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    Re: "Artificial Sights"
    From: Glendon
    Date: 2003 Mar 21, 09:43 +1100

    ----- Original Message -----
    From: "Fred Hebard" 
    > I wonder what a sextant stand would look like, specifically the part
    > that cradles the instrument; ie, how does one attach the sextant to the
    > tripod.  I also wonder what sort of sextant would allow one to read to
    > the nearest second.
    Anything can be improvised as a "stand"...I have used a vice holding a G
    clamp with the sextant in the G clamp. However it does seem they had a
    purpose built stand.
    > Regarding the method itself, it would seem to me that one would have to
    > know one's longitude in order to solve for time.  The author mentions
    > working "Observations for Time" on each of 20 or 30 observations to
    > arrive at synchronization to the nearest tenth of a second for the
    > chronometers.
    Reading between the lines, it seems to me that they knew exactly where they
    were when taking a land sight, and worked back from observed and calculated
    sextant altitudes to correct their chronometer. The author says they took
    these readings at any convenient time during the day, not necessarily at
    The accuracy of their adjustments...1/10 of a second of time...intrigues me.
    I haven't worked back what that means in terms of sextant accuracy and
    accuracy of location of their position, but it all seems pretty fine.
    Particularily with regard to location, with those locations being in a range
    of countries, not just the relatively well mapped US and UK etc. The author
    talks about the difficulty of finding a "flat" area to set up upon, but does
    not mention accuracy of location. The author
    recounts how on occasion the local police were used to hold up traffic while
    the party set up and took readings in a main street....suggesting to me that
    in some countries there were only a few locations of known accuracy
    sufficient for their purposes, and flat enough to work upon.
    > These guys also seem to be working sights to the nearest tenth or fifth
    > of a minute of arc, although I infer this from the story rather than
    > reading it directly.  I would imagine such accuracy would be very
    > helpful in finding cable 20,000 leagues under the sea.
    Interesting stuff.
    Lee Martin

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