MERIDIANS AND PARALLELS OF LATITUDE 33 



Although the exact determination of time is a difficult 

 task and requires great skill and very accurate instru- 

 ments, yet it is not very hard to determine quite satis- 

 factorily the length of a solar day. Before there were any 

 clocks, people told the time of day by sundial (Figure 12), 

 which consisted of a vertical " pointer " the shadow of which 

 fell upon a horizontal plane. From local noon, or the 

 time the sun cast the shortest shadow on a certain day, 

 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 ho- 

 rizon. As the lengths of 

 these solar days vary 



slightly, for reasons which cannot be explained here, we 

 now divide the mean length of the solar days for the year 

 into twenty-four 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 Island, San Francisco, and 

 telegraphed each day to different parts of the country. 



Experiment 7. On a day when there appear to be indications of 

 settled fair weather place a table covered with blank paper in an 

 open space where the sun can shine upon it. Make the top of the 



FIGURE 12. A SUNDIAL 



