PRACTICAL ASTRONOMY 143 



SOLUTION. (a) Here A, the eastern locality, is Washington 

 and B is San Francisco; also, d ff =S h 9 m 47 8 -5 h 8 m l s = 3 h l m 

 46 s . Therefore, applying formula 1, 



r a -9 h 3 m +3 h l m 46 3 = 12 h 4 m 46 s . 



(6) Applying formula 2, 



T& = 19 h 54 m 30 s - 3 h l m 46 s = 16 h 52 m 44 s . 



Standard Time. Time referred to the meridian of a given 

 place is called local time of that place. To obviate complica- 

 tions in comparing local times of different localities, for use 

 in ordinary affairs of life standard times have been adapted 

 for regions between certain longitudes. The United States is 

 divided into four zones, or sections of standard time. The time 

 in each zone is referred to the meridian passing through its 

 center. These central meridians are 15 or l h distant from 

 each other and are, respectively, 75, 90, 105, and 120 west 

 of Greenwich; or, in hours, 5 h , 6 h , 7 h , and 8 h west of Green- 

 wich. Each of these meridians controls the watch time of all 

 places within 73 on either side. This is shown as follows: 



8 h 3O m 8 h 7 h 3O m 7 h 6 h 30 m 6 h 5 h 3O m 5 h 4 h 3O m 



Pacific Mountain Central Eastern 



127 30' 120 1 12 30 1O5 97 3o' 9O 82 3O' 75 67 3O 



Time referred to the 75 meridian is called eastern time; 

 to the 90 meridian, central time; to the 105 meridian, mountain 

 time; and to the 120 meridian, Pacific time. 



To Change Standard Time Into Local Time and Vice Versa. 

 Standard time can be changed into local time or local time can 

 be changed into standard time by applying formula 1 or formula 

 2, according as the given place is east or west of the reference 

 meridian of the zone in which the place is located. 



EXAMPLE. The standard time, by a watch, at a place whose* 

 longitude is 81 37', is 9 h 37 m 45 s A. M.; what is the local time? 



SOLUTION. Since the longitude is 81 37', the place lies 

 within the zone of the 75 meridian; and being west of the 

 latter, formula 2 must be applied. In this case, T a = 9 h 37 m 45" 

 and <k=81 37'-75 = 6 37' = 26 m 28 s . Therefore, 2^ = 6 h 

 37 m 45 s -26 m 28 s = 9 h ll m 17 s A. M. 

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