DIURNAL MOTION OF FIRMAMENT. 



presented to him is not stationary. In the course of an hour, he 

 will observe that various objects which were visible above the 

 horizon on his right have disappeared ; and that, on the contrary, 

 a corresponding number of objects, which were not visible above 

 the horizon on his left, have come into view. By further atten- 

 tion he will perceive that the objects which were at the mid- 

 heavens, in the direction due south, are now no longer so, but 

 have descended towards the right, that is, towards the west, 

 while objects which were to the left of the mid-heavens will have 

 risen to that region. 



4. To assist our explanation, let us imagine the entire firma- 

 ment divided by a line or great circle, rising from the point of 

 the horizon towards which the observer is supposed to look, and 

 being carried vertically upwards to pass over his head, and to 

 descend behind him to the northern point of the horizon. This 

 great line of division, which is called the celestial meridian, 

 divides the whole visible firmament into two equal parts ; one 

 lying to the west, or to the right, and the other to the east, or to 

 the left, of the observer. 



By continuing his attentive observation of what goes on before 

 him, he will soon perceive that all the objects visible upon the 

 firmament are in motion. That they rise on the east side ; that 

 they ascend to the meridian ; and then, descending to the west, 

 pass below the horizon and disappear. 



5. Let us now suppose our observer to face round and direct 

 his view to the north. A different spectacle will be presented to 

 him. Supposing him to be placed in these climates, he will soon 

 ascertain that the chief part of the objects which are visible in 

 the firmament do not appear and disappear ; that is, they do not 

 rise and set. If, for example, any such object be observed upon 

 the celestial meridian over his head so soon after sunset as the 

 stars become visible, he will observe it from hour to hour to 

 descend on his left, that is, towards the west, and to depart more 

 and more from the meridian. So far, however, this is what 

 equally took place when he looked to the south, and had the west 

 upon his right. But after the lapse of a certain time he will 

 find different appearances to be manifested. At the end of about 

 three hours from the time the object referred to began to depart 

 from the meridian, it will be found to have attained a certain 

 limit of distance from the meridian, which will not be exceeded. 

 After this it will begin, on the contrary, again to approach the 

 meridian ; but, in doing so, will also approach the horizon, as 

 though it were ultimately destined to set. Such, however, will 

 not be the case ; for, at the end of twelve hours, if the return of 

 daylight be sufficiently retarded to enable our observer still to see 



i 2 147 



