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Alexandre,

This is so interesting.  Shall have to read the thread on Wulf's grid.

Following is a quote from DON QUIJOTE OF LA MANCHA, which I love....

  "And if I had here an astrolabe to take the
altitude of the pole, I could tell thee how many we have travelled,
though either I know little, or we have already crossed or shall
shortly cross the equinoctial line which parts the two opposite
poles midway."

   "And when we come to that line your worship speaks of," said Sancho,
"how far shall we have gone?"

   "Very far," said Don Quijote, "for of the three hundred and sixty
degrees that this terraqueous globe contains, as computed by
Ptolemy, the greatest cosmographer known, we shall have travelled
one-half when we come to the line I spoke of."

"Don Quijote of La Mancha" (Cervantes) Chapter XXIX

All the best,
Lisa

Alexandre Eremenko wrote:
> 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.
>
>


--
Kind regards
Lisa Fiene
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