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    Index error nomenclature
    From: Jeremy C
    Date: 2013 Mar 4, 07:01 -0800

    There has been some question as to what I mean when I post Index error "on the arc" and "off the arc." This is standard for those of us in the US Merchant Marine, and perhaps the other US schools of celnav, but might be a bit alien to other schools. This nomenclature is straight out of Bowditch 1984 Vol 1 page 427.

    When I determine index error, usually by super-imposing either the sun or other body, I find the error by reading the micrometer drum. I usually do this many times in rapid succession and take an average reading.

    When I read 0 deg 00.5' on my micrometer drum this is said to be 0.5' on the arc for index error. The index correction would be -0.5' in my reduction form as the measurements will always be too high.

    When I read -00 deg 59.5' on my micrometer drum, I would say that it is 0.5' off the arc. I would then add 0.5' onto my Hs to correct a sextant reading that is too low.

    I hope that this clears up any confusion.


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