There are two ways of looking at San Petronio, and they lead to two different articles. The first is from below, on piazza Maggiore, and concerns a facade that stops halfway. The second is from the floor, inside the left nave, and concerns a marble line 67 metres long which for two centuries was the most precise astronomical instrument in Europe.
They are the same story told twice. It is the story of a city that set out to build the largest church in Christendom, did not manage it, and with the space it had left made something else.
Entry to the basilica is free. The practical information is at the end, with the date on which I checked it.
The project that would have outdone St Peter’s
The building site opens in 1390, and it is not a bishop who opens it: it is the Comune. San Petronio was born as a civic church, built with public money and dedicated to the city’s patron saint, and that is why it faces the square rather than the bishop’s palace.
The design attributed to Antonio di Vincenzo called for a Latin cross of enormous dimensions: some reconstructions make it longer than the Vatican basilica. It was never built. After three centuries of work, and after building the naves alone without the transept, the site stopped: today San Petronio measures 132 metres by 60, remains the largest Gothic brick church in the world and is the fourth largest church in Italy, but it is a fragment of the design.
Why it stopped is still debated. The traditional explanation is that Pope Pius IV, in the sixteenth century, diverted the funds towards the nearby Archiginnasio to stop a municipal church from outdoing St Peter’s. The technical and financial reasons, less romantic, are enough on their own: a structure of those dimensions in brick, without stone buttressing, posed problems fifteenth-century Bologna did not know how to solve.
The facade: where to look, and what to see that is missing
The facade is the point where this story becomes visible to anyone, without needing to know it.
The cladding in pink Verona and white Istrian marble covers the first third, drawing the three portals and the panels; then it breaks off with a clean cut, and above it twenty metres of bare brick continue. It is not a ruin and it is not decay: it is the level at which the money ran out, frozen since the sixteenth century.
Between the sixteenth and the seventeenth century the city held competitions to complete it, and big names tried: Giulio Romano, Vignola, Palladio, Baldassarre Peruzzi. No design was ever built. The drawings are kept in the basilica’s museum, and it is a collection worth the visit on its own: they are the facades San Petronio never had.
On the central portal, on the other hand, there is something that was finished, and finished superbly. These are the reliefs by Jacopo della Quercia, carved between 1425 and 1438: along the jamb panels runs Genesis, with a Creation of Adam and an Expulsion from Paradise that Michelangelo studied on a visit to Bologna, and that reappear in the Sistine vault seventy years later.
Cassini’s meridian, which served to measure the year
As you go in, along the floor of the left nave, runs a strip of marble with numbers cut into it. Almost everyone steps over it.
It is the meridian of Giovanni Domenico Cassini, traced in 1655 and restored by Cassini himself in 1695. It works like this: a hole in the vault, about 27 metres up, lets in a ray of sunlight which at noon falls on the line. The position of the bright spot along the 67 metres gives the day of the year, because the height of the sun changes with the seasons.
The point is not the curiosity, it is the use. Cassini was measuring the obliquity of the ecliptic and the length of the tropical year, and the length of the line gave him a precision no mural quadrant could match: the longer the instrument, the smaller the relative error. With this meridian and with his later measurements, seventeenth-century astronomy came to establish that the year is not exactly 365 and a quarter days, and to confirm that the Earth’s orbit is not circular.
One curiosity: Cassini chose San Petronio precisely because it was unfinished. He needed a very long, tall nave with no transept interrupting the floor and a wall in which to open the hole. The failure of the building site handed him his instrument.
Going towards noon, the ray is visible. Shortly before 13.00 in summer time, to be precise, because the meridian marks local solar noon and not the time on the clock.
The nave, and how you look at a brick church
The interior disconcerts anyone arriving from Tuscan or Roman churches. There is no marble on the walls, no gold in the vault: there are clustered brick piers with white sandstone capitals, and very tall pointed arches alternating the red of the brick with the white of the stone.
