The gnomon of the Cathedral of Santa Maria del Fiore.
The measurement of cosmic time in the civil and religious heart of Florence
In almost every large western city, on the top of central buildings we can often see clocks. These are time measuring machines that keep the ‘temporal unity’ of the urban communities that host them. Consider Big Ben in London or the Prague Tower, but also the countless clock towers in Italy, both in large cities and small towns.
This ancient tradition comes from the Mediterranean civilizations. Even in the Agorà, the 'major' square of ancient Athens, we can still see today the marble 'Tower of the Winds’. It is an octagonal building, designed in the 1st century b.C. by the astronomer and architect, Andronico di Cirro. It is meant to mark different elements such as the cardinal points, the direction of the winds and it must accurately indicate the time of the day with the use of precise sundials. In Greek, it was called 'hōrológion', which means 'what the hour says'.
Tracing the time meant not only marking the life of the community according to the cosmic rhythms, but also ordering the calendar of religious holidays which, both in pagan times and then in Christian civilization, were arranged according to seasons and astral phases.
Also Florence, a city of Roman foundation and Christian since the 4th century, wanted to have a center of measurement of its space and time in the Middle Ages. At the beginning of the 13th century, the stoneworkers of the two major construction sites of the time - the Baptistery and San Miniato al Monte - placed two very similar zodiac wheels in marble inlay on the floor of both temples.
Although it is not historically ascertainable, the ancient chroniclers have told us how the zodiac flooring of the Baptistery used to mark the summer solstice on June 21st. The flooring was illuminated by a sun ray entering from the lantern through an instrument, as tradition says, designed by the captain of war and astronomer Strozzo Strozzi. Today this ‘astronomical clock’ would have unfortunately ceased to function because the zodiac wheel was probably moved over the centuries. Contrary, his ‘twin’ in San Miniato is still working and the portion of the zodiac decorated with the cancer symbol is perfectly illuminated by a ray of light that enters the Basilica at midday, marking the summer solstice. This suggests that the same might have happened in San Giovanni and that the chronicles’ testimonials are somehow true.
It was certainly believed so in the 15th century, when in the erected Cathedral dome, inspired by the Baptistery with its double roof and top lantern, they decided to place such a solstice measuring instrument called ‘gnomon’ (from the Greek gnṓmōn, ‘indicator’). It was the highest in the world at the time.
The gnomon is an indicator of the position of the sun consisting of a hole through which a well-defined ray of light seeps through. In fact, it projects a scaled image of the sun as it happens in a camera through the lens shutter, or the lens of a modern video projector. The cathedral gnome hole, called ‘bronzina’, is made of bronze and has a diameter of about 1/1000 of its height from the ground. This allows a clear projection of the solar disk on the floor. The ‘bronzina’ is located 90 meters high in a southern window of the Lantern. From the end of May to the beginning of July, the sun rays seep through it, hitting the floor of the ‘Chapel of the Cross’ on the north side exactly where a graduated meridian line and two ‘circular marks’ are located. At exactly midday on the solstice day, the solar image projected coincides precisely with the largest of these two superimposed marble discs, which has a diameter of 90 cm.
The designer of this astronomical machine was one of the greatest geniuses of the Florentine Renaissance: Paolo dal Pozzo Toscanelli (Florence, April 21st, 1397 - Pisa, May 10th, 1482), mathematician, cartographer, and astronomer. The gnomon started working between 1468 and 1475, when the dome lantern was completed. The relationship between Toscanelli and the cyclopean dome architecture which houses his instrument is profound: according to Vasari, Toscanelli was the one who taught mathematics to Brunelleschi and it was probably with his scientific support that the great architect conceived his extraordinary architecture. After all, the dome of the Cathedral is a ‘cosmic’ architecture inspired by the vault of heaven. What better place then to put an instrument for astral time measurement?
Do not forget that Brunelleschi himself was fascinated by the calculation of time: his biographers have told us about his passion for the construction of clocks and his is also the design of the mechanism of that in Palazzo dei Vicari in Scarperia (Florence). In Florence Cathedral counter-façade, stands the large 24h clock frescoed by Paolo Uccello, telling the time according to the ‘Italic hour’. The frescoed clock-face and its first clock mechanism were made in the 40’s of the 15th century, when Toscanelli and Brunelleschi were still alive.
However, that of Toscanelli is a more complex instrument which does not calculate the time of day but it aims at larger measurements, astral ones as we may say.
The purpose of the gnomon was, in fact, to accurately identify the astronomical date of the solstice and then precisely determine the duration of the solar calendar year. The purpose of this calculation was very important to maintain an accordance between the liturgical and civil calendar and the astronomical reality, in order to avoid further discrepancies as it was indeed happening at the time, due to the errors of the calendar in use, developed in ancient Rome.
Besides, being so tall, Toscanelli's gnomon perhaps responded to an even more ambitious goal: to determine the ‘ecliptic’, that is to verify whether the inclination of the Earth's axis remained constant; a question already debated by Arab astronomers and which in the years of Toscanelli had also raised interest in Europe.
In the following decades, the gnomon partially fell into oblivion and the protagonists of the calculation of astronomical time worked elsewhere in the city. The Dominican Ignazio Danti (Perugia, April 1536 - Alatri, 19th October 1586), thought of making measurements on the duration of the solar year starting the construction of a gnomon in Santa Maria Novella (together with the famous ‘armillary sphere’ placed in the counter-façade and marking the equinoxes). His studies in Rome led to the reform of the current calendar, called ‘Gregorian’, named after Pope Gregory XV who put it in place. Galileo Galilei’s observations (Pisa, 15th February 1564 - Arcetri, 8th January 1642) took place mainly in Arcetri with the use of his famous telescope.
Almost three centuries must go by after Toscanelli's death to start talking again about his gnomon and precisely 1754, when the great Jesuit astronomer, engineer and geographer Leonardo Ximenes (Trapani, 27th December 1716 - Florence, 4th May 1786), obtained funding for one of his research projects - again - on the problem of the ‘ecliptic’, for which he decided to use the ancient gnomon of the Cathedral. He therefore made the first archival studies on this instrument and perfected it, correcting slight geometric errors of design and location that gave it its current shape, and with it he carried out several measurements between 1755 and 1782.
Unfortunately, during the restoration works on the lantern of 1859, the ‘bronzina’ was dismantled and the gnomon disappeared. In 1864 the director of the Specola Museum Giovan Battista Donati (Pisa, 16th December 1826 - Florence, 20th September 1873) noticed it and decided to make new measurements. The following year the bronze ring was found in the Opera storage and was reassembled in 1865, despite at a slightly higher height.
From this moment, the ancient instrument of Toscanelli / Ximenes lost its function for astronomical studies as it had been overcome by more sophisticated telescopes in the meantime. However, it kept its value in controlling the stability of the Dome itself for some decades.
Eventually, the miracle of the ‘sun in the Cathedral’ has for some years been an event open to citizens and it attracts not only amateur astronomers. One cannot help but be enchanted by the appearance of the image of the sun in the sacred space conceived by Brunelleschi, and by its rapid and perfect running along the graduated sundial until it overlaps exactly the marble disk at the precise time of the ‘Florentine midday’. Those who watch are aware of being for a moment at the center of time and space of the city, as well as in the heart of the great history of Florence, made of genius and beauty.