MEASUREMENT OF TIME 



29 



until it cast the shortest shadow the next day, was con- 

 sidered a day's time, or a solar day, and was divided into 

 twenty-four equal parts 

 called hours. 



The direction of the 

 shortest shadow is a 

 north and south line, 

 since the sun must then 

 be halfway between the 

 eastern and western 

 horizon. As the lengths 

 of these solar days vary 

 slightly, for reasons 

 which cannot be ex- 



Fig. 11. 



plained here, we now divide the mean length of the solar 

 days for the year into 24 parts to get the hours. The 

 civil or conventional day begins at midnight, not noon. 

 The determination of the exact time is very important ; 

 for the United States it is done at the Naval Observatories 

 at Washington and at Mare's Island, San Francisco, and 

 telegraphed each day to different parts of the country. 



A day may be measured by the interval between the suc- 

 cessive passages of a star across the zenith. This would 

 be called a sidereal day, from the Latin word for star. It 

 might also be measured by successive passages of the 

 rnoon across the zenith. This would be called a lunar 

 day, from the Latin word for moon. 



If a person should start at noon and travel around the 

 earth from east to west as fast as the sun does, the sun 

 would be overhead all the time and no solar day would 

 have passed for the traveler, even though 24 hours would 

 be required for the trip. But when he reached home he 

 would find that it was the next day. Thus any one travel- 

 ing around the earth must drop a day if going toward the 



