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    Re: Horizons, was Summary of Bowditch Table 15
    From: Jim Thompson
    Date: 2005 Jan 30, 11:23 -0400

    > -----Original Message-----
    > From Jim Thompson
    > Trevor wrote in reply,
    > > The sensible horizon might be better understood as a plane,
    > > perpendicular to the direction of gravity acting on the observer and
    > > drawn through the observer's eye. It is parallel to the celestial
    > > horizon because that too is a plane perpendicular to the direction of
    > > gravity acting on the observer but drawn through the centre of
    > > the Earth.
    
    Jim wrote but meant to finish:
    > I have not yet found an independant reference to this idea that the
    > horizontal coordinate system's horizons are perpendicular to gravity.
    
    Sorry, Trevor, I meant to complete this thought before posting that message,
    but my trigger finger slipped.
    
    I have not yet found an independant reference to this idea that the
    horizontal coordinate system's horizons are perpendicular to gravity.  All
    the definitions I have found so far refer to the center of the earth, not
    the direction of gravity.  You were challenging my comment that the horizons
    are perpendicular to a line drawn through the center of the earth to the
    observer's position on the surface of the earth.  I think what you meant was
    that this would only be true if the earth was a perfect sphere and if
    gravity pointed to the center of the earth, but the earth is geoid, and so
    the direction of gravity is a more proper reference than the center of the
    earth.  Is that so?
    
    Jim Thompson
    
    
    

       
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