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That was more or less my point Frank.

I'd done a ****load of lunars using Bruce Stark's tables. I got to be pretty
good at it though I confess that I have not done any lunars for a long time.
A lot of rust has set in since then. My attempts yielded results that ranged
from one to two minutes, with my best being within 11 seconds and 21
seconds. Even at that, I think I got lucky on those ones. All of my
observations were taken from rock solid land. I doubt if I could obtain that
kind of accuracy on a boat, unless it were an aircraft carrier or a tanker.

I submitted an article to the Navigator's Newsletter ("A Field Assessment of
Stark's Tables for Clearing the Lunar Distance and Finding GMT by Sextant
Observation", Issue 65, Fall 1999).

I mention the above not to pass myself off as an expert; only as a fellow
who has done his fair share of lunar distances and thus may have something
to say on the matter.  My earlier point is that while lunars can be useful
for determining chronometer error, but they would only be good to determine
errors within minutes and not seconds. And even at that, using mechanical
devices from the deck of your average sea-going vessel would not lend itself
to high precision.

cheers,

Robert



----- Original Message -----
From: 
To: 
Sent: Monday, May 11, 2009 2:31 AM
Subject: [NavList 8220] Re: How Many Chronometers?



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.navlist.net?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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