THE EARTH AS A PLANET 351 



star in turn and moves on beyond them. This causes them to seem 

 to rise and pass across the sky. In due time our western horizon passes 

 them shutting them from our sight, and we say that they have "set." 



In like manner the earth's rotation brings the sun above the horizon 

 of an observer, causes it to pass across the sky, and then to disappear 

 from view as the western horizon passes it. Thus day and night as 

 periods in which the sun is above or below horizon follow each other 

 in endless succession, and will continue to do so as long as the earth 

 continues to rotate. 



Places at the equator are carried eastward by the earth's rotation 

 at the rate of over one thousand miles per hour. This velocity lessens 

 toward the poles where it has a zero value. Directly related to these 

 rotation rates are the directions of the great atmospheric and oceanic 

 currents. 



It is because of the rotation of the earth that we have a basis for use 

 of the terms axis, poles, equator, and parallels of latitude in connection 

 with earth studies. 



ROTATION COMBINED WITH REVOLUTION, AND 

 CHANGES IN SEASONS 



It is difficult for one who lives in the middle latitudes to 

 realize what is true of seasonal changes in the far north or in 

 the Torrid Zone. Experiences gained through travel assist 

 in comprehending them. ,To understand the revolution of 

 the earth about the sun, whether studied in Physical Geog- 

 raphy or in Astronomy, requires more than mere reading 

 about it. Long sustained observations of celestial phe- 

 nomena as " studies of the sky" when intelligently pursued 

 afford a good basis for such an understanding. 



The lessened meridian altitude of the sun at noon-time, 

 from June to December, and the consequent increase in 

 obliquity of the sun's rays, is accompanied by lessened 

 heating effects day by day. But the lowest temperatures of 

 the season occur later than December because of the cumula- 

 tive losses in solar heating. Accompanying these changes are 

 shortened periods of sunshine (shorter "days"), intensifying 



