The most beautiful doormat in the history of art is on the floor of the Baptistry, near the north door. It is a marble slab that has been trampled on for centuries without any thought, and which starting a few years ago – perhaps heeding some of my heartfelt recommendations – the Opera del Duomo has protected from walking on it.
This marble slab represents the heavenly vault with the sun in the center, and around it the eight spheres of the Moon, the planets and the fixed stars. In a geometric sense, the image is a map, meaning the orthogonal projection of a hemisphere (the starry sky) on a horizontal plane (the earth), and it is absolutely extraordinary to see that this image was made by correctly applying the theoretical principles of projective geometry as described by G. Monge in the 18th century. The circles then are crossed by spirals formed by curved triangles shaped one by one, which create a hypnotic effect of rotational motion – a masterpiece of art and science.
To think that this image was created in the Romanesque period is obviously untenable, and so is attributing it to some unidentified Middle Eastern schools from that time. Therefore how did they conceive, in Late Antiquity and in the context of the construction of this building, this singular marble piece and accomplish it with such fine workmanship?
Here are a few traces that might lead us towards a possible explanation.
In order to celebrate the glory of the Emperor after his victory against Radagaisus, the architect of the Temple thought of some sun symbols. One took the highest beam from the annual path of the sun and projected it at noon on June 21st at a specific spot inside the building. This is what is related by the chronist Villani; but there is no trace of this projection on the floor by the north door, where the sunbeam would inevitably have to fall.
Near this spot there is rather the abovementioned marble slab, which carries obvious solar symbolisms. If this slab was created to receive the projection, why don’t we find it in the right place?
The answer: when the architect made the design he was far away, probably in Greece or in the Middle East, and he didn’t know the exact angulation that the solar beam would have had in Florence. He made some calculations anyway and he prepared the marble for the projection, with the foresight that it could be placed to measure on site. Thus, he created in the floor a rectangular panel with a design that allowed him to slide the marble in once he found the right adjustment. This was something which for whatever reason was not done: perhaps in the hurry of not waiting until June 21st the workers limited themselves to putting the marble in the center of the frame, with a simple decorative function.
These traces do not completely exhaust the mystery of our doormat, but perhaps they can help in further investigation.
Above: the solar marble near the north gate, the projected concept used to design its image and a few details that show the supreme quality of its workmanship.
Left: the solar projection inside the Baptistry at noon on the summer solstice, as it would have happened according to the story of Giovanni Villani, in a graphical reconstruction by the Museum of the History of Science in Florence.