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    Re: Beginner with inaccurate results
    From: Geoffrey Kolbe
    Date: 2005 Aug 30, 17:43 +0100

    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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