The Sunlight and the Hidden Paintings in the Church of San Lorenzo in Torino

The Real Chiesa di San Lorenzo is a church designed and built by Guarino Guarini. This church, adjacent to the Royal Palace in Piazza Castello, is one of the most beautiful churches in Torino. It possesses a remarkable cupola, made by a series of nested octagons. The lower part of the church has no windows. The natural light is coming from large windows above the base of the dome and other smaller openings in it. Besides the high altar, we find in the church two lateral altars and four radial chapels. In the chapels, Guarino Guarini has hidden some paintings inside their double vaults. These paintings are visible only when the sunlight is passing through a specific oculus in the front of the chapel. This happens when the sun has the right azimuth and altitude. Actually, the architect had used his knowledge of astronomy and geometrical optics to create this effect. Here we discuss it and the required position of the sun, using software for evaluating solar altitude and azimuth