NavList:

   

Reply

What a privilege  it is to have on this list expertise such as Robert Eno's
to call on.

Thanks, Robert, for a well-argued (and completely convincing) comparison
between the relative merits of a theodolite and a sextant with artificial
horizon, for a polar explorer.

Further questions arise.

Is it part of the levelling procedure for a glass artificial horizon to
reverse the spirit levels, end-for-end, on the glass plate, in case the
levels themselves have become somehow deranged in the knockabout of arctic
travel?

Presumably, whatever the glass horizon is mounted on must be VERY rigid and
firm, to ensure that when the (slight) weight of the spirit levels is added
and then removed, its horizontality is affected by no more than a fraction
of a minute. Is that rigity easy to achieve? Any such difficulty might be
overcome, perhaps, by leaving the levels in place while an altitude is
taken, if room can be found between the levels on the glass surface for a
Sun reflection to be seen, but I have doubts whether this would be
possible.

George Huxtable.

==========================

Robert Eno wrote (snipped)-

>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.


------------------------------

george@huxtable.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
------------------------------

   

Reply