In place of clocks, a sundial. In the Basilica of San Petronio, it is impossible not to know what time it is. In fact, the structure houses the longest sundial in the world. It measures 67 meters and crosses the floor of the church since 1657. It was Domenico Cassini who made it; a professor of astronomy, who believed that the motion of the planets depended on the Sun. According to this theory, therefore, the only way to study planetary orbits was to investigate the solar’s one.
The basilica was a perfect place for his experiments: large enough and with an already present sundial. The first was in fact inserted in San Petronio between 1575 and 1576 by the Dominican Egnazio Danti, who had already worked on the sundial of Santa Maria Novella in Florence. This first example, however, was destroyed by some lengthening works of the church. To replace it came the Cassini sundial, who to complete his project drilled the roof of the left aisle to 27.07 meters in height. In this way it was possible to see the sun without getting in the way. On the ground was instead drawn an iron line with a marble paving on which the zodiacal signs, the months and the equinoxes were engraved. The sundial was then restored in 1775 by Eustachio Zanotti, who replaced the iron of the floor line with brass.
Cassini’s idea ultimately proved to be successful: not only did he succeed in proving that the Sun has two types of motion, a real and an apparent one, but its meridian, equal to the six hundred thousandth part of the Earth’s meridian, became an instrument of precision. An instrument that even today allows us to recognize the solar midday for the whole year, in any season.