Sunlight Simulation of the Church of Saint Nectaire in Virtual Reality: a Digital Time Machine

Abstract

The roman church of Saint Nectaire, like many churches in Auvergne, France, is richly decorated with carved capitals. Most of them represent figures or symbols of Christianity and are periodically illuminated by the sunlight that comes to strike them at different times of the day throughout the year. The periodicity of these occurrences, which seem to correspond to a "targeted" temporality around religious feasts, appeals to historians who foresee the possibility of a perpetual religious calendar marking the times of the Christian liturgy with regularity and precision. The observations made since 2009 by Daniel Tardy [Dan13] have made it possible to highlight the high number of luminous phenomena, particularly concerning the remarkable lighting of the choir capitals: this has made it possible to hypothesize peculiar coincidences between the day of the luminous event and the date of the Julian calendar (used from 46 B.C. to 1582 AD) corresponding to the Christian celebration of the illuminated figures. The presence of hills, however, recurrently masks the sun at the beginning and end of the day and prevents the illumination on a certain number of sculpted figures that one would expect, given the number of calendar occurrences already observed elsewhere. Considering its experience in the field of digital survey 3D modeling and real-time simulation in the field of heritage [ASL15], [MDSB14], [NMRS13], [SCN*13], [Sal18], the MAP laboratory created a complete numerical model of the church and to submit it to a virtual heliodon in order to predict the illumination of the interior decorative elements at "critical" moments throughout the year if the surrounding hills did not exist: the question of the primary location of the church is currently the subject of many conjectures. This experiment consists in a methodological approach whose purpose is to validate a solar simulation method on an existing building and to verify its validity by direct confrontation between the simulation produced and the observable effects in reality. This not only allows us to make hypotheses about the constructive history of the the church of Saint Nectaire, but also - in the near future- to apply this method to several nearby churches, similar in their history, their architecture and their religious iconography.