Plate |
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). |