Planespheric projection Planespheric projection is the technique used to map the celestial sphere onto the plates of an astrolabe or dial. The accurate engraving of planespheric projection requires a high degree of mathematical knowledge and considerable skill. London instrument maker Henry Wynne was particularly well respected for this skill. The process that allows the three-dimensional sphere to be represented on the two-dimensional flat plate is called stereographic projection. Although Ptolemy designed his armillary sphere based on a flawed representation of the universe, it is impressive that he explained the underlying mathematical theory of stereographic projection in his Planispherium, thus establishing that stereographic projection was a characteristic of the astrolabe as early as the 100s AD.
