A visit to Bologna certainly starts from Piazza Maggiore, one of the symbols of the city, a concentration of medieval art and architecture that will leave you amazed. Here it is absolutely worth visiting the Basilica of San Petronio, inside which you can visit the longest sundial in the world. It is located on the floor of the left aisle and is a strata made by Gian Domenico Cassini around 1656. It measures 66.8 meters, exactly equal to the six hundred thousandth part of the earth's circumference. Every day, entering from the hole placed at 27 meters high in the vault, a ray of sun intersects the line, marking the slow and inexorable passing of days and seasons.
In 1655 the Fabbriceria di San Petronio decided to entrust the project of a new sundial line to "Dr. Gian Domenico Cassini Genoese". Cassini had already been teaching Astronomy in Bologna for some years and was noted for the accuracy shown in astronomical observations, including those of the comet of 1652, which he showed to be far above the orbit of the Moon, contrary to current Aristotelian ideas, which believed that comets were fumes from the Earth's atmosphere and not celestial bodies.
Cassini presented a daring project but had to overcome considerable economic, logistic, technical and even "academic" difficulties. The naves of the great basilica, which had been deliberately built so as to overlook the town square, did not have a north-south orientation. The greatest technical difficulty, therefore, was precisely that of being able to avoid that the path of sunlight was interrupted by the columns, being able to use as much as possible the large size of the building.
After accurate observations of the path of the Sun, the gnomonic hole was placed in the fourth vault of the left aisle, at a height equal to 1000 ounces of the royal foot of Paris (27.07 meters) and on the day of the summer solstice of 1655 the first stone of the sundial line was laid. The length on the ground of the line from the vertical point to the gnomonic hole, as predicted by Cassini, was equal to the six hundred thousandth part of the earth's circumference (66.8 meters).
On the occasion of the solstice, Cassini published a poster inviting all the citizens and professors of the University to attend the final verification of the meridian line and the passage of the image of the Sun "between those columns, which was believed to prevent its description".