# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Plumb-line horizon vs. geocentric horizon
From: FS
Date: 2005 Feb 9, 06:36 -0800

```--- 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
> (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-
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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