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    Re: Lunar eclipses and other things
    From: Alexandre Eremenko
    Date: 2004 Oct 26, 22:04 -0500

    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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