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    Re: Lat/Lon by "Noon Sun" & The Noon Fix PROVE IT
    From: Frank Reed
    Date: 2009 May 13, 06:51 -0700

    Gary, you wrote:
    "But if you can determine the time of LAN within one minute then the 
    uncertainty in longitude is only plus and minus 15 minutes of longitude, plus 
    or minus 15 NM at the most."
    But can you really determine the time of LAN within a minute?? Sometimes, yes. 
    But under general circumstances, not even close. And also, we don't observe 
    the time of LAN. We observe the time of maximum altitude which is offset from 
    LAN. At the equinoxes, the motion of the vessel contributes an offset in the 
    longitude of 4*v*sin(Lat) n.m. where v is the speed of the vessel in the 
    North/South direction. So if v is 10 knots, Lat=45 deg, that gives 28 
    nautical miles offset EVEN IF the time of maximum altitude can be estimated 
    with no error at all.
    And you concluded:
    "Soooo, why go to the extra complication of the method discussed here, 
    just mark down the time of LAN, place an error band +/- 15 NM on the 
    longitude, and plan your landfall based on the most critical longitude 
    determined? One of the arguments made for this method is that it is easy 
    to relearn, but it is not nearly as easy to relearn as just taking the 
    time of LAN."
    Well, the problem here is that your 15 n.m. is around the lower limit for the 
    approach you're describing. You could EASILY be wrong by 75 n.m. when you 
    factor in the difficulty of guessing at the peak time of the altitude and the 
    offset due to the vessel's motion. The steps required to fix this up and 
    bring the error under 5 n.m. (on average, frequently much better) are not 
    much of an "extra complication" and they provide an easy method for getting a 
    real fix that navigators can learn at an early stage. Have you tried it, 
    Navigation List archive: www.fer3.com/arc
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