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    Re: Longitude by altitudes. was Re: How Many Chronometers?
    From: Marcel Tschudin
    Date: 2009 May 14, 17:53 +0300

    Sorry for the water-glass instead of water clock. When I looked for
    the translation of this word I found only sandglass and concluded
    wrongly that the one with water would be called correspondingly.
    Frank you wrote:
    > 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.
    Regarding the logistics problem: Having seen on TV a "documentary" on
    the various expeditions which were undertaken to observe the Venus/Sun
    transits of 1761 and 1769 at various places of the earth let me
    question whether their would have been much earlier an easy solution
    to "think of a way around that".
    I'm not studying Archaeoastronomy, but there are strong indications
    that sun and moon have been well observed in the past and this over
    longer time periods; e.g. the Saros cycle was according to Wikipedia
    known by the ancient Babylonian astronomers. The Sky disc of Nebra
    http://en.wikipedia.org/wiki/Nebra_sky_disk which is even much older
    seems also to contain particular astronomical information gained from
    longer periods of observations. Unfortunately the English version of
    the Wikipedia page doesn't show the possible interpretations; the
    German version is somewhat more detailed. If the Saros cycle was
    known, it could possibly also be used for predictive calculations such
    as for setting up a sort of ephemerides. If the knowledge did exist
    should be sufficient to reasonably assume that one used it in some
    form of tables; if we haven't found such a document doesn't mean that
    it didn't exist.
    Don't misunderstand me. I don't want to "proof" here something. It
    just was an effort to sketch how a possible method might have been for
    measuring longitude without the means which became available later.
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