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    Re: Back In Hobby: Some Questions, Please
    From: Alexandre Eremenko
    Date: 2007 Apr 26, 10:21 -0400

    
    Dear Ken,
    Tjhanks for an interesting piece of data.
    
    > Peter Fidler in 1791, using a "brass sextant of five or six inches radius
    > made by Ramsden"
    
    That is of a "toy size" by the standards of that time...
    
    > reports the following two latitudes (among many celestial
    > observations, but I'm pretty sure that I know where he was when he made
    > these two).
    >
    > 51�50'47" (actual latitude 51�02'55")
    > 52�21'37" (actual latitude 52�15'35")
    >
    > These are very bad.
    
    Indeed. The very first known latitude determination
    in history (very first known to me) is attributed to some
    Pytheas, a Greek from Marseille in IV century B. C.
    He determined the latitude of Marseille with some 10 miles accuracy
    (I cite by memory but can verify the number).
    What he used to measure the Sun alt at noon, I am not sure,
    probably gnomon (a vertical stick whose shade you measure).
    But this was long before "divided scales", before trigonometry
    and about 500 years before Ptolemy:-)
    
    > David Thompson used a 10 inch radius brass sextant made by Dollond. His
    > latitudes at Rocky Mountain House from 1800 and 1801 were:
    >
    > 52�21'29"
    > 52�21'27"
    > 52�21'35"
    > 52�21'32"
    >
    > The actual latitude is 52�21'20" but it should be remembered that Rocky
    > Mountain House is at an altitude of 3200' and Thompson did not account for
    > that in his calculation.
    
    Cook's companions did worse with their lalitude.
    If you average these you get 1".3 accuracy!!
    While the max error of the individual numbers is about 0.3'
    and I suppose this is as good as you can expect in general,
    with any sextant. These latitude data are probably not individual
    shots but the averages themselves, am I right? And the art horizon was
    used, correct?
    
    > Recall that I posted his lunars from this same
    > position about 2 years ago and after correcting for the poor data in the
    > almanac, his sigma was about 14' in longitude for 13 observations.
    
    Which again means roghly 0.5' accuracy in the measurement
    of the distance itself. Which is consistent with what Kelvin,
    and Kalivoda say.
    
    But we know that Frank does much better,
    as well as the "novices in the Mystic seaport conference of 2005"
    :-)
    
    Alex.
    
    
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