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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: bubble sextant index error PS
From: Frank Reed CT
Date: 2005 Dec 10, 22:03 EST
From: Frank Reed CT
Date: 2005 Dec 10, 22:03 EST
Hi Bill, you wrote: "As list gurus have pointed out, a plumb bob will not necessarily point exactly to the center of the earth, and since a bubble level also depends on gravity, there is another variable to fret over." No need to worry about this. It's true that a plumb bob doesn't point to the center of the Earth. The plumb line is tilted by a dozen or so minutes of arc in mid-latitudes relative to a line pointing straight to the Earth's center. But the plumb line IS perpendicular to the horizon. It's perpendicular to the astronomical horizon by definition (the zenith is straight up as determined by a plumb line and the horizon is the great circle whose points are 90 degrees from the zenith), and it's almost exactly perpendicular to the sea horizon and a very flat land horizon in practice. So a bubble level can in principle provide the same information as the sea horizon, just less accurate. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars
