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> -----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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