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Subject: Re: On polar nav
From: Robert Eno (enoid@XXX.XXX)
Date: Sun Sep 22 2002 - 18:17:33 EDT
Always a pleasure for me to discuss polar navigation.
First off, I should point out that I am by no means an expert on polar
navigation. I am certain that there are a lot of folks out there who vastly
outrank me in that regard. My knowledge of such matters is based, purely
and simply, on 20 years' experience and hundreds of sights taken under
Arctic conditions. I would never consider myself to be the final word on
this topic. Secondly, a minor point, I don't live above the Arctic circle -
close but a few degrees below that!
Now to the theodolite vs. artificial horizons.
I am quite familiar with both the mercury and glass horizons as I have used
both and under all kinds of conditions. Yes, indeed, the glass horizon must
be precisely levelled but this is not as difficult as it sounds. My
Freiberger artificial horizon is provided with two tubular spirit levels
that are sensitive to thirty seconds (30") of arc. They are placed on the
surface of the artificial horizon at right angles to one another, in a
capital "T" shape, after which one adjusts the three levelling footscrews on
the artificial horizon, until the bubbles in both spirit levels line up.
It takes me less than one minute to precisely level my artificial horizon in
this manner. A bit longer if it is particularly windy. Frozen fingers? You
betcha! But it is not insurmountable and believe it or not, one does get
used to it (like cold showers and paying taxes).
Using this system, I have been able to fix my position, consistently, to
within less than one half nautical mile of my actual position and often, to
within 2/10ths or less. Not too bad for a coarse instrument like a sextant.
Lest you think that I am boasting of my skills, I freely admit that I am not
what you would call a gifted navigator. I am as average as they come and
certainly not as good as my mentors or anyone else that I have sailed with.
I like to think that my enthusiasm for the art makes up for my lack of
prowess. Now think about what kind of results a highly skilled practitioner
can achieve.
One drawback to the artificial horizon is the degree of difficulty in
measuring low altitude bodies. Difficult but not insurmountable. One has to
be a bit of a contortionist and/or raise the level of the artificial horizon
to about chest level. I have measured the sun when it was as low as 3
degrees off of the horizon and got pretty decent results: about 1/2 nautical
mile from actual.
----------------------------------
A theodolite is levelled in a similar manner only I personally find it more
cumbersome to achieve and maintain this.
Because the theodolite must be mounted on a tripod, it presents a higher
profile to the wind. Contrast this to the low profile of an artificial
horizon. As it is, a theodolite is a very sensitive instrument; so much so
that vibrations from someone walking nearby will throw it off balance. Throw
in a good wind (in the polar regions, is it almost always windy) and you
start to experience problems. Again, contrast this to an artificial horizon,
which can be sheltered from the wind with a minimum of effort (like placing
a few snowblocks in front of it.).
Finally, a sextant, because it is -- mechanically -- a very simple device,
lends itself very well to field repairs and troubleshooting A theodolite
because of its relative mechanical and optical complexity, does not lend
itself to field repairs or to troubleshooting in the event that something
goes wrong. And...you have to lug that tripod around in order to operate it.
Keep in mind that in making the above statements, I am only arguing the case
for using a sextant over a theodolite for purposes of turn-of-the-century
polar navigation. There is no denying that a theodolite is far more precise
than a sextant and therefore will provide more accurate positioning
information.
Essentially, if you are conducting a geodetic survey, I would go for the
theodolite, however, if you had to traipse across a vast polar landscape in
search of the south pole and had to carry every single item that you
required to get there and back, alive, I would argue for a sextant and
artificial horizon as my means of position fixing. I strongly suspect that
this is what Amundsen concluded too.
So there you have it. My case for Amundsen's choice of sextant and
artificial horizon.
cheers,
Robert Eno
----------------------------
George Huxtable wrote: (clipped)
> Robert Eno, who tells me he lives within the Artic Circle (so he should
> know), answered-
>
> >Because he was measuring the angle between the Sun and its reflection,
> >Amundsen would record a value of twice the Sun's altitude. Scott's
> >navigator (Bowers) used a theodolite, which seems to me a more
appropriate
> >tool for the job than the sextant / artificial-horizon combination.
Again,
> >levelling needs to be very precise. In both cases, presumably the
observer
> >would have to take a hand out of his mitts for delicate adjustment of the
> >metal knobs: not much fun at polar temperatures! One wonders how the
> >instruments were kept free-to-move and the lenses clear of frost in such
> >extreme conditions.
--------------------
> >I have yesterday discovered a remarkable book by Peter Ifland (occasional
> >contributor to >this list) entitled "Taking the Stars", about the
> >instruments used in navigation. >Superbly illustrated and very
> >knowledgeable, it has the best information I have found >anywhere about
> >artificial horizons. Now I must search for my own copy.
>
> ===================
> As a postscript to the above, the Ifland book has a good picture of such a
> Norwegian artificial horizon, which consists of a disk of black glass
> (black so reflections come only from its upper surface) mounted on a frame
> which can be levelled using adjustable screw feet and tested with one or
> more sensitive spirit levels.
>
> If Amundsen's measurements were to determine his distance from the pole to
> within a mile or so, then this levelling had to be similarly precise, to a
> minute of arc. This is asking a lot of a spirit level, which would need to
> be as sensitive as that found in a theodolite. The levelling process would
> have to be very painstaking. However, having been levelled once, the
> assembly could then be left in place if its snow-pillar base was stable
> enough, and the observer could then follow the Sun around the horizon over
> a 24-hour period, measuring the angle between the Sun and its reflection.
>
> Robert Eno knows about such matters in cold conditions, and expresses a
> preference for a sextant over a theodolite as being lighter to carry and
> simpler to adjust in such a climate. But I wonder if he still holds the
> same view when taking account of the fact that such an artificial horizon,
> with its spirit level(s), has to be carried as well as the sextant, and
> then has to be so carefully adjusted before the Sun observations can even
> start.
>
> George Huxtable.
>
>
> ------------------------------
>
> george@XXX.XXX
> George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
> Tel. 01865 820222 or (int.) +44 1865 820222.
> ------------------------------
>
>
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