Abstract Summary
The slow changing position of the fixed stars with respect to the vernal point, directly observable through the stars’ changing ortive amplitude, were accounted for by diverging models during the Middle Ages. Ptolemy assumed a linear increase of stellar longitude over time. A text on the movement of the Eighth Sphere (the sphere of the fixed stars), sometimes attributed to Thābit ibn Qurra, circulated in the West and described a back-and-forth movement. In the Latin world this movement of “access and recess” was termed trepidation. The Toledan tables (11th c.) included this same movement of trepidation. In the Castilian Alfonsine tables (ca. 1270) trepidation of the Eighth Sphere is also considered. In Parisian Alfonsine astronomy those secular changes are described by combining a linear change (one revolution in 49000 years) with a trepidation movement. Very few spatial, material representations of these aspects of astronomical theories are known: today only about a dozen surviving armillary spheres are modelling the phenomenon of trepidation. Some such trepidation spheres from the 16th century are signed by the famous G. Arsenius. A systematic description and comparison of these spheres is still lacking. An outline of such a comparative study will here be proposed. It will have to tackle the relationship between competing theories, astronomical tables and canons on the one hand and visual representations, including diagrams and material models, like armillary spheres, on the other.
