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    Re: Advice concerning sextants
    From: Gary LaPook
    Date: 2011 Jan 7, 12:56 -0800
    (Since I am never sure how the formatting is going to work out I am attaching this same post as a PDF file.)

    What I meant by we could “have worked  the standard noon formula from the other end and
    compute what we would have measured if we had been at 25 degrees North” was to assume we
    were at 25 degrees north and then rearrange the standard noon formula to compute the altitude
    we would have observed had we actually been at 25 north. This is how it worked.

    Date: October 29, 2009
    Time: 13:18:46
    Declination 13̊ 36.4' south
    Height of eye: 33 feet
    Index error: +1.6'
    Lower limb observation

    Hs   51̊ 26.4'
    I.C.         -1.6'
    Dip         -5.6
    Ref.        -0.8'
    S.D.     +16.1'
    Ho   51̊ 34.5'

    ( For instructional purposes, I like to do the semi-diameter and refraction corrections separately
    so the student can see where the numbers are coming from. The total of these two corrections
    was + 15.3'. If using the sun correction table the combined correction for the lower limb shot was
    + 15.5'. Doing the corrections separately is more accurate since the sun correction table doesn’t
    use the actual S.D for the sun on the day of the observation but an average S.D. for a six month

    Normal noon sight computation

    90̊ = 89̊ 60.0'
    Ho   - 51̊ 34.5'
    ZD     38̊ 25.5'
    Dec  -13̊ 26.4'
    Lat     24̊ 49.1' north

    Introducing the Marc St. Hilaire method by calculating computed altitude using the rearranged
    noon formula.

                   A lat             25̊ 00.0' north
    Dec                    + 13̊ 36.4'
    Assumed  ZD     38̊ 36.4'

                89̊ 60.0'
    Assumed ZD     -38̊ 36.4'
              Hc            51̊ 23.6'
    Ho                       51̊ 34,5'
    Int                              10.9 nm toward

    Noon Zn is 180̊ so 10.9' south of the assumed latitude places the 90̊-270̊ LOP at 24̊ 49.1'
    north latitude, the same result as the normal noon sight.

    Introducing the usual way of doing the same computation with H.O. 229 for the general case:

    LHA           0̊
    A lat          25̊ north
    Dec            13̊ contrary name
    Tab Hc      52̊ 00.0'
    d                               -60.0' (per degree of declination change)
    d corr             -36.4'   
    Hc              51̊ 23.6'
    Ho              51̊ 34.5'
    Int                     10.9 nm toward
    Zn              180.0̊

    Using H.O. 249:

    A lat             25̊ north
    Dec               13̊ contrary name
    Tab Hc          52̊ 00'
    d                                 -60
    d corr                -36'
    Hc                51̊ 24'
    Ho                51̊ 35'
    Int                       11 nm toward
    Zn                180̊

    Using a calculator with the normal Sine - cosine formula

    Hc= arc sin( (sin lat x sin dec) + (cos lat x cos dec x cos LHA))

    --- On Wed, 1/5/11, Gary LaPook <glapook@pacbell.net> wrote:

    From: Gary LaPook <glapook@pacbell.net>
    Subject: [NavList] Re: Advice concerning sextants
    To: NavList@fer3.com
    Date: Wednesday, January 5, 2011, 10:14 PM

    Here is a link to the article, read the third from last paragraph.


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