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    Re: Noon sight for longitude
    From: Greg R_
    Date: 2008 Jul 10, 20:58 -0700

    --- "Richard B. Langley"  wrote:
    
    > For this article? 
    
    Link doesn't seem to work - I just get a generic Tiny URL page from it.
    
    --
    GregR
    
    
    
    --- "Richard B. Langley"  wrote:
    
    > 
    > 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
    > >>
    > >>
    > >>
    > >> Around the Emerald
    > >>
    >
    
    Isle
    > >>
    > >>
    > >>
    > >> The one in sixty rule
    > >>
    >
    
    
    > >>
    > >>
    > >>
    > >> Do not rely exclusively on
    > >>
    >
    
    GPS
    > >>
    > >>
    > >>
    > >>
    > >>
    > >> Customer Service
    > >>
    > >> Phone: (207) 772-2466
    > >>
    > >> Email:
    > >>
    >
    subscriptions@oceannavigator.com
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >> March/April
    > >> 2008
    > >>
    > >>
    > >>
    > >> [cid:part3.04010300.08010509@pacbell.net]
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    >
    
    Subscribe
    > >>
    > >>
    > >>
    > >>
    > >>
    > >> [cid:part4.01030109.02090403@pacbell.net]
    > >>
    >
    
    
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    >
    
    Renew
    > >>
    > >>
    > >>
    > >>
    > >>
    >
    
    [cid:part5.02090905.05050004@pacbell.net]
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    >
    
    [cid:part6.04010404.04010302@pacbell.net]
    > >>
    > >>
    > >>
    > >>
    >
    
    Archive
    > >>
    > >>
    > >>
    > >>
    > >>
    >
    
    [cid:part7.04090504.05010208@pacbell.net]
    > >>
    > >>
    > >>
    > >>
    > >>
    > >> Boats & Gear
    > >>
    >
    
    
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >> [cid:part8.06000602.02080908@pacbell.net]
    > >>
    >
    
    
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    >
    
    
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >>
    > >> This message was sent from Ocean Navigator to
    > >> glapook@pacbell.net. It was sent from:
    > >> Navigator Publishing, P.O. Box 569 , Portland, ME 04110. You can
    > >> modify/update your subscription via the link below.
    > >>
    >
    [cid:part9.02030605.00000705@pacbell.net]
    > >>
    > >>
    > >>
    > >>
    >
    
    [cid:part10.00030202.06070300@pacbell.net]
    > >>
    > >>
    > >>
    > >>
    > >>
    > >> To be removed click
    > >>
    >
    here
    > >>
    > >>
    > >>
    >
    ------------------------------------------------------------------------
    > >> It?s a talkathon ? but it?s not just talk. Check out the i?m
    > >> Talkathon.
    > 
    > >> >
    > >
    > >
    > >>
    > >
    > 
    > 
    >
    ===============================================================================
    >  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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