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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Longitude at noon
From: Paul Bedel
Date: 2020 Sep 10, 08:09 -0700
From: Paul Bedel
Date: 2020 Sep 10, 08:09 -0700
Hi Philippe,
The first calculation would be correct if the horary variation in Greenwich hour angle of the sun was exactly 15°. The 7th of september 2020 the horary variation is about 15,00354° so I find a longitude of 111° 28,1' W. The second method is more exact because the almanac (nautical, éphémérides nautiques etc) gives the true value of GHA at the time of the event.
This second method is described in "Traité de Navigation" by MM. Caillou, Laurent, Percier; Masson éditeur 1989, pages 329 et suivantes (point par hauteurs circumzénithales correspondantes). See also the instructions of "Nouvelles tables destinées à abréger les calculs nautiques" by Amiral Perrin (last edition in the sixties).
Regards.
Paul.
