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John Huth asked about astrolabes.

Yes, they do indeed call for a separate plate for each latitude. If there's
only one, then you can usually get an idea which locality it was made for.
Often there were two alternatives, achieved by turning the plate over.
Sometimes there was quite a set of them, often marked for 2-degree latitude
differences. Also, universal astrolabes were made that could be used over a
wide range of latitudes.

It does indeed call for a lot of work to inscribe these things. And also to
cut the fretwork of star-pointers on the rete.

Only some were made precisely. Others degraded, from one generation to
another, being copies of copies of copies. Some were indeed used for
prediction and understanding of Sun and star motions, and could achieve
reasonable accuracy when used at the right latitude. Others were more for
show, a sort of ritual going through the motions for astrologers, so the
precision didn't matter too much. Rarely, there were adaptations to allow
Moon and planets to be predicted also, requiring the use of tables.

Still available as a remaindered book is "Astrolabes at Greenwich", at �30,
compared with its original price of �116, from
www.psbooks.com.uk.

The Museum for the History of Science in Oxford has made its complete
collection of astrolabes, (the most complete one there is) available as
high-quality images, showing complete details.

A useful website is "Keith's astrolabes" at
http://www.autodidacts.f2s.com/astro/ which includes a working model you can
drive around.

Another is James E Morrison's website at
http://www.astrolabes.org/astrolab.htm .

George

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message ----- 
From: "Apache Runner" 
To: 
Sent: Saturday, October 03, 2009 4:18 PM
Subject: [NavList 10056] Re: facing Mecca


| Frank -
|
| When I'm up to it....I'm still trying to figure out how astrolabes work.
| It looks to me like they need a different plate for each latitude.   I'm
| also curious how this critters were made - it looks like a lot of work to
| inscribe these things.
|
| John Huth
|
| |
|


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