ASTRONOMY. 83 



The Temperate Zones, are those contained between 

 the Torrid and Frigid Zones. 



84. If a straight line be drawn from the cen- 

 tre of the sun to the centre of the earth, a great 

 circle on the earth's surface, perpendicular to 

 this line, will separate the enlightened hemis- 

 phere from the dark one ; all the places within 

 certain limits, will, by the revolution of the 

 earth on its axis, be made to cross this circle 

 twice in the course of a solar day ; at the one 

 of which points, the sun will appear to rise, and 

 at the other, to set ; the circle, also, which se- 

 parates the light from the darkness, will have a 

 vibration corresponding to the sun's annual mo- 

 tion, by which it will incline alternately toward 

 the north and toward the south. 



The circle separating Day from Night, or the light 

 from the dark hemisphere of the earth, is called 

 the Circle of Illumination. 



a. When the sun is in the equator, the circle of illu- 

 mination passes through both the poles, and every 

 parallel of latitude is one-half in the light and one-, 

 half in the dark ; so that day and night are equal. 

 This is the season of the Equinox, and the sun's 

 meridian distance from the zenith of every place is 

 , then equal to the latitude. 



b. As 



