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    Re: sextant index error measurement
    From: Richard M Pisko
    Date: 2006 Nov 3, 21:45 -0700

    On Sat, 4 Nov 2006 13:14:10 +1100, Peter Fogg wrote:
    > The experiment you are working you way towards carrying out
    > involves observations made over a distance parallel to the
    > ground, close to the ground. Refraction will depend on your
    > atmospheric conditions there, and could be different to the
    > refractive effect from sighting from eye height skywards.
    True, over longer distances or special conditions, and that
    explains a slight disadvantage in the US custom of using a
    vertical surveying rod for stadia derived distances.  The method
    of using a horizontal two meter subtense bar, common in Europe,
    has more potential accuracy ... according to the texts.
    So:  Place the sextant horizontally on a small but sturdy
    platform on a good surveying tripod.  Then both laser beams will
    be in the same layers of air.  Do not have either beam run close
    to a building or big tree, as there is such a thing as
    horizontal deflection also, and there are warnings against that
    practice in geodetic surveying.  Depending on the weather, you
    may find the beams quiver a bit due to local mirage.  Night
    observations are the best, especially if the ground and air
    temperatures are close to being equal, and the beam will be more
    Richard, 2006-Nov-03 9:28:15 PM GMT - 6 hours for summer
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