NavList:

"The battle was about a mile to the west, 53°12'N  000°02'W."

Aha. Thanks! Until you added this, I thought you were sending us on a snipe hunt. Is that a familiar name? According to the wikipedia article on snipe hunt, it's a traditional American "thing" (described as a prank... but also a fantasy game) for kids at summer camp [this could be the most over-footnoted article on wikipedia, which may be part of the joke].

Why did I think "snipe hunt"? Because when I zoomed in on the photo of the marker you posted, it references "Snipe Dales", which also has a wikipedia article, and in that article there's a photo of the Prime Meridian marker very similar to yours. I found this about ninety seconds after I saw your original post, but I didn't want to spoil the "hunt". :)

Back to navigation and navigation history:
The marker says: "The Greenwich Meridian, chosen as the Prime Meridian in 1884, passes through this point. Measurement of longitude was made possible by the development of an accurate maritime chronometer by Lincolnshire-based clockmaker John Harrison of Barrow-on-Humber".

Obviously the local boy gets his due, but I imagine there were some local lunarians c.1770 would have been appalled. And as far as 1884, the Greenwich Meridian was the de facto prime meridian thanks to, yes, lunars in the Nautical Almanac, a century earlier.

Finally, is the stone in the "correct" place? I would guess that the marker was originally placed in the pre-GPS era. Nearly as I can tell from some clues in Google Maps, the marker is about 320 feet west of GPS 0°00'00" longitude which is about 5" of longitude west. This appears to be very close to the displacement of the astronomical prime meridian at Greenwich to the west of the GPS prime meridian (roughly 330 feet?). So that fits. Given the high-quality of Ordnance Survey mapping pre-GPS, no doubt there were excellent pre-electronic tools available to place the marker. Or maybe it's post-GPS, and they placed it on the traditional Greenwich meridian for historical consistency.

Could we measure that small difference of longitude by sextant? You could set up a perfect artificial horizon, shoot the Sun on the prime vertical for a pure longitude sight, get all the corrections right... but the limiting factor is the same one that would have faced a fan of Harrison's chronometer or a fan of lunars. Five seconds of arc of longitude?? Good luck timing that!

We all know that four seconds of time corresponds to one minute of arc of longitude or 60 seconds of longitude.So if we want to detect a 5" offset in longitude we would need to time our sights to better than one-third of a second of time. And really to detect the longitude offset with any confidence we would need to time the observation ten times more accurately than that, on the order of a thirtieth of a second! Not humanly possible, but maybe with careful video of the sextant sight process. :)

Frank Reed