NavList:

   

Reply

At least one reason for the popularity, or convenience, of establishing
the Latitude at noon, or close to civil noon, is very simply that for
many years it was customary for both commercial and naval ships to
calculate their day's run, as well as other voyage records both deck and
engine, on a noon to noon basis. Establishing Latitude by the noon sight
became the traditional method of ascertaining most accurately one of the
components of the positions used for these calculations - the other being
the AM longitude advanced by DR to noon. The position was desired for
Standard Time, so obviously Latitude obtained at LAN had to be advanced
or retarded as necessary to obtain positions 23 - 24 - or 25 hours apart,
depending on whether clocks were adjusted for time zone between
observations. Naval ships recorded formal positions at 0800 + 1200 + 2000
hours, the 8:00 o'clock positions, AM + PM,  being based on morning and
evening stars and the 1200 position on the LAN Latitude and AM Longitude,
both run up or back to 1200.

Certainly the ship's position can be otherwise established by
simultaneous or running fix at any time celestial bodies can be observed
at appropriate azimuth differences, and when accurate time by
chronometer, or otherwise, is available, and this is normally done by
star sights. There are also frequent opportunities during daylight hours
to cross a Sun line with one calculated by use of the Moon or Venus -
this was often done and the position established advanced to 1200, but
the noon sight was also taken as a check on any error occasioned by
failure to make good course and distance used to advance the earlier fix.

The other traditional reason for observing the meridian transit of any
body for Latitude is to obtain that position component without the
availability of accurate time - obviously if you do not, the
establishment of lines of position, except for that represented by
Latitude, goes out the window.
For arguments sake, it can be said that the LOP represented by Latitude
was the most accurate obtainable if there were to be any question as to
time accuracy - on average, a 4-second time error would equate to a
1-mile LOP error.

The noon position is/was therefore a combination of traditional,
practical, statistical, and commercial factors, of concern primarily to
naval and merchant ship navigators, some of which transcend purely
technical considerations. I really don't think terming it overrated is
entirely appropriate.


Henry

On Sun, 5 Jun 2005 19:32:37 -0400 Robert Eno 
writes:
> An old friend of mine, who passed away a few years ago, and who was
> an
> experienced navigator for over 60 years, once told me that the noon
> sun shot
> was overrated; that all you had to do was to take a few shots with
> an
> interval of 2 - 3 hours then transfer one of the LOPs. Fred and a
> number of
> other list members seem to agree, and so too, do I.
>
> Mind you the noon sun shot is fun to do and very simple to
> calculate. For
> this reason, it should remain as one of the basic mainstays of
> astro-navigation, however, taking and reducing a sun shot at any
> other time
> is painfully easy with calculators and with HO 249 (AP3270 in
> Britain and
> Canada).
>
> George has a very good point about crossing the ocean without GPS
> and with
> someone who only knows how to take a noon sight. This is something I
> harp on
> when discussing GPS with the uninitiated. The argument that some
> students
> will be detracted from learning astro-navigation if things become
> too
> complex has merit, however, my response would be that I would not
> bother
> wasting my time in trying to train someone who is not committed to
> learning
> it properly. No pain, no gain.
>
> I won't delve into the "what ifs" (what if my watch went overboard
> etc.).
>
> Robert
>
> ----- Original Message -----
> From: "Fred Hebard" 
> To: 
> Sent: Sunday, June 05, 2005 11:42 AM
> Subject: Re: Latitude and Longitude by "Noon Sun"
>
>
> > On Jun 5, 2005, at 10:50 AM, George Huxtable wrote:
> >>
> >>
> >> I ask nav-L members if they would be happy to cross on ocean,
> without
> >> GPS,
> >> with a "navigator" who had learned his craft in that way, and was
> >> unable to
> >> handle any other observations than those of the Sun at noon.
> That's
> >> what I
> >> fear will happen, if such individuals can claim some
> qualification in
> >> "celestial navigation".
> >>
> >
> > In response, I don't find the intercept method any more difficult
> to
> > understand geometrically than a noon shot.  The basic flag pole
> > analogy, much derided here, seems adequate to me, only requiring
> that a
> > spherical surface and infinitely tall pole be substituted to
> achieve
> > reality.  While building the infinitely tall pole, an instructor
> would
> > get to talk about parallax.
> >
> > Using pen and paper methods of sight reduction, LOP navigation is
> more
> > difficult, but not with a calculator or computer.  Also, using the
> Air
> > Almanac, LOP sight reduction on paper is much easier than the
> Nautical
> > Almanac/HO-229 or predecessors.  The Air Almanac seeks 1.0'
> precision
> > while the Nautical Almanac seeks 0.1' precision.
> >
> > George's comments about taking a prolonged series of sights while
> > underway are well taken.  Not only would it take a long time to
> take
> > the shots, but a clear sky is needed.  Why not take a quick
> morning,
> > noon and evening shot?
> >
> > One point about a traditional noon shot is that the sun often
> peaks out
> > from behind the clouds at noon for a few moments even on days with
> > heavy overcast.  There is a story famous to me about a passenger
> on a
> > ship asking why all the officers had their sextants out under a
> cloudy
> > sky at noon, only to be informed by the captain that the sun often
> > would peak out for a bit, and sure enough it did.  That's also
> been my
> > experience at 36N in the mountains.
>

   

Reply