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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.

Marcel

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