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    Re: Using the sun's direct and reflected image to determine a sextant's IE
    From: Paul Hirose
    Date: 2020 Mar 9, 23:03 -0700

    On 2020-03-09 13:16, Tony Oz wrote:
    > To obtain the value you should ALWAYS ADD (*algebraically!*) to the H_S do:
    >    -- measure the Sun's width by touching the direct and reflected images 
    right-to-left (or upper-to-lower if you want) and then the left-to-right
    >    -- imagine that the zero on the arc is marked as 360° and therefore if 
    the index is to the left of it - read it as 360°mm', if the index is to the 
    right - read it as 359°nn'
    >    -- calculate 360° - (360°mm' + 359°nn') / 2
    
    The calculation is simplified if you imagine the -1° mark on the limb is
    0°. For example, if the sextant reads 1′ high and semidiameter = 15′,
    the limbs touch at 1°31′ and 0°31′.
    
    Index correction = 1° - (1°31′ + 0°31′) / 2
    = 1° - 2°02′ / 2
    = 1° - 1°01′
    = -1′
    
    If it seems more natural, you can reverse the operands in the
    subtraction. Then the result is index error, which is *subtracted* to
    get the corrected angle. In fact that's the method I prefer.
    
    I also adjust my sextant so an index error determination always produces
    a small positive result. I believe there's more chance of a blunder if
    IE is sometimes positive, sometimes negative.
    
    There's much confusion about the sense of IC vs. IE. A few years ago I
    tried to correct someone on this. He replied that many other people do
    the same thing, and seemed content with his understanding of the matter.
    

       
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