Marine Collimating-Compass 



Table 47. — CoTredion Factor for Ekvated or Depressed Scale, tan h cosec A. 



183 



in a greater ratio than sec h. The azimuth of the star, moreover, depends on the com- 

 puted local tune, which is subject to errors of the ship's run. If A^, S, q, and h represent 

 the azimuth, declination, parallactic angle, and altitude of the star, respectively, for the 

 hour angle t, at the instant of observation for magnetic decUnation, we have the well- 

 known differential formula of spherical astronomy, 



J . cos 5 cos q , 



dA, = r — at 



cos h 



from which it is Ukewise evident that the influence of an error in time on the azmiuth is a 

 minimum, as far as the altitude is concerned, when the star is in the horizon. If the alti- 

 tudes are measured simultaneously with the magnetic-decimation observations, an excellent 

 check on the azimuth is available through the equation 



cos nA = V cos s cos (s — p) sec <p sec h 



in which <p and p denote the latitude and polar distance respectively, and s = ^ {h + p +(p). 

 The azimuth obtamed from this formula is independent of the local time, and is affected 

 only by refraction errors in the low altitude, and by errors in the assumed latitude. 



