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    Re: Longitude by Sunrise
    From: John Huth
    Date: 2010 Feb 4, 08:25 -0500
    This is a favorite of mine when I vacation in Jamaica.   I check my watch regularly with NIST time, and then try to do an eyeball estimate of latitude every evening by sunset.   There are sometimes some rather profound thermal effects that give green flashes etc that can make sunset time vary.   I try to use the moment the lower limb touches the horizon as my time for sunset, which roughly accounts for average refraction.

    John H. 

    On Wed, Feb 3, 2010 at 8:50 PM, John Karl <jhkarl@att.net> wrote:

    Jeremy,
    Your sunrise longitude results don't seem out of bounds to me. Both these sextantless "longitude-by-sunrise" (or sunset) and my "zero-altitude" shot can easily suffer from the same possible anomalous refraction conditions. The difference is the zero-altitude sight is a true LOP, independent of assumed latitude. BTW, did you correct for temp & pressure? They can easily account for your results.

    Moreover, the Almanac's dip tables don't account for anomalous refraction effects which can throw the horizon off by 3' or more.

    So yes, these are somewhat emergency sights, but also possibly entertaining at sunset happy hour -- well, for certain types of sailors.

    You and Berson are lucky to live near Greenport, it looks wonderful compared to Wisconsin. Outside my window I see pickup trucks driving on my sailing grounds.

    My "Sextantless CN" article should come out in this summer's Ocean Voyager.


    JK
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