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    Re: Negative Hc?
    From: Bill B
    Date: 2006 Apr 30, 14:29 -0500

    Guy wrote:
    > I don't understand the implication of a negative Hc. Can someone explian this
    > please.
    For cel nav, this means the actual location of the center of the body (not
    apparent/observed location) is below the horizon.  Note that even when the
    sun or moon have a slight negative Hc, you can still see them because of
    refraction (apparent lifting of the body by atmospheric refraction) and
    their relatively large diameters (approx.. 30').
    You can confirm this for yourself by looking at the 0-10 degree
    refraction-correction table in the almanac. Check out the upper limb
    correction of the sun when it just hits the horizon/sunset.  (That will be
    0d observed.)  This correction indicates how far the center of the sun is
    truly below the horizon, even though you observe the upper limb as tangent
    to the horizon.  This correction is nominally -49'.7 (or star/planet
    refraction minus the sun's semidiameter).
    For a quick check, look at the star/planet refraction correction at 0d to
    see how far the body's center (in this case a point source or nearly so) is
    below the horizon in reality vs. apparent (-33'.8).  Please note that the
    list Gurus would tell you that (most?) stars and planets are not
    sufficiently bright to cut through all the atmosphere when they set, so they
    will "extinguish" before you can observe their actual setting.  They might
    also caution that refraction values are at best a crap shoot near setting,
    as the image has to cut through approximately 5400nm of atmosphere to reach
    you, and the chances of temperature and pressure along that path matching
    your conditions are slim at best.  For extra credit, add in possible thermal
    inversion and its affects.
    It is refraction that squishes the sun down so it starts to look more like
    an egg on its side than a circle near sunset.  The inability to accurately
    determine refraction (which you will note increases by -28.'5 from 10d to 0d
    for a star or planet, and only a bit over 0'.1 from 60d to 50d) is primary
    reason texts recommend you limit observations to bodies with a minimum of
    20d-30d elevation if possible.
    If you are using HO229, there is a explanatory section in the tabular book
    that explains how to deal with observations close to the horizon (which
    includes negative Hc's).
    Hope that helps

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