A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bruce J. Pennino
Date: 2016 Aug 19, 07:48 -0600
Regards to all
From: "Frank Reed"
Sent: 17 Aug 2016 15:33:35 -0700
Subject: [NavList] Re: Historical charts and mapping
Don Seltzer, you wrote:
"But which reference meridian would have been in use in 1847? Would surveys be referenced to the Washington meridian of that time, providing a different offset to WGS84 coordinates?"
Well, I don't think anyone was really talking about a Washington meridian in 1835, but by the time of the re-printing in 1847 someone may have worried about it. There's only one longitude referenced in this particular chart: the Stonington Lighthouse. Let's suppose for now that they performed actual astronomical observations there. They would have used terrestrial instruments, but we can imagine them using a high-quality nautical sextant and a Mercury-filled artificial horizon. An altitude when the Sun bears more or less east or west gives local time, and the chronometer provides Greenwich time. Subtract and you get your longitude. But how, exactly, do we know that the chronometer reads correct Greenwich time? It has to have its error and rate checked at some location with a known longitude. So really our measured longitudes are referenced to that known longitude. I suspect, but don't know, that the Coast Survey in this era would have checked their chronometer errors at the nearest major port, probably New York in this case. Tricky, huh?