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    Re: Beginner with inaccurate results
    From: Fred Hebard
    Date: 2005 Aug 30, 08:47 -0400

    On Aug 30, 2005, at 6:24 AM, Asbj?rn Djupdal wrote:
    
    > 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.
    >
    >
    
    It's more accurate to determine the index error by first touching the
    reflected lower limb to the upper limb, recording the result, then
    reversing and recording that result.  Index error is half the
    difference.  Usually three measurements on each limb is more accurate
    than one on each; measure one then the other three times.  As a check
    on your interpretation of contact, you also can calculate the sun's
    semidiameter from these measurements; it's one fourth of their sum.  It
    should be no more than 0.1 arcminutes of that recorded in the Nautical
    Almanac.
    
    Always turn the index screw in the same direction when bringing the
    limbs into contact.  Never back off on the index screw to perfect the
    contact
    
    Only adjust the mirrors if the index error is out by more than 1 or 2
    arcminutes.  This shouldn't be necessary very often.  You don't want to
    be constantly adjusting them.
    
    If you use the clear glass on the horizon, you can leave the same
    shades in place.
    
    Accurate timing is important for most shots.  Try to time shots to
    within a second.  It can be very accurate to set the sextant so that
    the contact is fairly close, then wait for it to become perfect and
    record that time.  Whether the two images overlap or are separated when
    you wait for the contact depends on whether the shot is in the morning
    or afternoon and whether you're shooting the upper or lower limb.
    
    I haven't checked your calculations.
    
    Fred
    
    
    

       
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