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Lisa,
I found a confirmation to you hint that I interpreted
as the "rete was engraved with a STEROGRAPHIC projection
of the celestial sphere".

It is in Cotter's "History of nautical astronomy".

So now I understand how the device worked!
It was based on the same principle as the
"Wulf's grid" which I mentioned once on this list
Sun Oct 17 2004 - 00:44:05 EDT

Planispheric astrolabia was indeed an instrument
combining angle-measuring device, almanac and computer.

On its history Cotter writes:

"The great Hypparchus is usually credited with the
invention of planispheric astrolabe...
If, in fact Hypparchus did invent is, the instrument
of his invention could have been but a primitive form
of the complex astrolabes of a later age.
It is to the astronomers of India, Persia and Arabia that
honor is due for the perfecting of the planispheric astrolabe".

In this account he apparently misses the period when
Theon and his daughter Hippatia worked. (This was a relatively
short period of aborted renaissance of science in Alexandria
that followed the decline caused by the Roman invasion,
and ended soon after Hippatia's death, with the coming of
the Dark Age, when practice of science was completely interrupted
in the West).

Then he explains that this instrument was not for navigation
but an "astronomer's piece". And the mariner's astrolabe
was a much simplified instrument designed to take altitudes
only.

Alex.

   

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