CHAPTER VI. AZIMUTHS 



57. Azimuth Z„. — The true azhrmth of a body is the true bearing 

 of that body relative to the north geographic pole, measured on the 

 horizon clockwise from 0° to 360° Magnetic bearings or azimuths 

 differ from true bearings or azimuths by the local variation. 



58. Azimuths of the sun. — The azimuth of the sun at any instant 

 can be determined by solving the astronomical triangle established 

 by the observer's position, the sun, and the elevated pole. Since ac- 

 curate compass bearings of the sun can readily be observed for com- 

 parison with the sun's azimuth to obtain the compass error, the sun 

 IS a valuable reference point for compass adjustment and compensa- 

 tion. The azimuths of other celestial bodies for any instant can 

 similarly be determined, but are less practical for compass work be- 

 cause of the poor visibility of stars and the more variable time rates 

 and declinations of the moon and planets. Hence, subsequent ex- 

 planations concern themselves only with the sun and its azimuths. 



59. Azimuth angle Z. — United States Navy Hydrographic Office 

 publications H. O. 71 and H. O. 214 tabulate the solutions of the as- 

 tronomical triangle discussed above. For convenience of tabulation, 

 the relative azirmiths given in these tables are measured either east or 

 west from the elevated geographic pole. Hence, certain rules must 

 be observed in obtaining a true azimuth, Z„, from an azimuth angle, 

 Z, in these tables. These rules are as follows : 



1. For North latitudes : 



(a) Zn—Z if the sun is east of the meridian. 



(&) Zn=^QO° — Z if the sun is west of the meridian. 



2. For South latitudes : 



(a) Z„=180°— Z if the sun is east of the meridian. 



(b) Z„=180°+Z if the sun is west of the meridian. 



It must be remembered that in order to obtain magnetic azimuths 

 from true azimuths the appropriate variation must be applied to the 

 true azimuths. 



60. Equation of time (Eq. T.). — Apparent time is measured by the 

 apparent motion of the true sun, and at any instant it differs from 

 mean sun time by the equation of time {Eq. T.). The Eq. T. is 

 tabulated in the Nmitical Almanac for every even hour of Green- 

 wich civil time {G. O. T.) throughout the year. 



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