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    Re: Plumb-line horizon vs. geocentric horizon
    From: Jim Thompson
    Date: 2005 Feb 9, 10:50 -0400

    Jeff, While waiting for Frank's reply, I wrote this as a direct result of
    his post (and others), seeking the same "why", I think:
    "Geodesy: The Earth is neither flat nor spherical, and the center of the
    earth is not where you thought it was"
    Jim Thompson
    Outgoing email scanned by Norton Antivirus
    > -----Original Message-----
    > From: Navigation Mailing List
    > [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM]On Behalf Of Fried Squash
    > Sent: Wednesday, February 09, 2005 10:36 AM
    > Subject: Re: Plumb-line horizon vs. geocentric horizon
    > --- Frank Reed  wrote:
    > > I don't know if this was mentioned this time around,
    > > but it's worth
    > > repeating. Local horizontal coordinates, as already
    > > discussed, are defined by  the
    > > plumb-line. The zenith is opposite the direction of
    > > local gravity and  therefore
    > > the horizon of the coordinate system is essentially
    > > parallel with the  sea
    > > horizon. How much difference would it make if we did
    > > it differently? If we
    > > defined the zenith to be opposite the direction to
    > > the Earth's exact center,  where
    > > would the horizon be? If you work it out, the
    > > difference is just about
    > > (12')*sin(2*Lat). At 45 degrees latitude, the
    > > direction to the zenith would be
    > > fully 12 minutes of arc shifted if we defined the
    > > coordinates in terms of the
    > > Earth's center. That's a substantial difference from
    > > the point of view of
    > > sextant observations.
    > Frank-
    > Your thread was propelled off on a tangent almost
    > immediately.  Can you explain more of the "why" of
    > your interesting statement?  I'm not sure if you're
    > stating something about Earth's oblateness or if the
    > center of gravity is that much different than the
    > geometric center for some reason.  And if it is the
    > latter, how is it that we can navigate as we do
    > without compensating for it?
    > -Jeff

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