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    Re: Precision of lunars
    From: Fred Hebard
    Date: 2007 Apr 18, 19:22 -0400

    Alex,
    
    Thank you so much for doing this.  Your three years of research is
    very admirable, in my opinion.  Your findings also dovetail with Jan
    Kalivoda's report of the findings of a German who investigated
    precision of lunars at sea in the late 1800s.  As I recall, the
    standard deviation of lunar distance reported by Jan was about 25
    arcseconds.
    
    I am wondering how much practice you estimate it took you to achieve
    this level of proficiency?
    
    Thanks,
    
    Fred
    
    On Apr 15, 2007, at 12:59 PM, alex wrote:
    
    >
    > What  accuracy can be really achieved with  the lunar method?
    > This question was discussed several times, and now I want
    > to contribute some data. Let me first cite a great authority,
    > Lord Kelvin, then I will post some of my recent observations.
    >
    > Lord Kelvin says:
    > "...to observe the moon's position accurately
    > to half a minute of angle.
    > This can be done, but this is about the most
    > that can be done in the way of accuracy at sea."
    >
    > And then:
    > "If he has extraordinary skill, and has bestowed an extraordinary
    > care on his instrument, he may, by repeated observations,
    > attain an accuracy equivalent to the determination
    > of a single linar distance within a quarter of a minute of
    > an angle, and so may find the ships place within seven miles
    > of east and west distance; but, practically we cannot expect that the
    > ships place
    > will be found within less than twenty miles, by the method od Lunars
    > in tropical seas
    > or within ten miles in latitude 60d;
    > and to be able to do even so much as this
    > is an accomplishment which not even a good modern navigator,
    > now that the habit of taking lunars is so much lost by the use of
    > chronometers,
    > can be expected to possess.
    >
    > (Sir William Thomson
    > Popular lectures and addresses,
    > London, McMillan 1891, vol. 3 Navigational affairs.
    > Navigation 1-138 (Lecture delivered in the City Hall,
    > Glasgow, Thursday, Nov 11, 1875. Pages 101-102 ).
    >
    > I also analysed Cook's maps and observations, comparing his Lunar
    > determinations
    > of the longitudes of known places (those that can be found on
    > Terraserver.
    > If someone is interested, I can post the results of this analysis.
    >
    > It is interesting to compare this with my own experience.
    > After 3 years of practice, I was able to eliminate the systematic
    > errors
    > in my SNO-T Lunars, and now I post the data which on my opinion are
    > consistent with what Kelvin says (in the next message).
    >
    > Alex.
    >
    >
    > >
    
    
    
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