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From: George Huxtable (no email)
Date: Sun Jun 05 2005 - 10:50:51 EDT
Frank's posting explains, as well as I've seen it explained, how to get the
best, in terms of lat and long, out of an observation of the Sun in the
period around noon. I have some quibbles about the way he adjust for ship's
North / South velocity, of which he makes rather heavy going, but will
defer those to a separate posting.
>First things first: I've put the phrase "Noon Sun" in quotes here because
>the set of sights required for this system goes a little beyond the standard
>procedure for shooting the Noon Sun for latitude only.
It certainly does. It extends the Noon sight to a protracted series of 9 to
13 observations extending over 40 to 60 minutes around noon. Can this still
be described as "a noon-sight"? In fact, the longer this period covers, the
more accurate the result will be.
It accords with my comment, in a posting "long and time at sea", in which I
wrote-
"I don't deny that a noon Sun longitude can be made to work, to some
extent. One distinct improvement would be to extend the period of
observation, to be significantly longer than the 10 minutes around noon
that Lu allowed himself. But if the time interval between the rising Sun
and the falling Sun becomes great enough, the question then arises: can you
call it a "noon observation" any more?"
>This short method of celestial navigation will get you latitude and
>longitude to about +/-2 miles and +/-5 miles respectively --more than
>adequate for
>any conceivable modern practical purpose.
I accept as reasonable those estimates of precision, that result from
observations made in the way Frank suggests, over the protracted time
period he proposes. But better is achievable using the standard navigator's
"intercept method", which provides roughly equal precision in lat and long
(presuming GMT is well known). And many members of this list seek the
maximum precision they can obtain. Why else all that recent interest in the
time-precision obtainable from a quartz watch? I accept that Frank's method
will do perfectly well for mid-ocean navigation; when making a landfall,
however, who would be content with ±5 miles in longitude when ±2 miles is
available?
Frank's proposal, though the bast way to do the job of finding longitude
around noon, is quite a protracted operation. On how many lightly crewed
small craft can a crew member be spared for a period of 40 to 60 minutes
around noon each day while he takes such a series of shots at 5-minute
intervals?
>You can cross oceans safely and
>reliably for years on end using this technique if it suits you to do so. Its
>enormous advantage is simplicity. It's easy to teach, easy to
>demonstrate, easy to
>learn, and also easy to re-learn if necessary. I mention this because most
>people who are learning celestial navigation today will quickly forget it.
>What's the point of learning something if you can't reconstruct your knowledge
>of it quickly when and if the need actually arises to use it? It's tough to
>resurrect an understanding of the tools of standard celestial navigation on
>short notice, but easy with this lat/lon at noon method. Additionally, this
>method does not require learning all the details of using a Nautical
>Almanac (you
>don't need one at all --only a short table of declination and equation of
>time, possibly graphed as an "analemma") and it needs no cumbersome sight
>reduction tables.
There may be some interesting implications in what Frank is saying here.
With this method, he is teaching a way of finding lat and long that is only
available if the Sun happens to shine around the moment of noon. That's the
position navigators were in prior to Sumner, when if they lost their noon
(or near-noon) observation, both lat and long were unavailable until the
next sunny noon.
So I ask; is he also teaching intercept methods to those same students?
Perhaps (though I hope not), at Frank's institution, the revolution in
navigation, which occurred with Sumner and Saint-Hilaire, has already been
discarded.
Does he teach how a position line can be obtained from any altitude of any
celestial body, at any time, all in exactly the same way, from an assumed
position? And the nuts-and bolts of how that has to be done, and the plot
that gives the resulting position? If all that has been lost (and I hope it
hasn't), it would be a very crippled 19th-century navigator that would
emerge from the course.
I can see that if students have never learned (and are never to learn) the
intercept method, then teaching them how to estimate longitude by the
inferior method of around-noon observation might provide a (poor)
substitute. However, if they are really going to learn, and understand,
intercept techniques, then why divert their minds away in another
direction?
Frank concludes-
>The
>days are gone when celestial navigation was essential and fixed curricula
>could be dictated for students to either take in their entirety or leave. This
>field has moved on to the stage of "a la carte" learning. It can be a pain in
>the neck for instructors accustomed to doing things the same way year after
>year but it's a real liberation for students and possibly also for more
>creative teachers and "information publishers".
Is this another hint, perhaps, that the intercept method is on its way out
in navigation teaching, or perhaps has already gone?
===================
In another posting, Frank adds, about the intercept method-
>That's a ten-week course of work and frequent repetition required to keep up
>the skill. By contrast lat/lon by noon sun is something that can be learned
>and re-learned in an afternoon. It's not quite as accurate (does that matter?
>depends on what you're trying to achieve) as full-blown celestial navigation,
> but fewer and fewer students are interested in toiling over the details of
>the Nautical Almanac's interpolation tables and the tedious study of H.O. 229
>or other sight reduction tables. They wanna play with their sextants and
>figure out where they are in the fewest possible steps (just in case something
>bad happens to GPS). Of course, there are still plenty of others who want to
>learn the complete methods of "apex celestial navigation" because that suits
>their personal goals and interests. We're even getting to the point where
>people are studying it out of historical interest...
Now I am really worried. Rightly or wrongly, I interpret Frank's comments
as implying that in today's teaching institutions, the intercept method is
no longer considered a necessary part of celestial navigation. And if
that's really the case, perhaps students have to be supplied with some
other proxy method, easy to understand, to allow them to claim that they
know how to find (under certain restricted circumstances) latitude and
longitude, perhaps for the purpose of meeting some syllabus requirement.
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".
If I'm right, then I understand Frank's advocacy of the "lat/long around
noon" method, in allowing students to claim some celestial competence,
while keeping from them the navigational changes that occurred 150 years
ago, and which have dominated celestial navigation ever since. Understand
it, but deplore it.
George.
================================================================
contact George Huxtable by email at , by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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