The astronomical orientation of the San Galgano Abbey

San Galgano Abbey is a Cistercian abbey in Tuscany, a few kilometres from Siena. The building of its great church started in 1218. Here we show that the direction of the nave was that of the rising of the sun and of the full moon on 14 March 1218 (Julian Calendar). Probably, it was this rare astronomical event that determined the direction of the future church.