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    Re: ex-meridian sights
    From: Nigel Gardner
    Date: 2001 Feb 14, 3:44 PM

    It should perhaps be remembered that when ex-meridians came into use from
    the mid-nineteenth century on, very little graphical chartwork was used.
    Latitude by mer-alt (or ex-meridian) and Longitude by long-chron were
    worked up by traverse tables to give the noon position.
    Regarding sights when the sun is near the zenith, zenith fixes are a handy
    way of getting a fix. Admittedly I have only done a couple, one in the
    Pacific near the Line islands and the other in the Indian Ocean (both from
    aircratf). The trick is to get three sights with the azimuths about 60deg
    apart, plot the sub solar point for each sight (GHA and dec) and using the
    corrected ZD of each as radii plot three arcs of position; transfer the
    p/l's for course and distance run and Bob's your uncle. The real problem is
    the glare and making sure the sextant is vertical.

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