142 PRACTICAL ASTRONOMY 



Likewise, 



365.2422 



1 sidereal day = = .99726957 mean solar day 



366.2422 



= 23 h 56 m 4.09 s , mean solar time 



The equation of time is a certain quantity that must be added 

 algebraically to the apparent solar time to obtain the corre- 

 sponding mean time. The value of this quantity for each day 

 of the year is given in the American Ephemeris, which is pub- 

 lished yearly by the United States Government at Washington, 

 D. C. 



Civil Time and Astronomical Time. By civil time is meant 

 the time that is usually reckoned in ordinary life. For astro- 

 nomical purposes, the day is considered to begin at noon, and 

 hours are counted from to 24. When time is reckoned in this 

 manner it is called astronomical time. The civil day begins at 

 12 o'clock at night, and the astronomical day begins 12 hr. 

 later. For instance, the date Oct. 17, 7 h 14 m 3 s , astronomical 

 time, means 7 h 14 m 3 s after noon of the civil date Oct. 17, and 

 is in civil time, 7 h 14 m 3 s P. M. The astronomical date Feb. 20, 

 18 h 6 m 12 8 means 18 h 6 m 12 s after noon of the civil date Feb. 

 20, or 6 h 6 m 12" after midnight of Feb. 20; that is, Feb. 21, 6 b 

 6 m 12 s A. M. 



Longitude and Time. The mean sun describes a complete 



360 



circle in 24 mean solar hours. In 1 hr. it moves over 



24 



= 15 of arc; in 1 min. of time, over 15' of arc; and in 1 sec. 

 of time, 15" of arc. 



Relation Between Time and Longitude. Let A and B be 

 two places on the earth's surface, B being west of A. Let their 

 respective longitudes be g a and gi,, and let the difference between 

 gb and g a , expressed in measure of time, be d g . Let, also, T a 

 be the time at A when the time at B is T&. Then, 



T a -T+d f (1) 



and T^Ta-dg- (2) 



EXAMPLE 1. The longitude of Washington, west of Green- 

 wich, is 5 h 8 m 1 s ; that of San Francisco, 8 h 9 m 47 8 . What is the 

 time at: (a) Washington when it is 9 h 3 m at San Francisco? 

 (b) San Francisco when it is 19 h 54 m 30" at Washington? 



