ASTRONOMY. 



433 



arrive at a part of the earth that couid not receive 

 any direct rays from the sun. 



It is the annual motion of the earth round the 

 sun whicli occasions the diversity of seasons. To 

 understand this, we must observe what has been 

 already mentioned, that the axis of the earth is in- 

 clined to the plane of its orbit 23£°, and that it 

 keeps always parallel to itself; that is, it is always 

 directed to the same star. 



Let Plate 24, fig. 1. represent the earth in 

 different parts of its elliptic orbit. In the spring, 

 the circle which separates the light from the dark 

 side of the globe, called the terminator, passes 

 through the poles n s, as appears in the position A. 

 The earth then, in its diurnal rotation about its 

 axis, has every part of its surface as long in light as 

 in shade; therefore the days are equal to the nights 

 all over the world, the sun being at that time ver- 

 tical to the equatorial parts of the earth. As the 

 earth proceeds in its orbit, and comes into the 

 position B, the sun becomes vertical to those parts 

 of the earth under the tropic, and the inhabitants 

 of the northern hemisphere will enjoy summer on 

 account of the solar rays falling more perpendi- 

 cularly upon them ; they will also have their days 

 longer than their nights, in proportion as they are 

 more distant from the equator; and those within 

 the polar circle, as will be perceived by the figure, 

 will have constant day-light. At the same time, 

 the inhabitants of the southern hemisphere have 

 winter, their days being shorter than their nights, 

 in proportion as they are farther from the equator, 

 and the inhabitants of the polar regions will have 

 constant night. 



The earth then continues its course to the 

 position C, when the terminator again passes 



f f 2 



