HOW TO OBSERVE THE HEAVENS. 



below the horizon, and therefore invisible, it also receives the 

 name pole, and the two points are distinguished, the visible 

 one as the North Celestial Pole, and the invisible one as the 

 South Celestial Pole. 



The motion of the celestial sphere here described is apparent, 

 not real, being merely an optical illusion produced by the diurnal 

 rotation of the earth upon its axis. But this being a point not 

 immediately connected with our present purpose, it will be 

 sufficient here merely to indicate it. The readers who desire to 

 see the explanation of the apparent diurnal motion of the heavens, 

 will find it in the "Museum," vol. iii. pp. 55, 56. 



For all the purposes of the observation of the heavens which for 

 the present occupy our exclusive attention, the celestial sphere is 

 to be considered as revolving on its axis once in about twenty-four 

 hours, carrying with it all the objects seen upon it. 



9. The objects scattered over this sphere in such vast numbers, 

 differing one from another greatly in their apparent splendour, and 

 being characterised by very various and often remarkable configu- 

 rations, astronomers have invented a nomenclature to designate 

 them, founded partly on their relative splendour, and partly on 

 their configurations. 



A catalogue of the stars being made, in which each star would 

 hold a place determined by its relative splendour, the more 

 splendid having the higher places ; if it were required to resolve 

 such a list into classes, according to their decreasing degrees of 

 brightness, it would be impossible to fix upon any points where 

 each succeeding class would end and the next begin ; the grada- 

 tions of brightness, when star is compared with star, being 

 altogether imperceptible. Nevertheless, a distribution according 

 to degrees of relative splendour being by the common consent of 

 astronomers of all ages deemed expedient, such a conventional 

 classification has been adopted, arbitrary as the limits of the 

 succeeding classes must necessarily have been. In this a certain 

 number of the most splendid stars visible in the firmament have 

 received the denomination of stars of thejirst magnitude ; others, 

 of inferior brightness, are called stars of the second magnitude, and 

 so on, the smallest stars visible to the naked eye being classed as 

 stars of the sixth magnitude. 



10. The number of stars of each succeeding magnitude increases 

 rapidly as their splendour diminishes. Thus, while there are 

 no more than 18 or 20 of the first magnitude, there are 50 or 60 

 of the second, about 200 of the third, and so on ; the total number 

 visible to the naked eye, up to the sixth magnitude inclusive^ 

 being from 5000 to 6000. "We shall see on another occasion that 

 this number, great as it is, is no more than an insignificant fraction 

 150 



