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    Re: Sextant errors
    From: Paul Hirose
    Date: 2024 Jan 25, 16:02 -0800

    We occasionally see celestial observations posted here, including the
    sextant reading and the index correction. While I think it's good
    practice to show all your numbers, that may be going too far since I
    never know if person understands the difference between "error" and
    "correction".
    
    Some years ago on these pages I pointed out to someone that he had the
    terms mixed up. He replied that other people do the same, and seemed
    satisfied with his understanding.
    
    A helpful mnemonic is to remember the goal is to remove error from your
    observations. Remove = subtract. Therefore, error is subtracted. A
    correction is added. For instance, the Almanac altitude correction
    tables have negative numbers for dip and star / planet refraction since
    the corrections make altitude smaller.
    
    I adjust my sextant so the index error check always produces an angle
    "on the arc." Hence, index correction is always negative. It's one less
    opportunity to blunder. Also, if the IE check is sometimes on the arc,
    sometimes off, you have to read the vernier backward in the latter case
    (if you use the vernier — I don't). E.g., at a micrometer setting of
    -0.2′, the 0.8′ vernier graduation is in coincidence. This isn't
    difficult — you read the micrometer drum backward too — but a mental
    lapse is always possible.
    
    --
    Paul Hirose
    sofajpl.com
    
    

       
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