San Petronio

01What it is

The Basilica di San Petronio fills the south side of Piazza Maggiore. It was begun in 1390 and dedicated to Petronius, the fifth-century bishop who is the city's patron saint — and it was built by the commune, not by the Church, as a statement of civic pride.

Look at the facade and you can see where the money ran out. The lower band is dressed in carved marble; everything above it is bare brick, waiting for a facing that never came.

02The door and the reliefs

Around the central door, the Porta Magna, are the relief panels of Genesis carved by Jacopo della Quercia in the fifteenth century. Michelangelo saw them as a young man, and it shows in his own work later.

03The line of light in the floor

Running diagonally across the left aisle is a brass meridian line, laid out by the astronomer Giovanni Domenico Cassini in 1655. A small hole high in the vault lets a disc of sunlight fall on the line at local noon, and where the disc lands tells you the day of the year. It is one of the longest instruments of its kind anywhere.

04A coronation and an unbuilt church

In 1530 Pope Clement VII crowned Charles V as Holy Roman Emperor here — the last imperial coronation performed by a pope.

Note The basilica was planned larger still, with arms reaching east. The story everyone in Bologna tells is that the papacy saw to it that it never outgrew Saint Peter's, and that the Archiginnasio, built right beside it in the 1560s, stands on the ground the extension needed. Treat the motive as legend; the building beside it is fact.