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On 9 Dec 2004 at 17:24, Fred Hebard wrote:

> Actually, I think the spread of positions in the data illustrates
> precisely why sailors could not put great faith in lunars while
> underway: they weren't in the same place twice.

Yes, but it still seems odd that they would trust their dead reckoning so
much that they wouldn't update their account when they took a lunar.

> But did Thompson
> measure more than one distance for every observation, so that each
> point is the average of several distances?

These are probably just one series for each position. Where two positions
are noted on the same date are probably the only instances of multiple
lunars. He usually takes about 7 readings for a lunar and follows
immediately with a time sight.

> Also, perhaps Thompson was not the best of lunarians.

He learned celestial navigation in 1788 and so had about 12-13 years
practice by the time of these measurements (taking lunars continuously
throughout that period). So he was reasonably well practiced by this time.
But also note that most of the sights at Rocky Mtn. House were taken in
winter; temperatures of -40?C are not uncommon at this location. That
might have had an impact on some his readings. It's hard to say how good
he was. In the 1820s he was appointed to survey the Canada-U.S. border to
the Lake of the Woods. No less a personage than J.B. Tyrrell called him
the "greatest land navigator who ever lived". Tyrrell later surveyed much
of the territory that Thompson covered and obviously thought that Thompson
had done a good job. Also, Thompson was primarily a fur trader; his bosses
indulged him in his hobby of exploration so long as he turned a profit on
furs.

> Kieran Kelly
> last year or so posted a series of lunar observations taken by an
> Australian explorer that were exquisite.  I can't imagine that the
> distance cleared from those would have been in error by more than 0.1
> or 0.2' of arc, given an accurate sextant.

Augustus Gregory. This was about 50-60 years after Thompson. I don't think
sextants improved much in the intervening period but the quality of lunar
predictions probably did. It could be that Gregory was just that much
better than Thompson. Thompson was blind in one eye but I don't know if
the vision in his other eye was impaired in any way.

Thompson's sextant was a good one but it was subjected to a great deal of
abuse. He recovered it from overturned canoes on several occasions (well
downstream of the accidents), the index error changes regularly (sometimes
by ~10 minutes), the people he worked and travelled with had no
understanding of delicate instruments. He seemed to be constantly sending
pocket watches back to England to have them repaired, so we can infer that
everything he had was treated somewhat roughly. Probably these, and the
uncertainty of the almanac that Frank wrote about, are more likely factors
than Thompson's lack of skill.

Ken Muldrew.

   

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