# 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: 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"
http://jimthompson.net/boating/CelestialNav/CelestNotes/Coordinates.htm#Geod
esy

Jim Thompson
jim2{at}jimthompson.net
www.jimthompson.net
--------------------
Outgoing email scanned by Norton Antivirus

> -----Original Message-----
> [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-
> 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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