82 OUTLINES OF NATUKAL PHILOSOPHY. 



quity of the Ecliptic. A Year is the time which the 

 sun takes to describe this circle. 



82. During the apparent diurnal revolution of 

 the heavens, the plane of the ecliptic does not 

 remain fixed in its place, like the equator, so as 

 to be always vertical to the same point on the 

 earth's surface. The places, however, over 

 which it passes, and through the zeniths of 

 which the sun must pass in his annual course, 

 are all contained within a zone, extending about 

 23 28' on each side of the equator. 



a. This tract, which is called the Torrid Zone, is 

 bounded by two parallels to the equator, called the 

 Northern and Southern Tropics ; the latitude of 

 each of which is equal to the sun's greatest decli- 

 nation, or to the obliquity of the ecliptic. From 

 them the sun appears to return back, and hence 

 their name. 



83. Each pole of the ecliptic is distant from 

 the corresponding pole of the equator, by an 

 arch equal to the obliquity of the ecliptic ; and 

 in the diurnal revolution, they describe the cir- 

 cles called the Polar Circles. 



These circles are also called the Arctic and Antarctic 

 Circles, and the spaces within them are the Fri- 

 gid Zones. 



The 



