38 A MANUAL OF TOPOGKAP11I0 METHODS. 



REDUCTION OF AZIMUTH OBSERVATIONS. 



The time of observation of a star is first to be corrected for the differ- 

 ence in longitude, assuming that railroad time has been used, and for the 

 error of the watch. It is then reduced from mean to sidereal time. From 

 the sidereal time of observation is to be subtracted the right ascension of 

 Polaris, if that star is used, which is given in the Nautical Almanac, the 

 result being the hour angle or the sidereal time which has elapsed since it 

 passed the meridian of the place of observation, given in hours, minutes, 

 and seconds. This result is to be converted into degrees, minutes, and 

 seconds. 



Then tan A — —. 



1-b cos t 

 where rt — sec (p cot <5, <jp=the latitude. 



b = - 



tan (f 



tan 6 S =the declination of star. 



izzhour angle. 

 A =r angle between north pole and the mark. 



This angle is to be corrected for level as follows: 



level corr.zz— A-Hw+ti/)— (e+e')|tan h. 



d being the value of a division of the level. 

 w+w', readings of west end of level bubble. 

 (■ + <■', readings of east end of level bubble. 

 /*, the angular elevation of pole star. 



