THE SEASONS AND THE SOLAR SYSTEM 175 



are called meridians or lines of longitude; both parallels 

 and meridians are, of course, circular when complete, as 

 on a globe. A degree (symbol, ) is the unit of length of 

 these lines. It is Keo of their total circumference. The 

 degrees are subdivided into minutes (symbol, '), each of 

 which is %o of a degree (see Fig. 64). 



Now the northernmost parallel at which (on account 

 of the inclination of the earth's axis) the sun ever reaches 



FIG. 64. Parallels and meridians. 



the zenith is called the tropic of Cancer; the correspond- 

 ing parallel of the southern hemisphere is called the tropic 

 of Capricorn. These parallels are, of course, equidistant 

 (23 27' + ) from the equator, and the belt of the earth 

 which lies between them is called the tropical zone. You 

 see then that the sun seems to oscillate from one edge to 

 the other of that belt of the heavens which lies directly 

 over the tropical zone. Actually, however, it is not the 

 sun that thus reels through the heavens; it is the earth 

 that reels. When a moving train passes closely by a 

 standing one in which you are seated, you get the im- 

 pression that it is yours which is moving. Similarly we 



