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    Beginner with inaccurate results
    From: Asbjørn Djupdal
    Date: 2005 Aug 30, 12:24 +0200

    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'
    
    
    
    Asbjørn
    
    
    

       
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