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    Re: Index Error - Real vs. Artificial Horizon
    From: Ridge White
    Date: 2013 Feb 28, 16:50 -0500
    You have discovered parallax. Extend an arm and raise your index finger. Look at it and the background first through one eye, then the other. The same thing is happening on a horizontal plane with your sextant because the index and horizon mirrors are slightly apart and "see" nearby objects from a different angle. The only valid reference for determining (or eliminating) index error must be at least several miles away. 

    We received a phone call from a customer whose sextant we had cleaned, lubricated, and adjusted because he thought we had not adjusted his sextant's mirrors correctly. It turned out he had been using an object in his back yard as a reference. He knows better now.

    Ridge White

    Robert E. White Instruments
    PO Box 775
    Medfield, MA 02052

    Tel: 617-482-8460
    Fax: 617-482-8304

    On Thu, Feb 28, 2013 at 4:32 PM, Paul Fithian <pcfithian@gmail.com> wrote:

    While playing around with the sextant, I've noticed a big difference in index error when viewing things far away vs. near.

    For example, if I adjust the horizon mirror so that a near (5' away) horizontal line matches up through the glass and the mirror images, these images don't line up when I view a horizontal line that is several hundred feet away.

    Which one do I choose to apply the right index correction for the artificial horizon readings? I know I have to take the Hs, apply index correction, then take 1/2 of this reading to get Ho.
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