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    Re: Beginner with inaccurate results
    From: Lu Abel
    Date: 2005 Aug 30, 10:56 -0700

    Looks like this was a simple case of getting d correction wrong.
    For those on the list who are not aware of it, a great source for
    checking one's Hc and Zn calculations (assuming Internet access) is the
    US Naval Observatory's interactive Nautical Almanac.
    Enter UT and assumed L/Lo, and it will give the Zn and Hc for every
    above-the-horizon navigational body for that time and position!
    Lu Abel
    Geoffrey Kolbe wrote:
    > Hello Asbj?rn
    > I checked your calculation for measured altitude with the data you gave
    > below. I get a sextant altitude, with refraction, of 29 degrees 20.2
    > minutes, which is in close agreement with yourself. However, I get a GHA of
    > 23 degrees 53.2 minutes and a declination of 9 degrees 11.7 minutes. (This
    > is calculated using the Stormy Weather software, which has all the bells
    > and whistles and is very accurate.) Your GHA is close enough - perhaps
    > there is a typo in your declination....?
    > Your procedure seems fine - but how are you levelling your coloured glass
    > artificial horizon?
    > Try some star shots and see if there is refraction in the shade. (You will
    > not need the shade for star shots.)
    > Geoffrey Kolbe.
    >> Hi.
    >> I have just startet learning celestial navigation, and have bought a
    >> Davis 15 sextant.  Even though it has been rather cloudy here in
    >> Trondheim after I got the sextant, I have managed to get some
    >> "shots".
    >> I am however not satisfied with my results, my observed height is up
    >> to 6.5' different from calculated height.  This is strange as I can
    >> easily notice a 6.5' index error.  And my observed height is always
    >> (with no exceptions yet) less than calculated height.  One would think
    >> that if I am just inaccurate, my results should be on both sides of
    >> the calculated values.  So I am starting to thing I do something
    >> wrong.
    >> Here is what I do, hopefully a kind person reads through it and finds
    >> an error:
    >> I use a Davis artificial horizon with colored glass.
    >> 1. First I adjust mirrors.  Horizon mirror is adjusted so that when
    >>    setting sextant to 0 degrees and looking at the sun, both reflected
    >>    and real sun overlaps completely.
    >> 2. Then I remove some filters from horizon mirror (could there be some
    >>    refraction here?) and measure height.  Lower limb on sun is taken
    >>    down to touch upper limb of sun's reflection in the artificial
    >>    horizon.
    >> 3. I note height, GMT and then check index error with same procedure
    >>    as in 1.
    >> 4. Then I calculate, this is an example I took yesterday:
    >>    date:     29/8-2005
    >>    GMT:      13-52-23
    >>    body:     sun, lower limb
    >>    GPS long: E10� 24.7
    >>    GPS lat:  N63� 25.4
    >>    Sextant heigth:  57� 58.4'
    >>    index error:            0'
    >>    halve height:    28� 59.2'
    >>    correction:          14.4'
    >>    observed height: 29� 13.6'
    >>    GHA hour: 14� 47.3'
    >>    GHA inc:  13�  5.8'
    >>    GHA       27� 53.1'
    >>    LHA:      38� 17.8'
    >>    Decl. hour:   N9� 12.4'
    >>    d-correction:      0.8'
    >>    Decl:         N9� 13.2'
    >>    Calculated height: 29� 20.0'
    >>    Height difference:      6.4'
    >> Asbj�rn

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