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    Re: Old style lunar
    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.
    >
    
    
    

       
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