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    Re: "Artificial Sights"
    From: Fred Hebard
    Date: 2003 Mar 20, 22:24 -0500

    I apologize for the indirect link to this story; I was trying to credit
    the source.  But Yahoo sites clearly are difficult.  Lee Martin gave
    the direct link previously.
    As I read the link, the cable-fixing sailors were trying to find the
    error in their chronometers, similar to the use of lunars, not to
    synchronize the chronometers.  The account is pretty clear that they
    were working backward from the sights to the time.
    I would imagine that the location of these cable stations would have
    been worked out carefully by astrometric means when the cable
    originally was laid
    Some of my questions originated from me confusing the timing errors of
    lunars with the timing errors of straight sights; lunars are less
    sensitive than standard sights, so that a reading accurate to one
    second of arc would "only" be accurate to one second of time, not a
    tenth of a second of time.  If they were working backward from a known
    position, they could get tenth of a second accuracy in time from
    measurements to the nearest second of arc, to answer one of Lee
    Martin's questions.
    But I wonder about reading to the nearest second of arc using a
    standard sextant.  If it were a pillar sextant as George Huxtable
    describes, that might be possible.  All the standard vernier sextants I
    have seen (mostly on eBay over the last few months) have read to the
    nearest minute or half minute of arc, with a few going to a ten
    seconds.  And most of the correction labels indicate they were not
    machined precisely enough to be accurate below ten seconds of arc.
    I think these sailors might have been using different techniques from
    those described by Bruce Stark, as they were working in the very late
    nineteenth century, right before radio, rather than the early
    nineteenth century.
    For me, the non-navigational portions of this story were certainly the
    most exciting.  This whole cable business is a great new source of sea
    stories for me.

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