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Subject: Re: Preston's paper on Lewis & Clark's Navigation
From: Fred Hebard (Fred@XXX.XXX)
Date: Sat Jun 07 2003 - 19:06:04 EDT
On Saturday, Jun 7, 2003, at 17:17 US/Eastern, George Huxtable wrote:
> The latitude discrepancies are even greater than Fred's quoted figure
> of 5'
> suggests. On 9 June 1805, L&C give a lat of 47° 29'N, where the modern
> position of that observation site (if identified correctly) is stated
> to be
> 47°55'45"N, nearly 27 minutes out. Similarly, on 10 July, L&C give
> 47°03'N,
> but the site is stated to be at 47°27'23"N, over 24' in error. How on
> earth
> could any competent observer get his noon altitudes so wrong?
I was referring to the average discrepencies. Yes, there were some
lemons in the observations. You also have to remember that L&C's own
calculation of latitude from their data, which calculations they
apparently made, might be better than those of Preston, since L&C were
closer to the data, and not using a computer (which will blindly
calculate with garbage that a human would discard). Finally, perhaps
the "modern" interpretation of where L&C were located is incorrect. A
similar phenomenon seems to have occurred with the famous Canadian
explorer, David Thompson, around some lake up in the Rockies.
> The table also compares L&C's calculated noon Sun altitudes (without
> telling us how they were calculated) with, in another column, those
> calculated by "Ellicott's method of 1803" (without telling us how they
> were
> calculated).
I believe Preston prevents these two methods previous to the tables,
and refers to those presentations in the explication of the tables.
> Observationally, we should consider this. An octant was used for all
> these
> measurements, we're told, and an octant in normal use can measure an
> angle
> up to 90° only (compare with 120° for a sextant).
> Use of an octant in backsight mode has one great disadvantage,
> however. In
> normal mode it's easy to check its index error by looking at a distant
> object and aligning the two images, when the scale should show the
> index
> error. But as far as I know, there's no way of checking index error
> when in
> backsight mode.
The 45 degree problem wouldn't have arisen early in the journey. But
this is an attractive hypothesis to explain the errors (of 5') in
latitude. Preston refers to index error in his discussion of his table.
Lewis & Clark carried both a sextant and an octant. Perhaps one used
one instrument and the other the second. Knowing this fact would help
resolve this question.
> In the meantime, an interim
> conclusion appears to be that Lewis and Clark were indeed hopelessly
> bad
> celestial navigators: the Emperors had no clothes. I bet that will stir
> things up. Anyone disagree?
Oh George, you're just trying to stir up the Yanks! (But it's working
on me).
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