192 



EARTH AND HEAVENLY BODIES 



however, to the distance of the earth from the sun. 

 Strange as it may seem, we who live in the north 

 temperate zone are actually about 3,000,000 miles 

 nearer the sun in winter than in summer. 



There are two reasons for the difference in tem- 

 perature at different seasons of the year. (1) During 

 the summer season a given portion of the earth re- 



NORTH 



WST\ 



yf 20' 10' o w 20 



30' -Vf SO' 60' TO' 80' V Krf IK)' 120' 



SOUTH 



NOKTH 



EAST 



ceives more heat and light than in the winter because 

 the rays of the sun strike the earth more directly. 

 During the winter season the sun's rays strike the 

 earth more slantingly. As a result they spread over a 

 greater amount of surface and are less intense. (2) In 

 summer the days are longer than in winter. Thus we 

 receive more hours of sunlight each day. 



Exercise. Explain why the countries in the South- 



ern Hemisphere have winter while we have summer and 

 summer while we have winter. 1 



To make it possible to locate places on the earth 

 and for convenience in reckoning time, the earth's 

 equator is divided into 360 equal parts called degrees 

 (). Each degree is divided into sixty minutes ('), 

 and the minute is further divided into 60 seconds ("). 

 See Figure 315. Imaginary lines extending from the 

 north pole to the south pole through each degree are 

 drawn on maps of the earth. These imaginary lines, 

 which are called meridians, indicate degrees of longi- 

 tude. By international agreement, the meridian pass- 

 ing through Greenwich (gren'ich), England, is called 

 zero meridian or prime meridian and is marked 0. 

 From this point the meridians are numbered east and 

 west from to 180, making a total of 360 degrees. 



Likewise at every degree north and south of the 

 equator imaginary lines are drawn parallel to the 

 equator. These lines, which are called parallels, indi- 

 cate degrees of latitude (Fig. 315). By means of paral- 

 lels and meridians it is possible to locate any spot on 

 the whole surface of the earth. Locate the position of 

 your own home on the earth by reading latitude and 

 longitude degrees on a map. 



FIG. 316. STANDARD TIME BELTS IN THE UNITED STATES 



How is time kept? Timekeeping is based upon the 

 earth's movements. The rotation of the earth, com- 

 pleted regularly in about twenty-four hours, deter- 

 mines our day and night. Astronomers have shown 

 that the time of rotation of the earth upon its axis has 

 not changed appreciably for thousands of years. The 

 revolution of the earth about the sun determines the 

 length of our year. 



Sun time is reckoned from noon to noon. Noon at 

 any spot on the earth is the exact time in the day when 

 the sun is nearest overhead. The meridian of that 

 place is directly under the sun, and all places located 

 on that meridian have noon at the same time. Places 



1 See workbook, p. 72. 



