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Re: Plumb-line horizon vs. geocentric horizon
From: Jim Thompson
Date: 2005 Feb 9, 10:50 -0400
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@jimthompson.net www.jimthompson.net -------------------- Outgoing email scanned by Norton Antivirus > -----Original Message----- > From: Navigation Mailing List > [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Fried Squash > Sent: Wednesday, February 09, 2005 10:36 AM > To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM > Subject: Re: Plumb-line horizon vs. geocentric horizon > > > --- Frank Reedwrote: > > 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