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    Re: Latitude of prime vertical sight
    From: Chuck Taylor
    Date: 2007 Jul 31, 22:09 -0700

    > Paul.Lee wrote: 
    >> "I'm new to navigation, and am doing some work on the usage of prime
    >> vertical sights. I know it gives longitude, but if you knew the time
    >> of sunrise, is it possible to obtain a value for lattitude too? "
    
    One approach is to treat sunrise as an ordinary sight, except that it is taken 
    without a sextant.  When the upper limb of the sun just becomes visible, then 
    the altitude of the center of the sun is approximately minus 0 degrees 50 
    minutes.  Record the time. Correct for height of eye (dip) and you have Ho. 
    Then reduce the sight as usual to get a line of position. The -50 minutes 
    accounts (approximately) for semidiameter, refraction, and parallax.
    
    As far as accuracy is concerned, I'd put this in the category of "lifeboat 
    navigation", i.e., the sort of thing one does when there is no better 
    alternative.
    
    If you advanced this line of position to the time of your prime vertical 
    sight, then the two lines of position would give you both latitude and 
    longitude (as would virtually any two lines of position).
    
    Prime vertical observations were treated as special before the days of Sumner 
    and St. Hilaire because they give you longitude. With line of position 
    navigation, there is no longer any need to regard sights taken at the time of 
    the prime vertical crossing as special, because you can get both latitude and 
    longitude from any two lines of position (provided of course that they cross 
    at an appropriate angle).
    
    Best regards,
    Chuck Taylor
      48 N
    122 W
    
    
    
    
    
    
           
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