The rhythm is that of Italian Gothic, which is to say broad rather than vertiginous: the bays are wide, and the eye runs along the length instead of upwards as it would in a French cathedral. The twenty-two side chapels are all square and all the same size, closed by marble screens or iron gates, and they are the basilica’s real museum: inside are works spanning five centuries.
On the ceiling and in the webs, on the other hand, look for nothing: the vaults are plastered and bare. This too is an effect of the interrupted site.
The Bolognini chapel and Giovanni da Modena’s Hell
Of the twenty-two chapels one should be sought out deliberately, and it is the fourth in the left nave: the chapel of the Magi, called Bolognini after the merchant who had it decorated around 1410.
Its walls carry a cycle of frescoes by Giovanni da Modena that ranks among the most impressive things in Italian late Gothic. On one wall is Paradise, on the other Hell, and Hell is the reason you go in: an enormous Lucifer at the centre, with three mouths chewing the damned, and around him a highly detailed taxonomy of torments organised by sin, in which usurers, sodomites, misers and simoniacs can be told apart.
What strikes an art historian is how Dantesque it is: the cycle is a century after the Commedia and follows its moral geography, not that of the medieval bestiaries. It is one of the first times a painted Hell stops being a catalogue of monsters and becomes an ordering.
Attention: the chapel is visited with the ticket that also covers the chapels of San Sebastiano and San Vincenzo Ferrer, and it costs five euros.
The museum, which is free and almost nobody visits
At the end of the right nave is the entrance to the basilica’s museum, and entry is free.
Inside are the drawings and wooden models of the facades never built, which is to say the documentation of four centuries of attempts; the illuminated choir books; the goldwork; and the wooden models of the original design, which let you see how large the completed church would have been. If the story of the facade has caught your interest, this is the room where it closes.
Practical information
Figures checked on 1 September 2026 on the official site basilicadisanpetronio.org. Hours can change for services: during celebrations tourist visits are limited.
Tickets. Entry to the basilica is free, and entry to the museum is free as well. The ticket for the three visitable chapels (Magi, San Sebastiano, San Vincenzo Ferrer) costs 5 euros full; the reduced ticket costs 3 euros and applies to 11-18 year olds, over 65s, guided groups and university groups. School groups pay 2 euros. Children up to 10, people with disabilities and members of religious orders go in free.
Hours. The basilica is open every day from 10.00 to 18.00, with last entry at 17.50. The museum is open every day from 10.00 to 17.30.
Attention: the panoramic terrace is permanently closed. The basilica states that “following the conclusion of the restoration work the panoramic terrace has been permanently closed”. If you find a tour on sale that includes it, the offer is out of date: check before paying.
Free admission. Access to the basilica and the museum is always free.
Getting there. The basilica is on the south side of piazza Maggiore, a fifteen-minute walk from Bologna Centrale station along via dell’Indipendenza, arcaded the whole way.
How long. The basilica with the museum needs an hour, and half an hour more if you add the three chapels. If you come for the meridian, it is worth arriving around solar noon, which in summer falls shortly before 13.00.
Booking. Needed only for groups of fifteen or more.
If you are building a wider itinerary, San Petronio is the first stop in What to see in Bologna in 2 days. For the other two great Bolognese stratifications there are Santo Stefano and the Seven Churches and the Pinacoteca Nazionale, where the polyptych by Giotto hangs. And there is a link worth knowing: once a year the Byzantine icon of the sanctuary of San Luca comes down in procession and stays on show for a week right here, in San Petronio.
Photo credits: facade of San Petronio by GiorgioGaleotti, meridian and nave by Rosapicci, the Jacopo della Quercia portal by Fabrizio Garrisi and the Bolognini chapel by Yair Haklai, Hell by Giovanni da Modena photographed by Fabrizio Garrisi (CC BY-SA 4.0); title page of Cassini’s treatise in the public domain. All from Wikimedia Commons.
