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Subject: Re: Reaching the pole. (was Nautical Almanac)
From: George Huxtable (george@XXX.XXX)
Date: Tue Jul 02 2002 - 19:34:35 EDT
On 24 June 02, Robert Eno wrote, under the thread-name "Nautical Almanac"-
>On a related issue, which may be of interest to the list, it is a little
>known fact that when Roald Amundsen embarked on his historic journey to
>the South Pole in 1911-1912, he attended to every little detail (in
>characteristic Amundsen fashion), except for one thing: he forgot to bring
>along the 1912 edition of the Nautical Almanac (ref. Roland Huntford:
>Amundsen and Scott). Because of this, he was compelled to reach the South
>Pole (and fix his position there with absolute certainty) before the close
>of 1911. As it turns out, he did, on December 17. Talk about cutting it
>close eh?
Then Rodney Myrvaagnes <rodneym@XXX.XXX> responded-
>It does come with a formula for using one year for the next, at least
>nowadays. He probably knew the correction formula even if it wasn't in
>the almanac.
===========================
Some comments about polar navigastion follow-
I don't know the Huntford work quoted by Robert Eno. I can find no
reference in Amundsen's own 2-volume account, "The South Pole", to his
failure to bring the 1912 almanac along, but then if he really did neglect
to bring it, pehaps he wouldn't advertise the matter.
However, like Rodney Myrvaagnes, I question the relevance of the 1912
almanac to the Polar expedition.
The Sun is the only celestial body that's relevant to a Polar explorer. The
year 1911 not being a leap-year, there is only a quarter of a day's worth
of change in the Sun's declination, and in the equation of time, between
the same dates (Jan 1st, for example) in the 1911 almanac and the 1912 one.
The available window for reaching the Pole was only a month or so (Scott
reached it on Jan 18 1912, already late in the season, which is one reason
why his party failed to complete the return journey). Within that window,
taking Sun positions from the corresponding date in 1911 instead of 1912,
without bothering to make any special correction, would involve errors in
declination and GHA of no more than 3 arc-minutes.
When Amundsen's party thought it had reached the Pole, the approach adopted
was to make camp and measure Sun altitudes each hour throughout a 24-hour
period. For Amundsen, it was Dec 17, close before the Summer solstice, so
the Sun's Southerly declination was still increasing slightly, by about 2.3
arc-minutes in the 24-hour period. 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. Note that this procedure requires no
knowledge of the Sun's predicted declination, and the almanac does not
feature at all.
From this, my conclusion is that if Amundsen had indeed failed to bring a
1912 almanac, it would not have handicapped him at all. So I don't accept
Huntford's argument, and Eno's. There were much stronger arguments for an
early return from the Pole, as Scott's experience showed.
====================
What follows are some observations of my own on polar navigation, just in
case anyone else is interested in the topic.
There are some interesting common features between Polar sledge navigation,
and navigation of a vessel at sea. Each sledge was fitted with a
"sledgemeter", a sort of trailing bicycle-wheel-like affair, with a turns
counter, to act just like the taffrail log on a boat. One or more sledges
carried a magnetic compass, which had to be corrected for the
(considerable) local magnetic variation. By keeping a log of heading and
distance, a DR plot could be kept, exactly as we do at sea.
The position of the South Magnetic Pole had been recently determined by
Mawson in 1909 at 72 deg 25 min South, 155 deg 16 min East. For Amundsen,
approaching the Pole along a radial track of 164 deg to 169 deg West (and
less so for Scott, travelling along 163 to 170 deg East) the paths were
sufficiently far from the Magnetic Pole that the magnetic compass provided
a sufficient direction-reference when the Sun was veiled. Sometimes, on
rough ground, the party would have to stop to give the compass a chance to
settle. From time to time, local magnetic variation could be determined
when the Sun was visible.
So much for DR. Now for celestial positioning, which was done at intervals
of a few days, when the party had made camp and the sky was clear enough
for the Sun to be clearly visible.
On both Amundsen's and Scott's expeditions, members carried several watches
between them. Some of these were "chronometer-watches" (whatever those were
at that date). Presumably all these would be pocket watches, deriving some
warmth from the wearer's body. I wonder what polar temperatures did to the
oiling of a watch. Amundsen's watches were set to show local time at their
base camp, where the party had landed on the ice.
