The Shape of Time
Three ancient sundials can be found in Gravedona (Como) on the southern side of the Church of San Vincenzo, near the lake that bears the same name. Those dials offer a unique opportunity to compare different time systems.
The varying shadow cast by the sun provides the “dimension” of time, which people have shaped differently over the ages, adapting it according to their needs.
Before the restoration, two of the dials were in poor condition, as their surface faded due to the exposition to sunlight and other weather conditions.
Both sundials measured time with the ancient Italic time system but in different variants, while the third one, still in good condition and located on the wall of a nearby villa by the lake, shows how time was measured shortly after the unification of the Kingdom of Italy when for the first time all the hours in Italian cities were standardised to Rome’s time.
The possibility of observing those diverse time-keeping methods puts visitors in a sort of time portal where the different forms time has taken over the centuries coexist.
Position of the sundials
Sundials with Italic Hours
The practice of marking the divisions of the day by Canonical Hours, a system that had been used since the Romans, was replaced around 1200 by another system that became so widespread on the Italian peninsula that it became known as the “Italic time system.”
Following the Canonical tradition, this system also marked the end of the day and the beginning of the next — i.e. the twenty-fourth hour — at sunset, regardless of the time of year. Therefore, the hours that marked dawn and sunset changed throughout the seasons.
This time system, which may seem rather peculiar today, allowed people to know how many hours of daylight remained before nightfall, and that represented a crucial piece of information for the society of that era, which was largely devoid of artificial light and relied on the natural rhythms of day and night.
Towards the end of the 1600s, a variation of this method called the “Bell Tower Italic time system” became widespread. In this version, the twenty-four-hour mark is not set at sunset but 30 minutes after, at the end of twilight, when night begins.
These time systems were used until the 19th century, despite an Austrian edict banning them in 1786 in favour of the “French” time system.
Even the mechanical clocks on buildings and bell towers were periodically adjusted so that the twenty-fourth hour always coincided with sunset. Their dials had only one hand and were divided into twenty-four hours.
A beautiful example of a mechanical clock with Italic hours was frescoed in 1443 by Paolo Uccello inside the Florence Cathedral. On this dial, the hand still moves counterclockwise, mimicking the movement of a gnomon’s shadow on a sundial.
Many of the complex astronomical clocks that adorn some of our cities also have dials with the twenty-fourth hour positioned horizontally on the right, just like sundials with “Italic hours.”
Astronomical Clock of Torre dei Mori in Venice with the “XXIIII” Hour on the Right of the Dial
As the French hour system became popular throughout Europe, this atypical way of measuring time was confusing for the “Grand Tur” travellers who stayed in our country. A beautiful testimony to this is the description that Goethe gives of the Italic hour, where he expresses both surprise and appreciation:
Johann Wolfgang Goethe
“Memories of travelling in Italy in 1786-87”
“Verona, 17 September 1786”
“… We Cimmerians hardly know the real meaning of day. With our perpetual fogs and cloudy skies we do not care if it is day or night. … But here, when night falls, the day consisting of evening and morning is definitely over, twenty-four hours have been spent, and time begins afresh. The bells ring, the rosary is said, … this period of time varies in length according to the season, and the people who live here are so full of vitality that this does not confuse them, because the pleasures of their existence are related, not to the precise hour, but to the time of day. If one were to force a German clock hand on them, they would be at a loss, for their own method of time measurement is closely bound up with their nature.”
That last passage sounds almost prophetic if we consider the change that took place throughout the Italian peninsula between the second half of the eighteenth century and the early nineteenth century when the “French” system was imposed. The transition was traumatic, and the people were against it, as evidenced by the words of Giacomo Casanova’s cousin in the following excerpt:
Giacomo Casanova
“The story of my life”
“ … We fell into an incredible state of confusion. For the last three months, no one in Parma has known what time it is. As far as I can remember, the sun has always set at twenty-three and a half hours, and at twenty-four o’clock, we would recite the Angelus … Now, nothing makes sense anymore. The sun has gone mad, and it sets at a different hour each day … They call this a regulation, but do you know why? Because now everyone knows they have lunch at twelve. Some regulation! In the days of the Farnese, we ate when we were hungry, and it was much better.”
Conservation and Functional Restoration
conducted under the supervision of the Soprintendenza Archeologica Belle Arti e Paesaggio for the province of Sondrio
Thanks to the commitment and contribution of Mrs M. G. Rezia
Since the two dials were frescoed on religious buildings constructed between 1000 and 1600 and were subject to protection constraints, the conservation and functional restoration benefited from the assistance and supervision of the Soprintendenza Archeologica Belle Arti e Paesaggio for the province of Sondrio.
The actual restoration of the dials was carried out by Dr Mariachiara Fois, who was responsible for the analysis of the surfaces and their cleaning, the subsequent filling and application of lime colour washes based on the traces found, and finally, the tracing of the lines, following the original engravings.
East-Facing Sundial
Italic Time System
Condition
The dial on the east wall of the San Michele youth centre, which measures 182 cm wide and 151 cm high, was in poor condition, with a deteriorated surface and rare traces of colour still present in some of the engravings. The gnomon (the steel rod whose shadow acts as the hand on the dial) was missing, and a rainwater downpipe had been installed over it.
The dial had a wavy and irregular surface, with traces of hour lines attributable to two different time systems overlaid at different periods.
Functional restoration
The lack of flatness of the dial and the uncertainty regarding hour lines made it somewhat difficult to ascertain the accuracy of the layout and reverse engineer it in order to reconstruct the original position and dimensions of the gnomon.
