300 



HYDROGRAPHICAL SURVEYING L<-'HAi>. xv. 



Use of 

 Polaris. 



The Pole Star may be used in the Northern Hemisphere to 

 obtain true bearings at night. 



Circumstances under which this is useful are related at 

 p. 213, which see. 



The Greenwich time must be laiown, and the angle between 

 the Pole Star and object whose bearing is required must be large. 



Measure the angle and take the time. 



Ascertaining the sidereal time of observation as in ordinary 

 Pole Star calculation, add six hours to it for a second sidereal 

 time. 



Out of Table I. in "Nautical Almanac," take the correction 

 A^■ith first sidereal time, which, applied Avith the reverse sign to 

 the latitude, Avill give the altitude at the time. 



Take out a second correction with second sidereal time, 

 which will be the rectangular deviation of Polaris from the 

 meridiam. 



To calculate the horizontal angle answering to this, the 

 formula is 



Sin horizontal angle = Sin correction x See alt., - 



which will give the true bearing of Polaris — east of meridian 

 when first sidereal time is between 13 hours 20 minutes and 

 1 hour 20 minutes, west m hen otherwise. 



