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    Re: Bowditch tables and sexant parallax
    From: Frank Reed CT
    Date: 2005 Apr 27, 02:44 EDT

    Bill you wrote:
    "My Astra IIIB has an aprrox. vertical distance of 2.25" between the  center
    of the horizon mirror and the index mirror.  Using a plane right  triangle,
    and knowing the distance to the object, I should be able to  calculate a
    useable correction for parallax.  For 100 yards approx. 0d  2' 9", for 0.1 nm
    approx. 0d 1' 4", for .5 nm approx. 13".
    Distance  must be known to calculate the above, and distance is what we want
    to solve  for using the tables. Is it reasonable to hold the sextant
    horizontally,  align the sides of the object between the horizon glass and
    mirror, and use  that angle (off the arc so, add to sextant measurment)
    plus/minus IC to  correct to the actual angle?"
    If I've understood you right,  yes.
    I've been thinking along similar lines for a "laboratory" test of  arc
    errors. The parallax within the instrument (because of that 2.25" distance  between
    the line of sight through the horizon glass and the index mirror that  you're
    describing) is something that we can calculate exactly and correct for.  For
    example, I could do an index correction by looking at a an index card across
    the room. If the top and bottom of the card are aligned (assuming that's the
    same as the distance between the line of sight through the horizon glass and
    the  index mirror), then my sextant should reading should be identical to the
    I.C.  based on objects observed at great distance (the standard method for
    checking  index correction). For other angles, I am guessing that the parallax goes
    as  d*cos(h) where h is the angle read off the sextant and d is that distance
    (2.25"  in your case).
    42.0N 87.7W, or 41.4N  72.1W.

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