NavList:

   

Reply

On 30 May 2004 at 20:37, Bruce Stark wrote:

> Thank you for the valuable David Thompson information. For a long time
> I've wanted to know something about him. Having an example of his
> navigation is worth a lot.

My pleasure. I'm going through some of Peter Fidler's notebooks right
now. They're a lot easier to read so perhaps I'll post some
navigational material from them as well. I just came across an
instance of using a lunar eclipse to calculate longitude in one of
Fidler's notebooks. This may be of interest to you since the eclipse
used by Lewis & Clark to get the longitude of the Mandan village is
so famous.

> Like George, I'm surprised he took six measurements of the sun's
> altitude for a time sight. Clearly, he didn't mind a little extra
> arithmetic.

I think that's the case.

> Also surprising is that he seems to have taken his courses from the
> sun, when it was out, rather than from a compass. That's pretty much
> standard for outdoorsmen when the sun isn't too near overhead, but
> Thompson must have been unusually good at it.

He travelled about 55,000 miles on his journeys, navigating all the
way. Lots of practice, that's for sure. Also, the sun never gets very
high overhead here in Western Canada. Too, this particular journey is
right at the Eastern edge of the Rocky Mountains; a pretty good
landmark for estimating direction.

> The business of proportioning for the distance that fit his supposed
> Greenwich time, and comparing his cleared distance to it, strikes me
> odd. At present I can't think what the advantage would be. But, then,
> I can't think of any disadvantage either.

The technique would have been that used by Philip Turnor who came to
Canada in 1778. I wonder if the advantage comes from a time when
lunar distances weren't tabulated in the Nautical Almanac, but had to
be calculated from right ascension and declination. If so, then this
technique would require only a single calculated distance, rather
than two for interpolation (I think). If that's the case, then the
use of stars not included in the group of 10 lunar stars would also
make this technique easier, but as far as I have seen, Thompson never
uses stars other than the 10 lunar stars.

> Probably the difference
> between the Greenwich time per dead reckoning and the Gr. time per
> lunar wouldn't be enough for the moon's actual, as opposed to her
> average, orbital speed to have much effect.

In my reply to George, I note that this assumption is no longer
warranted. He may have used an average speed, but I provided no
evidence of that. That being said, since his navigation was not being
used for a landfall on a dangerous coast, the accuracy required was
much lower than for a ship at sea, so he could have used some
simplifying assumptions without risking any lives.

> The beauty of Thompson's way is that it highlights something special
> about the old navigation. As you   say: "He never bothers with a
> corrected Greenwich time because he deals only in local time.
> Greenwich time is merely for taking values out of the Almanac."
>
> His concern is longitude, not the time at Greenwich.

I think that's it exactly.

Ken Muldrew.

   

Reply