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.
It's easy to see how, along the journey to the Pole, track was kept of
latitude by comparison with almanac predictions of the Sun meridian
altitude. It's not clear to me, from any of the accounts, how either party
ensured that it was keeping to its intended radial line to the Pole, by
measuring the longitude. As we have discussed on Nav-L before, to establish
longitude involves comparing local time by the Sun with the reading of a
chronometer, and determination of the moment of local noon is not something
that can be done in a short time around noon. It requires one or two
altitudes, taken well away from noon, together with the altitude at noon.
=======================
There's a telling passage in book 2 of Amundsen, on page 111, from December
7th, in which he says ..."We badly wanted to get a meridian altitude, so
that we could determine our latitude. ... Although the prospects were not
very promising, we halted at 11am and made ready to catch the sun id it
should be kind enough to look out. the sun appeared. ...it...was checked
by all, and the altitude written down. ...before we had finished our
work-that is to say, caught the sun at its highest, and convinced ourselves
it was descending again-it was shining in all its glory." This provided a
latitude of 88deg 16 min, just 44 miles from the pole, and exactly in
accordance with their DR. This is indeed a classic account of taking an
observation for latitude, but would be useless for determining longitude.
Nowhere can I find a mention of observing for longitude, at a time well
displaced from the moment of local noon.
At a lat of 88deg 16 min, the change of altitude of the Sun between "noon"
and "midnight" (to the extent that these words are relevant in the land of
midnight Sun) would be only 1 deg 28 min. During the 2-hour period from
11am to 1pm local time, the Sun's altitude will increase, and then fall, by
only 1.5 minutes. Not a good basis on which to estimate the moment of noon!
At such high latitudes, just a rough value for longitude would suffice,
however, because at 44 minutes from the Pole the length of a degree of
longitude has reduced to only .013 miles.
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?
===================
The theodolite observations made by Bowers, in Scott's expedition, provide
some other problems. Whenever the results were noted in Scott's journal,
three quantities were given: latitude, longitude, and magnetic variation,
the last two quoted to a precision of one minute of arc.
For example, on Jan 12, Scott records that Bowers observed for a "night"
position (time not stated) after making camp, giving values lat 88 deg 57
min 25 sec South, long 160 deg 21 min E, var 179 deg 49 min W. It would be
absurd to expect that any compass could be read in those latitudes (or any
other) to a minute of arc. A degree would be good going. Where, then, have
those compass variations, to a minute, come from?
Although the figures for measured latitude may be acceptable, for a 4-inch
theodolite with half-minute divisions, the assessment of longitudes to a
minute of arc seems absurdly over-optimistic in such high latitudes.
Measurement of longitude to a degree would be good going and quite
sufficiently precise. And how has Bowers obtained both latitude and
longitude from what appears to have been a single observation?
My copy of "Scott's last Expedition" is the cheap 1-volume edition of 1923,
and I wonder whether the earlier and fuller 2-volume edition has more to
say on the details of the celestial navigation.
The fact that Amundsen and Scott both arrived at the same spot on that
featureless wasteland and agreed that it was the South Pole seems at first
to be a testament to the ability of both navigators. However, both were
converging toward that spot, a month apart, along radii that were about 30
degrees different. Scott came upon Amundsen's tracks when he was still 18
miles away from the Pole, and could then follow his path. Whatever path
Scott had travelled until then, provided any error had been to the left
rather than to the right of his direct line to the Pole, he would have been
pretty certain to pick up his competitor's track at some point. So there
was a 50:50 chance of that happening.
Nevertheless, I accept that the navigation must have been to a high
standard in both cases, even if it has not been explained well to us in
detail. Certainly, Amundsen did a much more thorough job of surveying when
he reached the Pole. Scott's checks were more cursory, but then, as
number-two at the scene, he must have lost some enthusiasm at that point,
and he was desperately short of time for his return journey.
To me, the whole business was a tragic exercise in futility. That such
teams should devote their energies, and for some, their lives, to being the
first to reach an otherwise undistinguished spot in a barren wasteland,
seems such a waste of human endeavour.
George Huxtable.
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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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