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    Re: Longitude by altitudes. was Re: How Many Chronometers?
    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.
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