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    Re: How Many Chronometers?
    From: Robert Eno
    Date: 2009 May 11, 20:56 -0400

    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.
    ----- Original Message -----
    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:
    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.
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