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    Re: Almanacs, theory and use.
    From: Gary LaPook
    Date: 2007 Nov 21, 01:07 -0800
    Gary adds:

    Some more useful information:

    An easy way to practice with a sextant and to take a number of
    observations to allow you to determine its accuracy is to use the
    meridian transit of Polaris. When Polaris is crossing the meridian it
    is moving horizontally and its altitude doesn't change. For the period
    of more than 15 minutes both before and after meridian passage the
    altitude of Polaris changes less than one - tenth of a minute (0.1').
    For a period of 34 minutes before and after passage the altitude of
    Polaris changes less than one half minute (0.5'.) For 48 minutes
    before and after its altitude changes less than 1 minute.
    
    Calculate the time that Polaris is crossing your meridian and get out
    early and start shooting. It is about 10:46  pm local time now (Nov. 21, 2007) depending on your exact longitude..
     Calculate the altitude by adding your latitude to the polar distance of Polaris,
    now 41.9 minutes, and then ADD the refraction correction (yes ADD) which
    will then give Hp (precomputed altitude, this is how it is done by
    flight navigators) because this porceedure allows you to compare your
    Hs directly with Hp to determine the intercept immediately, or the
    error in the observation if taken from a known point. If shooting a
    lower transit subtract the polar distance from your latitude. You only
    have to update this Hp from time as the declination of Polaris changes
    slowly.
    
    
    Here is a link to a site that will give you celestial almanac data and compute Hc for you.
    
    http://aa.usno.navy.mil/data/docs/celnavtable.php
    
    
    
    
    


    Here is a link to a site where you can download for free the current
    copy of Bowditch as well as H.O 229, H.O 249 etc:

    http://www.nga.mil/portal/site/maritime/?epi_menuItemID=c56aa099e2bff...

    Here is a link to an online version of Bowditch:

    http://www.i-DEADLINK-com/bowditch/

    Here is a link to an online Nautical Almanac:

    http://www.tecepe.com.br/scripts/AlmanacPagesISAPI.isa

    Her is a link where you can download a celestial navigation program:

    http://www.tecepe.com.br/nav/

    Enjoy.



    Isonomia wrote:
    Gary,
    
    thanks a million for the link to the book with navigational astronomy
    calculations etc. although it's a lot to go through, it was exactly
    what I needed.
    
    Mike
    
    On Nov 20, 7:31 pm, glap...---.net wrote:
      
    Gary LaPook writes:
    
    Also attached is a link to an online version of The American Practical
    Navigator refrence work, go to chapter 19.
    
    http://www.tecepe.com.br/scripts/AlmanacPagesISAPI.isa
    
    http://www.i-DEADLINK-com/bowditch/
    
        
    learnt spherical geometry at University, I'm a little  rusty)
          
    
    
      


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