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Robert Eno, you wrote:
"Determining your longitude and/or time via lunar distances for example, is 
only as accurate as the observer and his sextant and with so many variables 
(temperature, observer error, instrument error, refraction etc.) is it 
realistic for one to expect that he can determine chronometer error by this 
means? Unless your chronometer is out by hours."

Even using lunars, historically it was not unusual to discover that the 
chronometer had gone bad. Here's a detailed historical example from 1849 
which I wrote up for the list a few years ago: 
   http://www.fer3.com/arc/m2.aspx?y=200507&i=024812 
It's a great story, too. In the early 19th century, it was common practice to 
treat a difference between lunar longitude and chronometer longitude of less 
than 30' of longitude as "undecidable" --maybe the chronomer is off, maybe 
the lunar observation was off. But above that level of error, it was common 
to trust the lunar. As you know well, 30' of longitude would be an error of 
two minutes of time on the chronometer.

-FER





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