In the heart of Turin’s old town, there is a hidden, almost concealed place behind the facade of an ordinary, beautiful but anonymous building, on the square facing Palazzo Reale, an extraordinary treasure of sublime architecture, the presence of which is revealed by its beautiful dome, soaring over the roofs.

The church hides a wonderful secret that was revealed to me some time ago by an excited Lara Favaretto: inside the building, which was designed by Guarino Guarini, there are hidden paintings, visible only at certain times of year, when the weather is favourable.

In the middle of the six-pointed star in each of the church’s radial chapels, there is an oculus. These openings, which were deliberately designed to remain in the dark, are lit by natural light only twice a year: once during the equinox in the spring, and once during the equinox in the autumn.

Looking carefully through the oculus in the Cappella dell’Immacolata, when it is lit by the noonday sun that enters from the central dome, you will see a fresco, concealed until that time, of the Blessing God the Father. The reflecting light also reveals, above the altar opposite the Souls of Purgatory, a fresco of Christ.

It is an extraordinary natural and artistic experience – the fruit of Guarini’s brilliance and architectural, mathematical and astronomic knowledge – that repeats every year to astonish and amaze those lucky to see it.