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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
>>
>>
>>
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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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