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    Re: March 22 Lunar Observations
    From: Bill Noyce
    Date: 2002 Mar 25, 08:35 -0500

    Thanks, Arthur Pearson, for sharing your data.
    I haven't yet studied it, but I hope to get a chance to.
    I'm not yet a sextant user, just playing with the math,
    so take these comments for what little they may be worth.
    When adjusting your sextant to measure the distance, do
    you always make the final turn of the wheel in the same
    direction?  Do you measure your index error the same way?
    For altitude measurements, I've seen a suggestion to set
    the sextant a little ahead of the body's altitude (rising
    or setting), and then wait until the body's image just
    touches the horizon, and record the time.  That would seem
    to give good accuracy, but for lunars it might lead to
    a long time waiting...
    For graphically averaging your observations, I don't think
    you can use the slope of the line from D1 to D2, because
    of the "parallactic retardation" George Huxtable described.
    Its effect is to make the slope of apparent distances
    shallower than the slope of actual distances.  It would
    probably be appropriate to "un-clear" the D1 and D2
    distances, using computed altitudes at those times, and
    applying refraction and parallax with reversed signs.
    I'm not sure how easy that would be to do with Bruce Stark's
    tables, though.  But then the un-cleared D1a and D2a
    should define a slope that ought to match the slope of
    a line through your observations.

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