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On 3 July, I wrote-

>After correcting for the declination
>change for each hour, any variation in Sun altitude over the day would
>indicate an offset in the observation position from the true pole, and the
>strategy to correct that offset would be to travel in the direction in
>which the altitude is greatest.

I sincerely hope that those words of mine haven't impelled anyone to set
off straightaway for the South Pole, because they were diametrically wrong.
The correct strategy for reaching the Pole is to travel in the direction in
which the Sun's altitude is least, not greatest. Sorry about that.

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

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.

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

I appreciate, and largely agree with, the full response that Robert Eno has
provided (especially his views on the competence of Amundsen, compared with
Scott). His comments have the benefit of his Arctic experience, whereas
mine is no more than book-learning.

To my question-

>> The question remains - how did Amundsen ensure that he was keeping to his
>> planned radial track toward the Pole, and wasn't veering off to one side
>or
>> another? The best moments for checking this by Sun altitude would be when
>> the Sun was due East or West, at around 6am or 6pm local time. Does anyone
>> know of an analysis of how this longitude navigation was done by Amundsen?
>>
Robert Eno responded-
>--------------------------
>It was never clear to me either but if my memory serves me right, Amundsen
>had his men and dog teams spaced out along line so that the man in the rear,
>by shouting out commands, could keep the men ahead of him from veering too
>far to one side or the other; in this way they were able to keep a more or
>less steady, straight track.

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

My reply

That doesn't accord with my reading of Amundsen's book (see pages 211-213
of vol 1). My guess is that this is the passage that Robert recalls.
Amundsen decribes a "forerunner" travelling on foot ahead of the party,
directed by shouts from the driver of the leading sledge behind him, of
left or right according to the direction of his sledge-compass. So the
direction was kept up entirely from those compass readings of the leading
sledge.

What Robert describes is a technique for keeping in a straight line, to
avoid wandering off-course (or, at the worst, in a circle). Fuchs, crossing
the Antarctic in snow-tractors in 1958 describes (in "The crossing of
Antarctica") how he was forced to do the same when part of his journey was
near the Magnetic Pole. At best, all this technique can do is to reduce,
but not eliminate, any curvature of the path, so is unlikely to hold valid
over a whole day's travel, without some means of re-establishing the wanted
direction. It works best when the travellers are well separated, and so
needs good overland visibility.

My guess is that the vibration of Fuchs' tractors upset the operation of
the compasses when the horizontal magnetic force was low, even if they were
well compensated for deviation (which I doubt). I have come across a
compass installation on a boat in which, even in UK waters, the compass
card would rotate anticlockwise, slowly and continually, whenever the
engine was running.

The technique Robert describes does no more than maintain the travel in a
straight line, more or less. It's still up to the navigator to ensure that
the the right straight line was chosen when setting off, using whatever
means are at his disposal at the time.

Amundsen's party were never so close to the magnetic pole that their
magnetic compasses would fail to work, although on rough ground it may
sometimes have been necessary to pause the travel to obtain a good reading.
He would need to obtain a value for local magnetic variation (which was
considerable, and altered rapidly as they travelled) when the Sun appeared.

I agree with Robert that Amundsen's journey could have kept on track by a
series of running fixes: this would require that some of his Sun
observations were made, not near his noon and midnight, but toward morning
or evening, when the Sun had an Easterly or Westerly component, just as
Robert suggests. It's a pity that Amundsen doesn't supply any details about
these aspects of the navigation. An important reason for precise navigation
was the need to locate depots of food and equipment where these had been
left beforehand, and marked by snow-cairns and flags.

George.


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