XX.] THE MOVEMENTS OF THE EARTH. 353 



21, it presents exactly opposite conditions of light and 

 shade to those represented in Fig. 113. In fact, the north 

 pole is now turned as far as possible from the sun, and the 

 north-polar regions revolve in darkness, while the southern 

 polar regions are enjoying uninterrupted day. 



During the remaining half of its revolution, from No. 3 



Fig. 114. — The Earth at an equinox. 



back to No. i, the earth passes through successive stages, 

 similar to those just described, but in an inverse order. 

 When she has reached the position indicated at No. 4, 

 which she will do on March 22, there is again twelve hours 

 sunshine in all parts of the world. 



It will now be understood that, at two periods in the 



A A 



