THE EARTH 23 



(Figure 19). Everywhere in a given belt the clocks 

 are an hour ahead of those in the next belt west and an 

 hour behind those in the next belt east. When it is noon 

 at New York, it is 11 A.M. in Chicage, 10 A.M. in Denver, 

 and 9 A.M. at San Francisco. The accurate standard 

 time is sent once a day at twelve o'clock, by telegraph, 

 from the Naval Observatory at Washington to all cities 

 in the United States. By electric connection many clocks 

 are thus set exactly right each day. 



Revolution of the Earth. The earth moves once 

 around the sun in 365J days. The path in which it travels 

 is elliptical in shape and is called its orbit. The revolu- 

 tion of the earth around the sun gives us our measure 

 of time called a year and causes the sun to appear to 

 shift its position in the heavens from day to day. 



The axis of the earth constantly points in the same 

 direction whatever its position in its orbit. The axis 

 is inclined to the plane of that orbit 66| or 23^ from 

 the perpendicular. This inclination of the axis causes the 

 earth to assume quite different positions with reference 

 to the sun at different times of the year. To this is due 

 the varying lengths of day and night and the change of 

 seasons. 



Change of Seasons. From spring to autumn when 

 the north pole is inclined toward the sun (Figure 20), 

 the sun's rays are received vertically at places in the torrid 

 zone north of the equator and obliquely at places within 

 a distance of 90 north of the torrid zone. During this 

 time the sun's rays cover the north pole continuously, 

 causing the very long days in the frigid zone. It is then 

 summer in the northern hemisphere, the days being 

 longer and warmer there than in the southern hemi- 

 sphere, because the rays of the sun are more nearly 



