Art Meets Astronomy in Florence: The Tools and Masterpieces of Danti and Masaccio

Art Meets Astronomy in Florence: The Tools and Masterpieces of Danti and Masaccio

Although celebrated primarily for its breathtaking artwork and ancient architecture, Florence is a city with a great deal of significance in a number of fields. When it comes to astronomy and astrology, Florence is, undeniably, one of the world’s most important destinations, home to some of the greatest names in astronomical research and discovery. Among these was Egnazio Danti, whose instruments and tools allowed for the understanding of the Earth’s journey around the Sun. An astrolabe attributed to him Galileo Galilei himself used for astronomical calculations (for this reason it is known as "Galileo's astrolabe"), and is preserved at the Museo Galileo.
But more than this, Danti’s work enhanced an arguably more essential aspect of Florence: its art. With the introduction of his sundials, quadrants, and ‘armillae’, Danti was able to highlight the masterpieces of artists like Masaccio and Vasari; particularly those found within the marble walls of the Santa Maria Novella Basilica.
Keep reading to delve beneath the cobblestone surface of Florence and unveil its rich artistic and astronomical history.

The Masterpiece of Masaccio

The Santa Maria Novella Basilica in Florence is home to a historic masterpiece: the Trinity. The Trinity is a fresco by Tommaso Guidi (also known as Masaccio), who is considered the father of Renaissance painting. The fresco is found in the third arcade of the left nave in the Santa Maria Novella Basilica.
Here, you can see the Eternal Father, the Holy Spirit below him as a dove, and Jesus Crucified between the Virgin and St. John the Evangelist. On either side, below, are the figures of the donors, a husband and a wife – possibly a widow, given the black veil covering her entirely.

It’s all in the (Renaissance) perspective

In 1570, Giorgio Vasari covered the fresco with a stone altar and a painting depicting the Madonna del Rosario. The fresco was rediscovered in good condition during a restoration which took place in the 19th century; it was removed from the wall and reassembled on the inner wall of the facade, between the central and the eastern portals. During another restoration in 1952, a fresco depicting a skeleton lying beneath the Holy Trinity was discovered underneath the plaster. Because of this, the detached mural was put back in its original place. Over the skeleton reads the memento mori WHAT YOU ARE, I ONCE WAS; WHAT I AM, YOU WILL BECOME.
This magnificent painted architectural creation is the first example of Renaissance perspective; we can see that the cross becomes the centre of the cosmos that it reveals.  Picture a virtual pyramid with the base at eye level, the apex corresponding to the vanishing point (usually in the middle of the composition, on a horizontal line) and the pyramid rays meeting at that point/apex, so much so that, for the first time ever, the space is rendered in a measured way. This means one theoretically simple yet astonishing thing: human rationality can understand and overcome reality, and place it in order. Giorgio Vasari describes the “barrel vault drawn in perspective, and divided into squares with rosettes which diminish and are foreshortened so well that there seems to be a hole in the wall".
Jesus Christ’s anatomy can be found in Brunelleschi’s Crucifix in the same church, although stripped of the final remnants of Gothicism. In this depiction, he breathes in greater humanity by broadening the chest and increasing the bend of the legs. Classical figures had already helped to guide painters like Masaccio towards a fuller understanding of the human figure and its muscular structures. The portraits of donors are earliest examples of convincing likenesses of people other than saints and rulers, who were often depicted in a fairly stereotypical way.

Art, illuminated

Cast your eyes to the majestic chapel, reminiscent of a Classical triumphal arch. It boasts fluted pilasters, crowned by Corynthian capitels, the entablature moulded with dentils and decorated by an elegant frieze. Admire the arch's pendentives (circular, dome-like structures), and the steps on which the two patrons are kneeling. All of these elements become real with the light falling from the left, casting shadow over some parts and creating a third dimension, carving them into the wall and making them break in the nave. As you look, space recedes into the picture plan. You, the viewer, are the point of reference.
But there’s something else about this work of art; something which has only recently been discovered by cartographer Simone Bartolini and architect Francesco Sgambelluri. From the 25th to the 31st March, at nine-thirty in the morning, it has been noted that a beam of light which enters from the second window in the main nave lands directly on the Trinity. It then gradually illuminates an area with the exact diameter of Jesus Christ’s outstretched arms, which correspond exactly with the arc of light.
Nowadays, this doesn’t take place during the Spring Equinox, but at the time the fresco was painted, from 1425 to 1426ish, there were eight or even nine days of difference between the ‘calendar’ equinox – the 20th to 21st March – and the ‘astronomical’ equinox. Back then, the Julian calendar lost one day every 128 years.

A calendar discrepancy

In fact, the Ancient Egyptians called the five days following the 360 they calculated when looking at the moon the “five days after the end of the year.” This was based on the Sumerians, who had already divided the year into 12 lunations. When looking at the sun, however, they realised how the day was divided up, i.e. into 24 hours. Actually, it doesn’t take the Earth exactly 365 days to complete an entire lap around the sun, but 365 and 6 hours circa.
When Julius Cesar, in 46 B.C., wanted to reform the calendar – which was then known as the Julian calendar, later reformed by Emperor Caesar Augustus in 8 A.C.– he turned to an Egyptian astronomer named Sosigenes. It was Sosigenes who introduced the idea of the leap year, so that the calendar could make up for those six extra hours. But the addition of a single day during the leap year wasn’t enough to fully bring the calendar up-to-speed. Why? Although the Egyptians were very close to the true duration of the tropical year (a calculation from one Equinox to the next), the duration differed by another 11 minutes and 14 seconds – hence why a day was ‘lost’ every 130 years.

