10 Astronomy. [Lecture 23. 



which lie between the equator and the poles, it is 

 subject to considerable variations. 



To these parts of the globe the artificial day 

 is only of 12 hours' length when the sun is in one 

 of those points called the equinoxes, where the 

 equator intersects the ecliptic. At other times, 

 it is either longer or shorter, according to circum- 

 stances. To those who live between the equator 

 and the north pole, the day becomes longer than 

 12 hours, in proportion as the sun continues to 

 advance from the equator to the tropic of Cancer, 

 that is, from the time of the vernal equinox ; on 

 the contrary, it becomes shorter than that pe- 

 riod, in proportion as the sun advances from the 

 equator towards the tropic of Capricorn, which 

 takes place after the autumnal equinox. The 

 contrary happens to the inhabitants of the south- 

 ern hemisphere. Thus, in all the regions on 

 either side of the equator there are only two 

 days in the course of the year when the day and 

 night are equal : and throughout the earth, ex- 

 cept in the frigid zones, the longest and the 

 shortest day together make 24 hours. 



Such is the duration of the artificial day to 

 the different inhabitants of the earth, if we re- 

 gard only the actual presence of the sun above 

 the horizon. But there is one cause, which 

 serves to prolong the day-light, which we have 

 not yet noticed, and that is refraction. After 

 what you have read in the optical lectures, you 



