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    Re: Practical difficulties
    From: Gary LaPook
    Date: 2011 May 13, 17:43 -0700

    As an example of precalculation look at these examples:


    File:



    File:



    File:



    File:



    File:


    This is a standard way of doing in flight navigation.

    https://sites.google.com/site/fredienoonan/topics/precomputed-altitude-curves

    Just pick a convenient A.P. in the area where you plan to do your sailing for the day. Do the computation for even 4 minute intervals so that the GHA of the sun will have advanced exactly whole degrees so the same A.P. can be used for the computations. In flight navigation 20 minute intervals are commonly used resulting in 5 degree changes in the sun's GHA. Although Aries moves at a slightly different rate, for practical navigation you can use the same method for a selected star. If you are using H.O. 214, H.O. 218 or H.O. 249 you can do all the computations by just running your finger down the column. H.O. 229 is not so convenient, you must keep flipping pages. ( I still don't understand why they went to this format.)

    After you have the Hcs tabulated, apply the normal sextant corrections with the signs reversed to arrive at Hp (precomputed altitude) and plot Hps on the graph.

    You then can just compare your raw sextant altitude with the graph of Hp and you have the intercept instantly.

    To show how this works:

    Hs= 15°45'

    I.C +3'

    Dip -4'

    Ref. -4'

    Ho = 15°40'

    Hc = 15°30'

    INT = 10 toward
    ---------------------

    Signs reversed sextant corrections.

    Hc = 15°30'

    I.C -3'

    Dip +4'

    Ref. +4'

    Hp = 15°35'

    Hs= 15°45'

    INT = 10 toward


    Same intercept either way.

    See:

    http://www.fer3.com/arc/m2.aspx?i=116311&y=201104

    gl

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