12 Astronomy. [Lecture 28. 



term to the duration of light, it must also include 

 the twilight, and the length of the artificial day 

 will be proportionably extended. 



Twilight is that portion of light which the sun 

 diffuses through the atmosphere before his rising 

 and after his setting. The morning twilight, which 

 the ancients distinguished by the name of Aurora, 

 introduces to us the dawn of morning when the 

 sun is about 18 degrees below the horizon ; and 

 the evening twilight disappears when the sun has 

 descended about 18 degrees lower than the hori- 

 zon. Thus the line of twilight may be regarded 

 as a circle parallel to the horizon, and about 18 

 degrees below it. 



The morning twilight continues to augment 

 as the day advances ; that of the evening, on the 

 contrary, decreases gradually till it totally disap- 

 pears. I have said the twilight is the effect of 

 the dispersion of the solar rays in the atmo- 

 sphere, by which they are refracted and reflected 

 in various directions. To understand this, let 

 T (PI. II fig. 5) be the earth, A A A the atmo- 

 sphere, H H the horizon^ C C C a vertical cir- 

 cle, which serves to measure the height of the 

 sun, S the sun below the horizon either before 

 his rising, or after his setting. The solar rays 

 S,s, S s, S $, S s, are directed towards the points, 

 B B B B, and they would pursue this direction 

 but for the interposition of the atmosphere, which 

 having more density than the medium through 

 which they were last transmitted, and the rays 



