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Ptolemy's work ANALEMMA (Gr. Peri analemmatos: Latin: De analemmate)
discusses the details of the projection of points on the celestial
sphere onto three planes at 90 degree right angles to each other - the
horizon, the meridian, and the prime vertical.

In another book, the PLANISPHAERIUM, Ptolemy discusses the principles of
stereographic projection, whereby 3 dimensional components of the
celestial sphere can be projected onto a flat surface. (Hypatia would,
most likely, have come across his works).

After a quick look on the net, I've found:

http://www.hps.cam.ac.uk/starry/ptolsund.html
http://www.chass.utoronto.ca/~ajones/ptolgeog/astroworks.html

For an actual illustration of the basic principle, I can fax you
something, if you have a fax #.  Just email me direct with your fax, if
you like.

All the best,
Lisa




Alexandre Eremenko wrote:
> Lisa,
> Thank you for your explanation.
> But what does this have to do with the stereographic projection?
> (This was my question). You seemed to mention such connection
> with a reference to Ptolemy in your first message on Hyppatia.
>
> As I understand from your description, this was not only
> a measuring instrument but also a star almanach and
> an (analog) computer in one device. The pictures I see in my books
> do not correspond to your description completely.
> I suppose there were different types of Astrolabia.
>
> Was this "rete" a stereographic projection of the Celestial
> sphere??
>
> Alex.
>
>
>>PS I know a man in Tasmania who builds workable Astrolabes,
>>however this
>>is for the Northern Hemisphere only,
>>so I've never been able to ever
>>actually use one.
>
>
> I've seen some astrolabia(s) (ae?) on the web.
>
> In  the usual navigation equipment stores.
> Made of plastic and cardboard.
> Maybe not so nice as the ones handcrafted in Tasmania,
> but I believe they are workable.
> Don't know about Southern hemisphere, but I can find the
> web references for you.
>
>

   

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