PRACTICAL ASTRONOMY 151 



Washington. Take from the Ephemeris the declination corre- 

 sponding to the preceding Washington noon and add algebraic- 

 ally the product of the hourly change by the lime elapsed since 

 Washington noon. 



EXAMPLE. Find the true declination of the sun for 9 A. M. 

 Jan. 5, 1903, at Philadelphia. 



SOLUTION. Jan. 5, 9 A. M., civil time = Jan. 4, 21 h , astro- 

 nomical time. The longitude of Philadelphia is 7 m 37 s 

 = .127 h . The Washington time corresponding to 9 A. M. is 

 21 h -.127 h = 20.873 h . From the Ephemeris the declination at 

 Washington at noon Jan. 4 is -22 47' 43", and the hourly 

 change is 15.06". The algebraic increase is, therefore, 15.06 

 X 20.873 = 5' 14"; thus, the declination at 9 A.M. is 22 47' 

 43" +5' 14"= -22 42' 29". 



DETERMINATION OF LATITUDE, AND CORRECTIONS 



FOR ALTITUDE 

 Approximate Determination of Latitude From Polaris. In 



nearly all methods of determining the true meridian, the lati- 

 tude of the place of observation must be known, at least 

 approximately. In the majority of cases, the latitude can be 

 taken from a map or book of reference. In case this cannot be 

 done, a sufficiently close value may be obtained by measuring 

 with a transit the altitude of Polaris, which is very nearly 

 (within about 1 ) equal to the latitude of the place. 



This method of determining latitude is founded on the fol- 

 lowing very simple and useful principle: 



Principle. The latitude of any place on the earth's surface is 

 equal to the altitude of the pole with respect to the horizon of that 

 place. 



For more accurate work, the tables given in the Ephemeris, 

 entitled, For Finding the Latitude by Polaris, may be used. 

 The simple directions for using them are there given in full. 



Latitude by Solar Observation. Latitude may be deter- 

 mined by measuring the sun's altitude, with the sextant or tran- 

 sit, at the instant of its passage across the meridian; that is, at 

 apparent noon. The time of apparent noon may be determined 

 by adding algebraically the equation of time to the noon-of local 

 mean time, as previously explained. Then begin the observations 



