184 EXPERIMENTAL GENERAL SCIENCE 



the sun, the position of the earth's axis does not change. It 

 is the change in relative position as the earth travels around 

 the sun, that causes this appearance, and also produces the 

 changes in the seasons. When the sun and earth are in such 

 positions that the sun is exactly overhead at the equator, its 

 farthest rays just reach both poles. At this time, the days 

 and nights are equal everywhere on the earth. This condi- 

 tion occurs twice every year, at the vernal equinox on March 

 21, when spring begins, and at the autumnal equinox on Sep- 

 tember 23, when autumn begins. In spring, the sun appears 

 to pass north of the equator, and at the beginning of summer 

 is 23H degrees north of it. Here it appears to stop and turn 

 southward again. This point marks the summer solstice , at 

 which time, in the Northern Hemisphere, the days are longer 

 and the nights shorter than at any other time. The circle 

 over which the sun appears to stop is called the Tropic of 

 Cancer. At this time, the sun's rays lack 23}^ degrees of 

 reaching the south pole and the circle which they touch is 

 called the Antarctic circle. At the winter solstice, the sun is 

 23^ degrees south of the equator over the Tropic of Capri- 

 corn, and its rays then just reach the Arctic Circle 23^ degrees 

 from the north pole (80). 



Practical Exercises 



1. Does the size of the circle make any difference in the number of 

 degrees in it? 



2. How does the size of a circle affect the size of a degree? 



3. What is the great circle called that is located midway between the 

 poles of the earth? 



4. In what part of the world is Greenwich, from whose meridian 

 longitude is usually measured? 



