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Re: Longitude by altitudes. was Re: How Many Chronometers?
From: Frank Reed
Date: 2009 May 13, 10:07 -0700
From: Frank Reed
Date: 2009 May 13, 10:07 -0700
Marcel, you wrote: "I guess that their are still various snags in this "virtual" procedure." I would say that the procedure you've come up, at least as far as I have understood it, is more complicated than necessary. Why not just go with Ptolemy's suggestion using a lunar eclipse? It was apparently well-known, and it's possible that it was known much earlier than Ptolemy's era. It has logistical problems, as I've mentioned, but maybe you can think of a way around that. Importantly, it requires NO advanced astronomical knowledge beyond what we know they definitely had, and it requires NO special instruments. By contrast, lunar distances or lunar altitudes (which are only different in details, not in principle) necessarily require EITHER an accurate ephemeris based on sophisticated astronomical knowledge of the Moon's motion OR a very long series of highly accurate recorded observations AND ALSO require some sort of instrument for measuring angles quite accurately, like a sextant. And we have no evidence for any of these anywhere in the world before the 18th century in Europe. Meanwhile... in the land of the weird... the ever-eccentric Gavin Menzies who contends that the Chinese explored the whole world, skipping Europe, in the early 15th century (c.1421) has apparently dropped his idea that lunar eclipses were used for longitude by those explorers. Instead, he says that they used the eclipses of the moons of Jupiter. He's not suggesting that they invented telescopes, which are generally required for observing the eclipses of those moons. No, he suggests that some Chinese had really, really good eyes... and they observed the eclipses with the unaided eye! Ya see, he's heard that some people with excellent vision can, under good circumstances, detect the outermost of the four great moons of Jupiter near their greatest elongation from Jupiter, which is true, and he's heard that the moons of Jupiter can be used to determine longitude. So he put two and two together and got twenty-two. Oy! Being able to catch sight of the outer moons when they are at their greatest separation from Jupiter is irrelevant to determining longitude. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---