NavList:

   

Reply

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.

   

Reply