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    Re: Instrument Error
    From: Fred Hebard
    Date: 2005 Apr 29, 12:40 -0400

    On Apr 28, 2005, at 3:03 PM, Bill wrote:
    >> Fred wrote
    >> Here are the data with the mean date and time, UT1, and the altitudes.
    >> The location was 36*46.8N, 81*50.7W.  The observation for 18 April is
    >> listed as for 17 April in the original table, where the separate dates
    >> are zone time rather than UT1.  Most of the the time the sun and moon
    >> are higher than 20* altitude.
    > Fred
    > Your data does not seem to support my hunch that the error gets greater
    > later in the day.  Must be pilot error on my part.
    > The frustrating part is that the above are representative of other
    > series.
    > Late afternoon series with error STDEV as low as 0.18', but mean of
    > long
    > 27.78 off.  Early afternoon series with error STDEV of 0.36' but mean
    > longitude 3.75' off.
    > Bill
    Were you always looking at the sun through the clear part of the
    horizon mirror or at it's reflected image?  I generally aim the
    telescope at the lower body, as it's easier to hold the sextant.  If
    you're reversing the body at which the telescope is aimed between early
    and late afternoon, that might account for the difference.
    If reversing, part or all of it might be shades.
    Whether reversing or not, the gap in late afternoon might be caused by
    not aiming straight and not centering the horizon body on the optical
    axis.  Those sorts of errors always cause the distance to be too great.
    The data also suggest more practice is needed, as you know.

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