40 OUR OWN WORLD 



on its axis as it moves around the sun, therefore, accounts for 

 our changing seasons. This inclination also accounts for the 

 varying length of our days and nights. 



We locate places on the earth's surface by means of imagi- 

 nary circles drawn around the earth, which are called merid- 

 ians and parallels of latitude. From the equator in either 

 direction to the poles is a quarter of a circle or 90. From 

 a zero meridian we measure a half circle, or 180, east, and 

 180 west. 



From the time the sun casts the shortest shadow one day 

 until it casts the shortest shadow the next is a solar day. 

 Solar days differ slightly in length ; and so, for convenience, 

 a calendar day is the average of the solar days of the year. 

 To avoid the endless confusion that would be caused by each 

 community having its own local time, the United States is 

 divided into belts 15 wide. Throughout one of these belts, 

 standard time is the same, and each belt differs by one hour 

 in time from a neighboring belt. The International Date 

 Line (about the 180th meridian) is the line which for con- 

 venience marks the beginning and ending of a calendar day. 

 Setting the clock ahead one hour during the summer months 

 gives more daylight during working hours. This is called 

 daylight saving. 



The earth has a north and a south magnetic pole. These 

 do not correspond with the poles of the earth's axis, nor do 

 they remain stationary. The attraction of these poles for 

 the magnetic needle or compass enables mariners always to 

 determine direction. 



QUESTIONS 



What simple reasons are there for believing that the earth is 

 round? 



Draw circles illustrative of the size of the earth, moon, and sun. 



