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Frank, without having gone into details my thoughts went in a
direction as outlined below. For this I assume certain knowledges
which I presume that they should have been known from long periods of
observations. I just tried to find a way how it might have been
possible to obtain the longitude without the means which were in use
later.

Based on the assumptions, that
- one possibly could produce predictions of sunrise/sunset for a
reference place.
- one possibly could also produce predictions for the position of the
moon relative to the sun at the moment of sunrise/sunset for a
reference place.
- one possibly could measure the latitude from the sun's position and
- one possibly could also calculate the differences resulting from
changing the latitude such as changed sun-moon position due to changed
moments of sunrise/sunset.

At this stage one would then be able to calculate a new reference
value corresponding to the longitude of their reference location
corrected for their current latitude which might be different to the
reference location.

- It would remain now to measure the difference between the sun and
moon positions at the moment of sunrise/sunset at their actual
location and the calculated (corrected) one of the reference place.
It's here where I thought that the water-glass might become useful for
measuring the time difference between the two moments one being the
actual sunrise/sunset and the other one being the moment when sun and
moon have the angular position of the reference place.

I guess that their are still various snags in this "virtual"
procedure. In addition I have no idea on the attainable accuracy.

Marcel

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