14 Astronomy. [Lecture 23. 



of the twilight must be greater in proportion as 

 die pole is more elevated above the horizon, or 

 in proportion as the place has a higher latitude. 

 Thus, if the latitude is such that the sun at mid- 

 night on the 21st of June cannot have descended 

 eighteen degrees below the horizon, as is the 

 case in our climate, the evening twilight will not 

 be finished before that of the morning com- 

 mences ; so that at this period of the year we 

 have, properly speaking, in clear weather, no 

 night at all. This is indicated in our best Alma- 

 nacs, in which the length of the morning and 

 evening twilight is given, for every fifth day in 

 the year. 



It follows also, that if there were any inhabit- 

 ants directly under the poles, who would, you 

 know, have a parallel sphere, the twilight would 

 be perceived by them two months before the sun 

 appeared above their horizon, and would also 

 endure two months after the disappearance of 

 the sun. In a parallel sphere^ therefore, there 

 would not be more than two months of actually 

 dark night; besides that, during these two 

 months, the moon would be twice visible above 

 the horizon for fourteen days at a time. 



The astronomical day commences at noon of 

 true time, .that is, at the instant when the sun is 

 in the meridian. In astronomy, the twenty-four 

 hours of the day are counted from noon to the 

 following noon again ; so that instead of calling 

 the hour after midnight one o'clock in the morn- 



