CH. VI] 



TROUBLES WITH SUNLIGHT 



163 



Fortunately, in many parts of America the sun can be counted 

 on in the daytime ; and for those parts the use of sunlight for 

 projection of all kinds is strongly recommended. 



North Pole 



South Pole 



FIG. 86. DIAGRAM OF THE CELESTIAL SPHERE WITH THE PLANES OF THE 



CELESTIAL EQUATOR AND OF THE ECLIPTIC; AND WITH THE APPARENT 



POSITION OF THE SUN AT DIFFERENT SEASONS. 



Earth This is shown as a small black sphere at the center. 



North Pole, South Pole The two poles of the celestial sphere. They are at 

 an infinite distance from the earth. 



West, East East and west points of the celestial sphere. The plane of the 

 celestial equator touches these points. 



Equator The plane of the celestial equator (shaded in lines) dividing the 

 celestial sphere into a northern and a southern hemisphere. A plane at right 

 angles to this traversing the north and south poles would divide it into an 

 eastern and western hemisphere. 



Ecliptic The plane (shaded in dots) around the outer edge of which the 

 sun seems to move during the year. It is inclined to the equator at an angle 

 of 23 27.' 



Equinox When the sun appears at the equator the days and nights are of 

 equal length (March 21, Vernal or Spring Equinox, and Sept. 23, Autumnal or 

 Fall Equinox). 



Solstice The point on the Ecliptic the farthest north or south of the Equa- 

 tor. (Summer Solstice, when north of the equator, June 22 ; Winter Solstice, 

 when south of the equator, Dec. 22). 



(See also fig. 87). 



