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    Re: On polar nav
    From: George Huxtable
    Date: 2002 Sep 22, 21:51 +0100

    Away in France for a few days, and off-watch from Nav-L, I was pleased to
    find on return an interesting thread "On polar nav"
    Peter Fogg said, on 12 Sept.-
    > Another little mystery is why they used sextants rather than theodolites,
    >which would >seem to be more useful for the purpose.
    Robert Eno, who tells me he lives within the Artic Circle (so he should
    know), answered-
    >I can answer the last question:
    >Theodolites are heavy instruments to lug around on a trip to the pole. But
    >most importantly, ever try to use one at -30 C?  It ain't fun, believe me. A
    >sextant is much easier and simpler, to work with under extreme conditions
    >and compared to a theodolite, a little more robust.
    >I think, but am not entirely certain, that Scott took a theodolite with him.
    >I could be dead wrong though.
    What follows is a quote from my posting to the list on 3 July 02, in a
    thread entitled "Reaching the pole".
    >Amundsen carried sextants and "artificial horizons", for measuring Sun
    >altitudes. These comprised one "Mercury horizon" and two "glass horizons".
    >What temperature does Mercury freeze at? I wonder what, exactly, these
    >glass horizons were. Amundsen refers, when near the Pole, to constructing a
    >snow pillar on which to place the artificial horizon, so it can't have been
    >an attachment to a sextant. Scott records that when he reached Amundsen's
    >tent, which had been left at the pole, it contained an abandoned sextant
    >and a "Norwegian artificial horizon". My guess is that it was a plane glass
    >reflector with means for precise adjustment of its levelling using very
    >sensitive spirit levels. An error of a minute of arc in the levelling would
    >give rise to a corresponding error of 1 mile in the position of the Pole.
    >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.
    Next day (4 July) the expertise of listmembers enabled me to add the
    following add the following-
    >Thanks to those who pointed out that Mercury freezes near -40 degrees
    >Celsius (or -40 >degrees Fahrenheit). As Trevor Kenchington emphasises,
    >this would often render a Mercury >artificial horizon completely useless
    >for polar explorers. Amundsen took with him to >Antarctica one Mercury
    >horizon and two Norwegian glass ones, which presumably were used >with
    >sensitive spirit-levels with a strong alcohol filling to avoid freezing.
    >The >evidence seems to be that the glass horizons, not the Mercury ones,
    >were taken on the >sledge journey to the Pole, and one of them was left
    >there to be found by Scott.
    >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
    George Huxtable.
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.

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