During the summer solstice, an amazing spectacle can be witnessed in Florence Cathedral. The sun’s rays fall through a small opening in Brunelleschi’s dome, forming a disk of light that overlaps perfectly with the marble engravings on the floor of the chapel of the Cross, to the left of the high altar.
This phenomenon has occurred in the Duomo since 1475, the result of an ancient astronomical instrument known as a gnomon, which tracks the sun’s position in the sky and determines the length of the calendar year. A bronze tablet with a small opening, it is placed horizontally in the south window of the dome, 90 metres above the floor. The sun’s rays pass through the hole and hit the floor of the church for a few minutes shortly before and after noon. Documents in the Opera di Santa Maria del Fiore archives suggest that the gnomon was installed by Florentine mathematician Paolo Dal Pozzo Toscanelli.