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    The Prime Vertical
    From: Greg Rudzinski
    Date: 2010 Aug 30, 17:00 -0700

    During spring and summer the Sun can be observed morning and afternoon on the prime vertical. When using the traditional sight reduction tables of Pub 208, 211,Weems, or Trig Log a special case arrises where the navigator is relieved of azimuth calculations (azimuth is either due East or West). Predetermination of P.V. needs to be derived by formula ARC SIN(SIN DEC/SIN LAT) which is satisfactorily performed by slide rule. Once the P.V. is approximated then an observation of the Sun within plus or minus a degree of P.V. altitude has the beautiful advantage of being plotted by divider alone just as the LAN observation. Combine the P.V. and LAN to get a unique running fix. Pub 249 can be used to approximate P.V. easily with table entries of assumed whole Latitude and nearest whole Declination then scroll to Z=90° to get the corresponding Hc of P.V. (see attachment for todays P.V. by Pub 249 for Lat. 34° N , Dec 9° N)

    Greg Rudzinski
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