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    Re: Plastic vs Metal Sextants
    From: Richard B. Emerson
    Date: 1999 Aug 25, 6:40 AM

    Lu Abel writes:
     > I personally wonder if this IE drift would be an issue for a voyager, who
     > is hopefully surrounded by both decent breezes and the thermal mass of the
     > ocean.
    I found Larson's assumptions somewhat suspect or at least procedurally
    weak.  For example, I would have more confidence in his conclusions if
    he had each student using both sextant types.  There doesn't appear to
    be any control for the optical and design (e.g., mirror size and type)
    But, procedure aside, he's otherwise reporting the obvious.  Take a
    sextant from one environment to another and it's subject to thermal
    stress.  Some materials and designs handle this stress better than
    others.  Keep a sextant in a constant environment and there's less
    shift due to thermal stress.
    Thermal damping from the ocean won't completely eliminate or offset
    local thermal inputs from insolation.  It will help to smooth out the
    overall air temperature in the course of the day, of course, but
    sights taken on land in 85 deg. air will see the same heating (all
    other factors remaining constant) as sights taken at sea in 85
    deg. air.  Air movement will have some influence, of course, but, for
    example, a breeze in 85 deg. air is still not as effective as removing
    heat in 70 or 60 deg. air.
    Bottom line: sextant materials and design are still an overwhelmingly
    important factor.
    S/V One With The Wind, Baba 35

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