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    Re: Noon sight for longitude
    From: Richard B. Langley
    Date: 2008 Jul 10, 23:45 -0300

    On Thu, 10 Jul 2008, Gary J. LaPook wrote:
    
    >Gary LaPook writes:
    >
    >I pointed that out when I read the article (see attached post to Ocean
    >Navigator. ) BTW, has anyone seen the current issue of Ocean Navigtor?
    
    For this article? 
    
    -- Richard Langley
    
    >gl
    >
    >Post to Ocean Navigator:
    >
    >Subject:
    >Re: Newsletter on Celestial Navigation
    >From:
    >"Gary J. LaPook" 
    >Date:
    >Wed, 02 Apr 2008 22:56:59 -0700
    >
    >To:
    >Ocean Navigator 
    >
    >
    >That method has been known for a long time and the longer time before
    >and after LAN the more accurate the derived longitude. But what you
    >really have is a running fix and you have to adjust for movement of the
    >vessel between the two shots. If you are moving at all north or south
    >then the time of LAN is NOT the mid points between the shots. You must
    >also adjust for the movememt east and west. These are especially
    >critical when doing celestial from a fast moving plane such as a B-52.
    >
    >gl --
    >
    >
    >
    >Mike Burkes wrote:
    >
    >>
    >> Hi Gary, thanks for that and oddly enough, unless I missed something,
    >> this method is not mentioned in Hew Schlereth's "Latitude and
    >> Longitude by the noon Sight" and other books! One otherwise excellent
    >> book in particular "Celestial Navigation" by Tom Bottomley does give
    >> the classic AM and PM curves with Hs and time but NOT equal altitudes.
    >> He relies on the peak of the curve. On the equal alt method several
    >> pairs are taken. I do not remember which book this was in possibly Hew
    >> Schlereth's "Cel Nav in a Nutshell"?
    >> Mike Burkes
    >>
    >>
    >> ________________________________
    >>
    >> Date: Wed, 9 Jul 2008 23:13:45 -0700
    >> From: glapook@pacbell.net
    >> To: NavList@fer3.com
    >> Subject: [NavList 5769] Noon sight for longitude
    >>
    >>
    >> Here is another article on this subject from Ocean Navigator.
    >>
    >> gl
    >>
    >>
    >>
    >>
    >> 
    [cid:part1.00020205.04070108@pacbell.net]
    >>
    >>
    >>
    >>
    >>
    >>
    >>
    >>
    >>
    >>
    >>
    >> April 2008
    >>
    >>
    >>
    >> Celestial Navigation
    >> Another method of obtaining longitude from a noon sight
    >>
    >>
    >>
    >> In the last Navigation newsletter, I wrote an article explaining how
    >> to obtain longitude from a noon sight using the GHA method. Basically
    >> this method is based on the fact that at the time of meridian passage
    >> of the sun, Greenwich Hour Angle (GHA) is equal to the DR longitude of
    >> the observer. The resulting Local Hour Angle (LHA) is thus equal to
    >> zero degrees ? which is a definition of meridian passage. This method,
    >> although reliable, has its drawbacks. This is because the hanging time
    >> of the sun, when it reaches the meridian, is difficult to determine.
    >> It follows that the exact time of meridian passage is questionable.
    >> Fortunately, there is another method of finding longitude from a noon
    >> sight that eliminates this problem.
    >>
    >> Although I scoured Mixter?s, Dutton?s, Leaky and Blewett for
    >> information concerning this technique, it was only in Bowditch that I
    >> found any mention of the procedure. I learned this application from my
    >> friend and navigational mentor, Eben Whitcomb, years ago while
    >> shipping aboard the schooner Harvey Gamage.
    >>
    >> Instead of relying on the GHA we instead take at least two timed shots
    >> of the sun, while it is ascending, and when it is descending. I?m
    >> certain that there are many variations on the theme, but I will
    >> explain the procedure that I use.
    >>
    >> Roughly 15 minutes (it doesn?t need to be exactly 15 minutes) before
    >> the calculated time of Local Apparent Noon (LAN) I take either a lower
    >> or upper limb shot of the sun and mark the time. I then record the
    >> sextant altitude and the time.
    >>
    >> I record LAN as usual, so I can obtain my latitude. Then after the
    >> time of LAN, I pick up the sextant ? which I then set to the angle of
    >> the shot I took 15 minutes before LAN ? and when the sextant altitude
    >> of the sun is the same going down as it was when it was rising, I mark
    >> and record the time. I then put the sextant away and prepare to
    >> calculate the exact time of LAN.
    >>
    >> The procedure for finding the exact time of LAN is simple: just add
    >> the two times of the two shots and divide the result by two. This will
    >> yield the time of LAN that you can use to enter into the almanac to
    >> find the necessary information to get the longitude. Let?s do an example:
    >>
    >> The day is April 15th. We are at a DR position of 35� 25? N and 60�
    >> 18? W. We want to calculate the longitude from the meridian passage of
    >> the sun. We first see on the daily pages that the time of LAN is 12
    >> hours and no minutes. This would be for the standard meridian of a
    >> time zone (0�, 15�, 30�, etc.) We think we are at 60� 18? so we have
    >> to see how long it takes the sun to move 18?. Entering the Arc to Time
    >> Conversion table we find that it takes 1 min. 12 sec., so we can
    >> estimate that the time of LAN for our DR will be at 12 hours 1 min, 12
    >> sec. I always convert the local time to GMT so we add 4 hours to the
    >> time of LAN, making it 16:01:12. At 15:45:08 GMT we take a sextant
    >> sight and record the altitude at X�. After we find LAN altitude of the
    >> sun, we reset the sextant to X� and at 16:29:10, the sun is once again
    >> at that altitude.
    >>
    >>
    >> We next take the times and add them and then divide by two:
    >>
    >> 15:45:08
    >> +16:29:10 =
    >> 32:14:18 / 2 =
    >> 16:07:09. This is the time of LAN.
    >>
    >> We next go to the daily pages of the Nautical Almanac for 16 hours on
    >> April 15:
    >>
    >> 16 hours = 60� 01.3? GHA
    >>
    >> +07 min 09 sec +01� 47.3? =
    >>
    >> 61� 48.6?
    >>
    >> Longitude at time of sight = 61� 48.6?. Remember the GHA is equal to
    >> the longitude of the observer at the time of LAN so we can convert GHA
    >> into longitude. It is also interesting to notice that the longitude
    >> puts us further to the west of our DR, and we should adjust our plot
    >> accordingly.
    >>
    >> We will notice that if this method is used, as it is, we have not
    >> discussed latitude. Latitude from a noon sight is easy to obtain, but
    >> the point of this discussion is to see how we can establish longitude
    >> from the noon sight. I welcome your comments.
    >>
    >>
    >> About the Writer
    >>
    >> [cid:part2.00080305.05090602@pacbell.net]Contributing Editor David
    >> Berson writes the Nav Problem page in every issue of Ocean Navigator.
    >> He is also the owner and operator of
    >> 
    Glory,
    >> an electrically powered excursion boat, in Greenport, N.Y.
    >>
    >>
    >>
    >> Question for David?
    >> editors@oceannavigator.com
    >>
    >>
    >>
    >>
    >>
    >>
    >> More Ocean Navigator Articles
    >>
    >>
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    >> Around the Emerald
    >> 
    Isle
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    >> The one in sixty rule
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    ===============================================================================
     Richard B. Langley                            E-mail: lang@unb.ca
     Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
     Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
     University of New Brunswick                   Fax:      +1 506 453-4943
     Fredericton, N.B., Canada  E3B 5A3
         Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
    ===============================================================================
    
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