264 Astronomy. [Lecture 17. 



the boundary of our sight, and which seems to bring the 

 apparent arch of the heavens in contact with the earth. 



The rational horizon is a great circle, parallel to the 

 former, but which would divide the globe into equal 

 portions. 



A parallel sphere is so called because under it the equa- 

 tor coincides, or is parallel to the horizon. The poles 

 are in the zenith and nadir ; that is, one pole is directly 

 over the head of the spectator, and the other directly 

 under his feet. The inhabitants of this sphere would be 

 those, if it were habitable (which, however, we may ven- 

 ture to decide in the negative, from the extreme cold), 

 that lived under the poles, who could have but one day 

 and one night in the year. The day continues six months 

 while the sun appears to pass through six signs of the 

 zodiac, and the night six months, while he appears to 

 pass through the other six. The day, under the north 

 pole, begins when the sun enters aries, and continues till 

 he reaches libra ; when night commences, and continues 

 the other six months. 



Under the south pole the direct contrary happens, it 

 being day there when it is night in the former situation, 

 and the contrary. But at both the poles there is a long 

 continuance of twilight, both after the sun has departed, 

 and before he appears. 



The polar inhabitants (if there are any) see the sun for 

 half the year, moving continually round above the hori- 

 zon, in a spiral line ; the first round skimming the skirts 

 of the horizon ; the second, higher ; and so on, till, by 

 ninety revolutions, he has reached the tropic, his utmost 

 declination; after which, by ninety more revolutions, he 

 again reaches the horizon, and then rong winter night 

 begins. 



A right sphere is so called, because under it the equator 

 cuts the horizon at right angles. The poles will lie or be 

 in the horizon. The equator will be in the zenith and 

 nadir. 



