San Petronio: The longest covered sundial in the world

Piazza Galvani, 5, 40124 Bologna BO, Italia

Simona Bertolaso



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The sundial, also known more correctly as sundial or sciatere, cadran-solaire-d-aiguilles-restaure-par-remi-potey-382is an instrument for measuring time based on the detection of the position of the sun. It has very ancient origins, and in its most general sense refers to sundials on the walls of buildings. Strictly speaking, a sundial should only be understood as an indicator of the passage of the sun at noon. The needle of the sundial is the stylus, called gnomon; it is the rod that, through the shadow cast on the plate or dial of the clock itself, allows the indication of the time.
The longest covered sundial in the world. This was one of the first things my teacher explained to me on a trip to Bologna. The sundial is inside the Basilica of San Petronio, on the floor of the left aisle, and dates back to 1655. It measures a good 66.8 meters, equal to exactly the six hundred thousandth part of the earth’s circumference.

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