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Re: "Artificial Sights"

Subject: Re: "Artificial Sights"
From: Fred Hebard (Fred@XXX.XXX)
Date: Thu Mar 20 2003 - 22:24:40 EST

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.

Fred



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