84 The Earth considered as a Planet. 



era boundary of our sight, when the light 

 begins to decrease, and gradually dimin- 

 ishes till he is 18 degrees below the hori- 

 zon, when dark night commences. This 

 intermediate light is called the crepuscu- 

 lum, or the morning and evening twilight. 



As the sun enlightens only one half of 

 the earth at once as it turns round its 

 axis, he rises to some places at the same 

 moments of absolute time when he sets 

 to others ; and when it is mid-day to 

 some places, it is mid-night to others. 



To every place 15 degrees eastward 

 from any given meridian, it is noon an 

 hour sooner than on that meridian ; be- 

 cause their meridian comes to the sun an 

 hour sooner ; and to all places 15 de- 

 grees westward, it is noon an hour later 

 because their meridian comes an hour 

 later to the sun, and so on : every 15 de- 

 grees of motion causing an hour's differ 

 ence of time. Therefore they who have 

 noon an hour later than we, have their 

 meridian, that is, their longitude, 15 de- 

 grees westward from us : and they who 

 have noon an hour sooner than we, have 

 their meridian 15 degrees eastward from 

 ours ; and so for every hour's difference 



