THE EARTH 



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parallels, or imaginary lines running around the earth 

 parallel to the equator, which is the largest of these 

 circles and is considered as 0. The other parallels 

 are numbered so many degrees north or south of the 

 equator. 



Latitude and Longitude. .The distance north or 

 south of the equator is called latitude and is measured 

 in degrees on the meridians by the parallels. Distance 

 east and west of the prime meridian is called longitude 

 and is measured in degrees on the parallels by the me- 

 ridians. 



Local Time. The period of the earth's rotation 

 furnishes a natural unit of time, the day, easily recog- 

 nized and everywhere constant. 

 Before there, were any clocks, 

 people told the time of day by 

 the sundial (Figure 18). This 

 consisted of a vertical rod, the 

 shadow of which fell on a hori- 

 zontal plane. From noon or the 

 time when the sun cast the 

 shortest shadow on one day 

 until it cast the shortest shadow 

 on the next day was considered 

 a day's time or a solar day, and 

 was divided into twenty-four 

 hours. Before the advent of 

 railroads and the telegraph, each 

 community used the mean solar 

 time of its own meridian. 



Standard Time. When railways extending east and 

 west became numerous in the United States, it became 

 very inconvenient to use local time, for the traveler 



FIG. 18. Sundial. 



