a certain point as a common centre; that they all com- 

 plete their revolution round that ]x>int in twenty-three hours, fifty-six minutes, 

 four seconds, and nine hnndredths of a second. All the stars complete their 

 revolution round'this* point, in exactly the same time, however different their 

 distances from it may be, and as they so revolve they preserve their relative 

 position with respect to each other. It has been shown on a former occasion 

 that this appearance is an optical illusion, caused by the rotation of the eaith 

 upon its axis. 



At the point which is the common centre of this motion, and which is called 

 the north celestial pole, no star is found ; but there is a star sufficiently bright 

 to he distinctly visible to the naked eye very close to it, which is therefore 

 called the POLE-STAR. The method of recognising this star we shall presently 

 explain. 



Even the most inattentive observer, when assuming the position we have 

 here described, will be immediately struck with a combination of seven con- 

 spicuous stars arranged in the relative positions exhibited in the annexed 

 diagram. 



Fig. i. 



This combination, or group of stars, presents to us the first and most striking 

 example of what is called a CONSTELLATION. 



The peculiar configurations affected by this and other groups scattered over 

 the firmament, give an impression that some physical relation connects the 

 component stars with each other. But a more exact acquaintance with stellar 

 astronomy proves that such impression is destitute of any good foundation. 

 The stars which compose the constellations are casually scattered over the 

 firmament, and it is the imagination only which groups them and invests the 

 collections thus associated with the fanciful figures of bears, lions, goats, 

 dogs, warriors, and mythological personages. Unreasonable as such a sys- 



Bm must be allowed to be, it is not without its use as a means of reference, 

 id an artificial aid to the memory. That a better system of signs and sym- 

 >ls might have been devised for these purposes may be admitted ; but when 

 is considered that the names and forms of the most conspicuous constellations 

 ive had their origin in remote antiquity that they were handed down from 

 e Chaldeans to the Egyptians, and from the Egyptians to the Greeks, and 

 ;)in these to the moderns that they are referred to in the works of every 



