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Thanks, Ken.
The "true position" was determined in modern times?
Alex.

On Thu, 9 Dec 2004, Ken Muldrew wrote:

> In the "No Lunars Era" thread, Alex was wondering about the precision of
> lunar distances, among other things, and whether that played a role in
> navigators not putting much faith in their lunar calculations.  While
> talking about land navigation from the same era, I offered to provide a
> sample of actual lunar longitudes as an example of the capabilities of the
> art at that time.
>
> On 6 Dec 2004 at 21:21, Alexandre Eremenko wrote:
>
> > On Mon, 6 Dec 2004, Ken Muldrew wrote:
> >
> >> If you like, I can provide you with some land based lunars taken by
> >> explorers in the Canadian West ca. 1800.
> >
> > Yes. Sure. I would like to see this.
>
> Below are a series of latitudes (by meridian altitude of the sun) and
> longitudes (by lunar distance) that David Thompson took at Rocky Mountain
> House in 1800 and 1801. He used a sextant by Dollond (9" arc) and a common
> pocket watch. The true position of Rocky Mountain House is given at the
> bottom where Thompson's readings are averaged.
>
> Latitude:
> 1800   9-Apr  52?21'29"
> 1801  20-Feb  52?21'27"
>       21-Feb  52?21'35"
>        5-Mar  52?21'32"
>
> Longitude (from lunar distance):
> 1800  17-Apr  115?12'00"
>       18-Apr  114?57'45"
>       22-Dec  115?11'00"
> 1801  17-Feb  114?57'15"
>       28-Feb  114?52'15"
>       28-Feb  114?59'45"
>        1-Mar  115?11'00"
>       18-Mar  114?44'15"
>       17-Feb  114?39'00"
>       24-Feb  114?36'00"
>       24-Feb  114?13'00"
>       25-Feb  114?28'30"
>       25-Feb  114?26'45"
>
> Thompson's average position:
> 52?21'30"
> 114?48'20"
>
> true position of Rocky Mnt. House:
> 52?21'20"
> 114?58'50"
>
> You can see that the spread of lunars covers a full degree but his final
> position was pretty close. As far as I have been able to see in his
> journals, Thompson always updates his account when he takes a latitude or
> longitude reading. In addition, he updates all the entries in his account
> log proportionally since the last reading to correct for a systematic bias
> in his reckoning. He took this latter step because he was intending to map
> everywhere he travelled, but he clearly put more faith in his celestial
> observations than did many of the ocean navigators that Frank has written
> about.
>
> Ken Muldrew.
>

   

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