THE EARTH AS A PLANET 359 



distance and its direction now being measured from the opposite edge of 

 the surfs disc. 



Easter is the first Sunday following the first full moon that occurs 

 next after the Vernal Equinox (time in the spring when the days and 

 nights are equal). 



TIME, AND TIME-KEEPING, AND STANDARD TIME 



Time-keeping resolves itself into a choice of convenient 

 units as subdivisions of natural time periods, and the use of 

 devices for counting and registering these smaller units 

 with an unfailing accuracy. Since the time of Gal-i-le'-o 

 (1564-1642) use of the vibration periods of pendulums (or 

 those of coiled springs in watches) has displaced the hour- 

 glass and other devices of the ancients for measuring time. 

 They had the years and the days for large time units the same 

 as we, but accuracy in the measurement of smaller time units 

 waited upon the discovery of the laws of the pendulum. 



It is quite indispensable that the time-pieces of a com- 

 munity its homes, shops, factories, mills, and schools 

 shall be so set and regulated that they show at all times the 

 same readings. Railway service in all parts of any section 

 of the country requires the same time standard by all con- 

 cerned in its administration, and travelers must adapt their 

 times to this standard. 



Since the earth rotates through 360 of circumference in 

 twenty-four hours, its rate of rotation is 15 per hour. Any 

 place 1 5 eastward of an observer has noon by the sun (that 

 instant when the sun's centre is on the meridian of the place) 

 just one hour earlier than noon with the observer, or four 

 minutes earlier for every i of distance. Places westward 

 of the observer have their meridians come to and pass the 

 sun four minutes later for every degree farther westward. 

 Observers living east and west of one another will have 

 different "local time" whether their time-pieces are kept 



