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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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