[NavList 5789] Re: Noon sight for longitude
From: gregr_ingest---com
Date: 10 Jul 2008 20:58
--- "Richard B. Langley" <lang---ca> wrote:
> For this article? <http://tiny.cc/QCNVG>
Link doesn't seem to work - I just get a generic Tiny URL page from it.
--
GregR
--- "Richard B. Langley" <lang---ca> 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? <http://tiny.cc/QCNVG>
>
> -- Richard Langley
>
> >gl
> >
> >Post to Ocean Navigator:
> >
> >Subject:
> >Re: Newsletter on Celestial Navigation
> >From:
> >"Gary J. LaPook" <glapook---net>
> >Date:
> >Wed, 02 Apr 2008 22:56:59 -0700
> >
> >To:
> >Ocean Navigator <newsletter---com>
> >
> >
> >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---net
> >> To: NavList---com
> >> Subject: [NavList 5769] Noon sight for longitude
> >>
> >>
> >> Here is another article on this subject from Ocean Navigator.
> >>
> >> gl
> >>
> >>
> >>
> >>
> >>
>
[cid:part1.00020205.04070108---net]<http://click.icptrack.com/icp/relay.php?r=9506120&msgid=108193&act=SZV8&c=179770&admin=0&destination=http://www.oceannavigator.com>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >> 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
> >>
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> >>
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> >>
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> >> 2008
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===============================================================================
> Richard B. Langley E-mail: lang---ca
> Geodetic Research Laboratory Web:
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