174 Trees, Stars, and Birds 



degrees. Likewise a star one third of the way from 

 the horizon to the zenith has an altitude of 30 degrees ; 



one that is two thirds of the 

 way, 60 degrees ; one that is 

 four fifths of the way, 72 

 degrees. 



The celestial equator. A 

 wheel rotates about a line 

 passing through its center. 

 The line about which it 

 rotates is the axis of rotation. 



FIG. 114. J 



The earth rotates about its 



axis, an imaginary line passing through the north and 

 south poles. The plane passing through the center of 

 the earth and perpendicular to the earth's axis is the 

 plane of the equator. The line where this plane comes 

 to the surface of the earth is the terrestrial equator, and 

 the line where it comes to the celestial sphere is the 

 celestial equator. Get a globe or use an apple or an 

 orange to represent the earth. Then study out carefully 

 what has been said in this paragraph. Cut an apple 

 in halves and lay a circular sheet of paper between the 

 halves, as shown in Figure 114. This may help you to 

 understand the celestial equator. Be especially careful 

 to understand what is meant by the word. " plane." 



The ecliptic. In the course of a year the earth re- 

 volves once about the sun. During this time the sun 

 seems to move around the heavens in a circular path. 

 This path is called the ecliptic. It is not the same 

 as the celestial equator. The plane of the ecliptic is 

 inclined to the plane of the equator 23^ degrees, so that 



