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    Re: Basics of computing sunrise/sunset
    From: Bill B
    Date: 2009 Jun 27, 02:49 -0400

    > From: "Christian Scheele"
    > Many thanks, Frank, for the clarifying words on the limits of computing
    > sunrise/set times. Being a function of athmospheric conditions at any
    > possible point on earth, refraction values, I admit, remain estimates,
    > however good. But in theory: Assume you know the athmospheric conditions to
    > "the  horizon" and beyond right up to GP of the sun, i.e. you are able to
    > monitor these conditions for this entire stretch; say you have an imaginary
    > string of weather balloons thousands of miles long. Would that solve
    > refraction? Then other problems remaining would include anomalies in the
    > curvature of the earth's surface, errors in the observer's height and
    > sextant errors.
    This series of weather balloons to the GP of the Sun setting on my horizon
    caught my interest, but I have not had the time (or mental horsepower) to
    give it the necessary thought to follow up. Despite answers from Frank et
    al, they seem like overkill to me.
    My questions. If I recall the GP would be approximately 5400 nautical miles
    (nm) away when I see Sun set (using appropriate eye protection of course).
    In my mental picture, I am not wrapping my vision along the surface of the
    Earth. I have a line of sight from my eye position to the Sun, which will
    run out of atmosphere *long* before 5400 nm or the 93 million miles to the
    Sun.  Therefore I would want a series of hypothetical weather balloons along
    my line of site.  But would that help with thermal inversion/abnormal lapse
    rates causing refraction anomalies at my visible horizon?  Do I need a
    series of balloons up from my horizon?
    Any help in clearing up my confusion would be appreciated.
    Bill B.
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