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Re: Old style lunar
From: Alexandre Eremenko
Date: 2004 Dec 9, 16:09 -0500
From: Alexandre Eremenko
Date: 2004 Dec 9, 16:09 -0500
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. >