Lesson xvj.] TWILIGHT. 8T, 



us from seeing the Sun ; the atmosphere (whose 

 reflecting part reaches to the height of about sixty 

 miles) will still be illuminated by that luminary : 

 so that for a while the whole heaven will have 

 some of his light imparted to it. But as the 

 Earth pursues its revolution, the Sun retires 

 farther below the horrzon ; and so much the less 

 is the atmosphere illustrated by him : until trie 

 Sun is about eighteen degrees below the horizon^ 



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when he no longer enlightens our atmosphere, 

 and then all that part thereof which is over us be- 

 comes dark. 



Likewise in the morning, as soon as the Sun 

 comes within eighteen degrees of the horizon, he 

 begins again to enlighten the atmosphere, and to 

 diffuse his light over the sky : so tbat its bright- 

 ness does still increase, till the Sun rises and makes - 

 full day. This kind of illumination between day 

 and night, which is observed' in the morning be- 

 fore the Sun's rising, and 'in the evening after his 

 s ettihg, is called Crespusculum or Twilight. It is - 

 longest in England, from May 24th to July 23d; 

 during which period there is " no real night;" and 

 shortest on March 2d and October 1 2th, when its 

 duration at London is about 1 hour 55 minutes. 



The terrestrial atmosphere also refracts those 

 rays which fall upon it from the Sun and Stars, 

 and changes their directions, by propagating the 

 light in other lines, and thus making the apparent 

 places of the celestial bodies different from their 

 true places. This refraction causes the Sun to be 

 yisible before he has risen above the horizon., and i 

 E 5. io 



