Astrolabe Glossary and Notes


Plates are the plates that usually bear the stereographic projection of the heavens. The most common stereographic projection used in constructing astrolabes is one that projects onto a horizontal plane, usually coplanar with the cestial equator, as viewed from the south celestial pole. Such a projection varies with any change in the observer's latitude on earth. Thus, most instruments contain a number of plates, each engraved with a projection for a specific latitude. Because of this latitude dependence, plates have often offered some clue as to where the astrolabe might have been used. A person who lived in London would have certainly found a plate engraved for 52° (the value found on one of the instruments in the collection) more useful than one engraved for 40° (the latitude of Toledo Spain).

Occassionally, plates will contain other types of scales (e.g., Tablets of Horizons) or other information (e.g., astrological tables or gazetteers).

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