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    Re: Old style lunar
    From: Fred Hebard
    Date: 2004 Dec 9, 17:24 -0500

    On Dec 9, 2004, at 3:29 PM, Ken Muldrew wrote:
    
    > 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:
    ....
    > 114?48'20"
    >
    > true position of Rocky Mnt. House:
    ....
    > 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.
    >
    
    Actually, I think the spread of positions in the data illustrates
    precisely why sailors could not put great faith in lunars while
    underway: they weren't in the same place twice.  But did Thompson
    measure more than one distance for every observation, so that each
    point is the average of several distances?
    
    Also, perhaps Thompson was not the best of lunarians.  Kieran Kelly
    last year or so posted a series of lunar observations taken by an
    Australian explorer that were exquisite.  I can't imagine that the
    distance cleared from those would have been in error by more than 0.1
    or 0.2' of arc, given an accurate sextant.
    
    Fred
    
    
    

       
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