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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunars using Bennett
From: George Huxtable
Date: 2008 Jul 4, 08:15 +0100
From: George Huxtable
Date: 2008 Jul 4, 08:15 +0100
Perhaps, I'm starting to understand better what Ken Muldrew has been telling us about these Canadian surveyors' astronomical observations. Although Ken has clearly stated that Sun altitude was never measured, for the lunar, he is less specific about the Moon, but I will assume that the Moon was being treated in the same way. Let me summarise the matter in my own words, and then if I still have at wrong, Ken can correct me. To clear a lunar distance observation to get Greenwich time, the navigator needs the altitudes of the two bodies, for example Moon and Sun. But they don't need to be actually measured for that purpose; they can be calculated instead, from a rough assessment of the lat and long, well enough to arrive at a good estimate of Greenwich time, and hence a much better value for long. Because these navigators were travelling slowly over land, they always knew an approximate position, by reference to their previous position assessment. So they NEVER needed to measure Sun or Moon altitude (for the lunar) and never in fact did so. Their Sun and Moon altitudes, for the lunar, were always calculated instead.. Having found a value for Greenwich time, so that their watch-error was then known, two altitudes were then needed, one near or at the meridian to define latitude, and one (a time-sight) as near as possible to East or West, to define longitude. But these altitude measurements played no part in the lunar distance calculation, and didn't need to be taken anywhere near the same moment as the lunar. ===================== If I have that right, then it differs from Ken's final paragraph, which states- " My point was just to say that to get Greenwich time from a lunar you need to measure two altitudes and the lunar distance, they just don't need to be the altitudes of the moon and the other body used for the lunar distance." But those altitudes were NOT measured "to get Greenwich time from a lunar", were they? As Ken explained, the altitudes, for the lunar, were calculated, not measured. Those altitudes that Ken refers to were measured, not to get Greenwich time, but to get position, having determined Greenwich time. They could have been altitudes of any convenient body, not necessarily the Moon and Sun, just as he says. I'm trying to clear the matter up, not to make it more complicated, but I wonder if I've got it right? Perhaps Ken will say. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---