The astronomical tools of Egnazio Danti

At the end of the 16th century, a solution to this problem was suggested. Cosimo I of the Medici House called astronomer Egnazio Danti from Perugia to court, giving him the official title of ‘cosmographer’: he was actually a cartographer, meteorologist, and anemographer. Born to a family of goldsmiths -he engraved his scientific tools himself-, a member of the Dominican order, Danti, who claimed he owed his knowledge of mathematics to his aunt Teodora, drafted the maps that still decorate the cupboards of the Sala delle Carte Geografiche (the Maps Room) of the Palazzo Vecchio today. He also made notable contributions to the improvement of scientific tools, as well as penning a number of widely circulated texts on the construction and use of the astrolabe and on astronomical topics.
In 1572-1574, knowing the city of Florence's latitude, he mounted a number of astronomical instruments (a quadrant and two equinoctial ‘armillae’) on the façade of the Santa Maria Novella basilica, so to better catch the sun beams.
The finely embellished quadrant to the right is made of several smaller solar quadrants with six sundials, each of them measuring time in six different ways. These include: the “oriolo all'Italiana” (Italian dial), which begins at midnight and counts 24 hours; the “oriolo boemico” (Bohemian dial), which begins at dawn; the “astronomical dial”, which begins at midday and counts 24 hours; the “oriolo comune de' Franzesi, Tedeschi, Spagnoli” (the dial used by the French, Germans, and Spaniards), which begins at midday and counts 12 hours, beginning again at midnight; a planetary dial; a canonical dial. They date back to 1572, while the ‘armillae’ date back to 1574.
Cosimo I of the Medici House, who was the newly appointed Grand Duke of Tuscany, had the quadrant engraved with these words: “Cosimo de' Medici, Grand Duke of Tuscany, himself devoted to the study of the noble arts, gave this [dial] to the students of astronomy in the year of Our Lord 1572.

Projecting the Equinox

One of the equinoctial armillae lies parallel to the Celestial Equator plane. Understanding, as he did, Florence’s latitude (43° 40'), Danti tilted the armilla so that it would align perfectly. The ring perpendicular to the wall is the meridian line, from North to South. When the Sun is at zenith with the Celestial Equator – i.e., when the rays of the sun are perpendicular to the Equator itself – we have an Equinox. This is the day of the year when night is equally long as daylight (from aequus, which means ‘equal’, and nox, which means ‘night’).
At midday of the Solar Equinox, this projects a perfect cross-shaped shadow on the church’s façade. It was essential to know the day of the Spring Equinox to be able to calculate the tropical year – especially when it came to Easter, with its ancestral Christian significance of sunrise and rebirth. Unfortunately, those armillae weren’t able to measure the Equinox but with a discrepancy of eight hours.
In that same church, Danti began the construction of two gnomons (a type of sundial), but was unable to finish them. After the death of Cosimo I, he was forced to leave Florence in circumstances which were never fully clarified; he moved first to Bologna, then to Rome.

Sundials and sunbeams

The two darkroom sundials are constructed of two pinholes – one in the façade’s rose window, and the other in the marble above it – and two metallic strips on the floor. These strips are positioned from South-North; the strip associated with the hole in the rose window represents the movement of the light beam in the time between the nadir and the zenith, and is around 41 metres long. In fact, most darkroom sundials in the world are actually found in Italy. The highest sundial in the Santa Maria Novella Basilica is also the third-highest in the world.
The metallic strips, however, are not original, but were constructed in 2016, inspired by the sundial created by Egnazio Danti in the Vatican’s Torre dei Venti (Tower of the Winds). This is Danti’s only remaining sundial. The pinhole in the rose window was reconstructed following the instructions provided by Leonardo Ximenes in his treatise, when the glass was dismantled and restored in 1982. In this way, the pinholes in the walls were reopened in 1983, before being tinted and refilled.
The Sun – not one of its rays, but the full projection of its image – penetrated the church through two pinhole gnomons, which were only a thousandth of the size compared to their height above the floor (20.45 and 26.56 metres). These were both opened primarily to calculate the Spring Equinox.
With the help of a white sheet of paper, it was once possible to see a black dot which floated on the border of the sun’s image; this dot was Venus..!

But in the end, the adjustment of this calendar with the Gregorian calendar (adopted by Gregorio XIII in 1582 with the papal bull Inter Gravissimas) was experimental. The centuries divisible by 400 were left as ‘leap’ centuries – such as 1600 and 2000 – and the 4th October passed directly to the 15th.


Not only is Florence home to some of the world’s most celebrated works of art and their artists, but is also known as an epicentre of astronomy and astrology. A city where art and astronomy go hand-in-hand, each enhancing the other, Florence is best explored on foot – ideally with an expert guide! Book my Florence Science Walking Tour and uncover a side to Florence overlooked by so many. Get in touch today and let’s chat.

Giovedì 06 gennaio 2022