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    Re: Latitude by Noon Sun for Beginners
    From: Jean-Philippe Planas
    Date: 2009 Apr 29, 14:20 -0700
    Note that on this day, at 12h00, the sun GHA is 356° 56.9', not 360°. Therefore that day LAN occurs at Greenwich at 12h 12m 12s.

    --- On Wed, 4/29/09, JKP{at}obec.com <JKP{at}obec.com> wrote:
    From: JKP{at}obec.com <JKP{at}obec.com>
    Subject: [NavList 8103] Re: Latitude by Noon Sun for Beginners
    To: NavList@fer3.com
    Date: Wednesday, April 29, 2009, 3:19 PM

    Thanks very much for your in-depth explanation. I find that it was the
    declination giving me trouble, and I finally realized I was subtractng 30'
    for the "d" correction, not 0.5' as I should have. (Half a degeree
    instead of half a minute!) So for the time of the sight I (eventually) got a
    declination of 18d 59.8'S and the correct latitude of 55d 37.8'S. Whew!

    As an aside, you asked why I thought there was a 12-minute time span between
    the time of the shot and LAN for the DR longitude. I converted his DR longitude
    of 68d 25' to time, and got 4h + 32m + 100s, or 16:33:40. I figured that
    meant that the sun would be over his meridian at that time. The shot was made
    at 16:46:20, which is 12m and 40s later. Did I go wrong on that reasoning? Did
    the navigator make his shot when he did, rather than when I would have advised
    him to had he asked me, simply because he recognized that his DR was not likely
    to be close enough to make the difference significant?

    Or, as another reader suggested, was this perhaps meant to be an
    "ex-meridian" sight? In that case, as I eventually found in Jeff
    Toghill's "Celestial Navigation," the observed altitude should be
    corrected for the time difference by factors found in Bowditch. (The tables
    referred to were 29 and 30, though in the 2002 Bowditch they are now 24 and 25.)
    I plan to rework the problem from that perspective and see what I get.

    Thanks again for your patience and thoroughness. This list is like having
    dozens of expert tutors on call!


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