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The sundial of the basilica of San Petronio in Bologna

A work of genius, capable of harnessing the sun on the earth

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Where is


Bologna BO, Italia (0m s.l.m.)


What it is and where it is

It almost risks going unnoticed, immersed in a setting overflowing with wonders like that of the basilica of San Petronio in Bologna. Yet the sundial kept there is nothing short of extraordinary: very large, incredibly precise and full of curious details that make it one of the best in the world.

Why it is special

It takes the form of a 67.72-meter-long metal line that runs along the floor of the basilica. On either side of this line, several marble slabs bear two sets of numbers, corresponding to two different systems of time measurement, the zodiac and solstices and equinoxes slabs. In essence, an extremely detailed and spectacular calendar-clock.

Not to be missed

Despite everything, however, the floor structure would be an end in itself if it were not for the light. This penetrates into the basilica through the gnomish hole in the ceiling, consisting of 2 concentric cones, the end of which is joined by a 27.1-mm-diameter pertussis. This particular composition of the hole, causes an even more incredible spectacle of the precision and size of this sundial to spill onto the floor of San Petronio. The point of light that marks the passage of time, in fact, is not a simple ray of light, but the very image of the sun, however inverted, captured by the hole that acts like the shutter of a camera obscura.

A bit of history

Our sundial dates from 1655 (in truth, another had already been drawn almost 100 years earlier) and is the work of Giovanni Domenico Cassini, who at only 25 years of age was called to occupy the chair of Astronomy at the University of Bologna and in 1666 was called to France by the Sun King himself, where the scientist died. after also holding the chair of Astronomy and Mathematics at the Sorbonne. His Bolognese work was admired by many over the centuries. Among them was Charles Dickens, who said that if the Basilica lacked attractions, the Sundial is 100 percent so.


The sundial of San Petronio gets people talking about mathematics and astronomy, but it also leaves room for legend. When the sun hits the columns, because of their grooves, it takes the shape of a heart. Tradition has it that the spinster who sees it will take a husband within a year and return to see the heart with her spouse. It is interesting to observe the visitors to the Sundial, accompanied by the guide who provides all the explanations, and when they have understood the mechanism , they never leave, rather they return to see it again and again.

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Susanna Leardini

Bologna is phenomenal!


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