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    Re: Calibrating a sextant scale
    From: Frank Reed
    Date: 2007 Nov 25, 23:07 -0500

    Alex, you wrote:
    "Of course, there would be no difficulty to detect such large errors using
    either Lunars or stars distances. By the way, this shows that the
    sextants you experimented with were of poor quality. The arc error of a good
    sextant is not supposed to change by 1'8 even if you ship it or transport in
    a car, is it?"
    The sextants in question were both over thirty years old. I have no idea
    what their history was. I don't know if they were thrown around like so much
    loose baggage, or were they treated with care and respect. That's the
    problem for most of us on this list. We are much less likely to have new
    sextants that we have had under our complete control from day one than
    practicing navigators a few decades ago. We are much more likely to buy used
    sextants, thirty years old, sixty years old, even a hundred and sixty years
    old. They often come from ebay merchants who claim they've cquired them at
    estate sales, but we really have no idea where they've been. There's no
    reason whatsoever to believe that the certificate which accompanies an old
    sextant represents its current condition. That doesn't mean a sextant is
    "poor". It just means the certificate may be irrelevant.
    And you wrote:
    "When, answering George's question, I said that "I failed", I was talking
    about STAR-STAR distances only. Which are less precise than the Lunars."
    Yes, and I agree. Based on my personal experience, star-star distances are
    less effective than lunars for judging arc error at the finest level of
    observation (tenths of a minute of arc). But this is all a matter of
    "degree". Some people aren't worried about that very fine level of
    measurement. Mike was talking about a plastic Ebbco sextant. For an
    instrument like that, star-to-star angles should provide all the necessary
    accuracy. I'm assuming here that the normal inaccuracy with a plastic
    sextant from one sight to the next is about 1 minute of arc (that's based on
    my own experience --your mileage may vary). So we don't have to worry about
    any differences smaller than that. So in his case, star-star distances will
    serve nicely. Also, if you're not worried about differences under 0.6
    minutes of arc, you can also ignore stellar aberration which changes the
    star-to-star angular distances during the course of the year.
    "My Sun- and Jupiter- Lunars(taken in long series and averaged) generally
    agree with the most recent certificate I have within 0'2. As you can see
    from the statistics I posted on my web site. Determining the arc correction
    from Lunars is a long-term enterprize, this cannot be done in one night."
    It's certainly not something that you could do *every* night, but yes, it
    could be done in a relatively short period of time (a night or two?) maybe
    once every five or ten years by using special apparatus (e.g. higher power
    scope, equatorial mount with clock drive, etc.) or a special location (a
    site with four distant lighthouses spaces ninety degrees apart, etc.). When
    I purchase a sextant, which is "new" to me, but invariably "used", I don't
    assume that the certificate can be trusted.
    And you wrote:
    "So this procedure would be pretty useless in practice, if the arc error
    changes during a car transportation."
    Obviously, I wasn't suggesting that sextant adjustment changes *every* time
    you stick one in a car. Otherwise, I wouldn't drive with them at all, right?
    But I do believe that it's fairly common knowledge that rough treatment will
    *eventually* change the adjustment of a sextant and require
    re-certification. Does anyone have a sense how often this was recommended
    historically? I remember reading a report of an eclipse journey by Newcomb
    in the 1880s or somewhere around then where he brought along a sextant to
    measure lunar distances for longitude up in central Canada. He made a point
    of re-measuring its arc error, stating that it had not been tested in twenty
    years (if I remember correctly), before hitting the road. But that's just
    one case.
    And you wrote:
    "Fortunately, in my sextant it does not seem to change: I transported it a
    lot by cars, trains and airplanes (in the luggage). Even IC does not seem to
    change much. It is between -0.3 and -0.5 since I bought it."
    I'm not surprised. One of the things that I really like about the SNO-T is
    its very sturdy design. I do expect that your sextant could survive more
    banging around than average sextants.
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