Solar phenomena in Santa Marta de Tera and San Juan de Amandi

Abstract

Romanesque churches usually have a canonical orientation to the east, so the path of the sun successively illuminates the east, south and west facades. The interior is usually dark and the lighting gaps are reduced, so the sun's rays illuminate the interior of the temple in a focused manner. Sometimes the ray of light can illuminate a singular element, which is usually called a solar phenomenon.

 Many of these phenomena have been known for a long time and always arouse the amazement of the spectators at the supposed astronomical knowledge of the medieval builders. In this article, the objective conditions that allow distinguishing a deliberate solar phenomenon from a purely casual one will be defined. Solar phenomena will be analysed with currently available calculation tools, but at the same time it will be shown that extremely simple means can also achieve this goal. They are simple techniques known since antiquity and were undoubtedly available to medieval builders.

This study will be particularized for two extremely interesting Spanish Romanesque churches such as Santa Marta de Tera in Zamora and San Juan de Amandi in Asturias. Solar phenomena already known as the «Equinoctial Light» of Tera will be analysed, but other leading lines that cause interesting solar phenomena and that have not been analysed up to now will also be studied.