Dal rilievo all’analisi di superfici complesse: il caso della pseudo-cupola di Galla Placidia

Manuela Incerti

«Archeologia e Calcolatori» 2022, 33.1, 55-72; doi: 10.19282/ac.33.1.2022.04

Abstract

Previous studies conducted on the Mausoleum of Galla Placidia (between AD 432 and AD 450) have allowed us to identify the geometric rule used for the creation of its starry sky and to propose the initial considerations about the morphology of its pseudo-dome. At first sight, the turns of the stars describe regular circumferences that seem to lie on a plane, in reality their trend is a space curve resulting from a projection on the surface of the ceiling from a point centrally located, approximately at the height of the base of the windows. The geometry underlying this tracing is based on a series of cones whose angle at the vertex increases progressively, and whose materialization is possible thanks to the knowledge of the operating principle of the astronomical instrument called Triquetro and Ptolemy’s table of chords. Apparently irregular, the ceiling is set on a parallelogram and is not traceable to operations of revolution of a curve, a cross vault or a pavilion vault. The section of the mesh with a set of horizontal planes with a distance of 10 cm has generated a set of curves, all different from each other, which we perceive connected by an unusual rule, not yet investigated. An initial analysis allowed to verify that we are dealing with hyperbolas. The present work proposes to continue the investigation of these curves and to hypothesize a construction technique that can be adopted in the context of a building site.

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