RISING AND SETTING OF SUN AND MOON. 



52. Now all these circumstances must be taken into account if 

 we desire to predict by calculation the portion of the heavenly 

 panorama which will be presented to the view of spectators at any 

 given place, at any given time, and the objects, whether they be 

 sun, moon, or planets, which may happen to be upon that portion 

 of the panorama. And this is precisely what astronomers do 

 when they compute those tables of the rising and setting, and the 

 meridional transits of these objects. "Without going into the 

 technical details upon which such computations are based, it will 

 be evident that if the position of a place upon the earth's surface 

 be given, the aspect under which the heavens will be seen from 

 that place, shifting from hour to hour, can be ascertained before- 

 hand, and the positions in which all objects upon it will be seen 

 at any given hour, minute, and second, or the hour, minute, and 

 second at which they will have any proposed position on the 

 visible hemisphere, can be certainly and exactly predicted. 



These, then, speaking generally, are the principles upon which 

 the numbers given in those columns of the calendar to which we 

 have just referred have been computed. 



53. We see this vast spectacle, however, not immediately, but 

 by the intervention of a medium which produces upon it certain 

 optical effects. Our station is at the bottom of an ocean of trans- 

 parent fluid, about fifty miles deep. This fluid is called the 

 atmosphere, and it is by looking upwards through it that we see 

 the heavens. Such a medium, however clear and translucent it 

 may be, has always a certain distorting effect upon the objects seen 

 beyond it. It is as though we saw the heavens through a thick 

 sheet of glass, the external part of which is convex, and the 

 internal concave. The celestial objects are by this, therefore, 

 more or less distorted in form, and disturbed in their position in 

 relation to the horizon. It is true that owing to the air being a 

 very light and attenuated fluid, and especially so at great heights, 

 this distortion and derangement are so inconsiderable, that except 

 in particular cases they can only be perceived by astronomical 

 observers, and by them only with the aid of good instruments, by 

 which very small differences of direction and position can be 

 ascertained. 



Nevertheless, there are cases in which this curious atmospheric 

 influence is palpable to the sight. Every one who has observed 

 the fiery orb of the sun, or that of the full moon just before 

 setting or soon after rising, when they are seen through a thick 

 mass of air at low altitudes, will have noticed that they do not 

 appear round as they ought to be, but oval, the longer diameter 

 of the oval being horizontal. Now this is a distortion of their 

 form produced by the mass of air through which they are seen. 



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