The surveys revealed an orientation of the dial at 90°13′ east. Subsequently, the analysis of the engravings, compared with that information, allowed us to determine the gnomon’s length of 37 cm and to identify the ancient Italic time system within the lines engraved in the plaster. Those lines were later erased by applying lime putty on the dial, allowing the tracing of new hour lines in order to overlay the Bell Tower Italic time system.
Since the more recent Bell Tower time system had been heavily deteriorated by wear and exposure to weather conditions, Dr Fois decided to uncover the older layout of the Italic time system underneath.
The Italic time sundial starts counting the hours from sunset until sunset the following day.
In this type of sundial, the time is indicated solely by the shadow of the tip of the steel rod. Throughout the day, the shadow of the gnomon’s tip moves across the dial, touching the hour lines and indicating how many hours have passed since the previous day’s sunset and, by simple subtraction, how many hours of daylight remain before the next sunset.
The meridian line is absent on this east-facing dial, as the dial itself corresponds to the meridian plane.
Comparison between the existing hour lines, which are not very precise, and the correct layout.
On the dial, one can distinguish the horizon line, the inclined equinox line — traced by the shadow of the gnomon’s tip during equinox days — and most importantly, the spring equinox, which for Catholics determines the date of Easter. Even though the hour numbers were no longer visible, numbers were added to facilitate reading the instrument. A few remaining segments of the engravings from the more recent Bell Tower Italic time system are still visible.
South-Facing Sundial
Bell Tower Italic time system
Condition
The dial on the south wall of the Church of San Vincenzo, measuring 200 cm wide and 210 cm high, was also in poor condition, with a deteriorated surface and rare traces of colour still present in some of the engravings. The gnomon (the steel rod whose shadow acts as the hand on the dial) was still in place but bent.
This dial, too, lacked a regular flat surface.
Functional restoration
The lack of flatness in the surface made it difficult to accurately determine the orientation of the dial and is perhaps the reason for the error in the original diagram. The conducted surveys showed an orientation of the surface at 0°10’45” west, making it almost perfectly south-facing, in line with the axis of the church, which was built on an east-west axis. However, the analysis of the original diagram indicated a layout designed for an orientation of 04°05′ west.
Regardless, the dial was restored while preserving the original diagram, thereby retaining the chronometric errors resulting from this discrepancy. This decision was made to prioritise the preservation of the ancient work in its original state, also using the opportunity to highlight how, in the past, time was measured and accepted in a much more approximate manner than it is today.
The existing gnomon, projecting 36.5 cm and made of wrought iron with a square section of 10 x 10 millimetres and twisted decorations, was repositioned halfway between the position in the erroneous diagram and the correct one in order to minimise chronometric errors.
A Bell Tower Italic time Sundial, which starts counting the hours from the beginning of the night until the beginning of the following night.
In this type of sundial, the time is indicated solely by the shadow of the tip of the steel rod. Throughout the day, the shadow of the gnomon’s tip moves across the dial, touching the hour lines and indicating how many hours have passed since the beginning of the previous night and, by simple subtraction, how many hours of daylight remain before the next night.
Since this dial is facing south, it includes the meridian line, marked with the letter “M”. When the shadow of the gnomon’s tip passes over the meridian line, it indicates the moment the sun crosses the meridian of Gravedona.
Comparison between the existing hour lines, which are not very precise, and the virtually correct layout.
On the dial, in addition to the hour lines of the Bell Tower Italic time system, one can distinguish the horizon line, the inclined equinox line — traced by the shadow of the gnomon’s tip during equinox days — and most importantly, the spring equinox, which for Catholics determines the date of Easter.
Sundial set to Rome Mean Time
Description
This sundial, mounted on a wall slightly facing west, was designed to indicate Rome Mean Time through lemniscate lines drawn at half-hour intervals. It features a double dial — the left side reads the time in winter and spring, while the right one reads the time in summer and autumn. Both dials are equipped with gnomons with pinhole apertures.
There is no construction date on the sundial. However, its unique hour diagram, designed to indicate Rome Mean Time, dates it between 1866, when the newly formed Kingdom of Italy adopted Rome Mean Time, unifying the hours of all Italian cities to Rome’s time for the first time, and 1893, when Italy joined the international time zone convention, aligning its time system with Central European Time.
It can be considered a modern sundial, as it no longer indicates the local solar time but rather a conventionally established time, aligning with mechanical clocks and the changing needs of a society and economy transitioning from agriculture to industry.
Historical Background
In the past, sundials typically measured the local solar time based on the movement of the sun across the sky as it passed over the meridians of different cities. For example, when the sun is on the meridian of Milan, it marks true solar noon there, while in Trieste, located further east, noon has already passed. Conversely, in Turin, located further west, noon will occur later. That is why the true solar time indicated on ancient sundials is referred to as “true local time,” as it remains consistent only for places on the same meridian but varies for locations on other meridians.
In the 17th century, as mechanical clocks became sufficiently precise, it became evident that the solar day was not always exactly twenty-four hours. Depending on the season, it could vary by 20 to 30 seconds daily. Thus, the passage of the sun over the meridian, and hence the hours, were subject to regular cyclical fluctuations caused mainly by the orbit of the Earth around the sun and the variable speed at which the Earth travelled that orbit. Those variations combined swung systematically about 14 minutes ahead or 16 minutes behind, depending on the season. The concept of “mean time” was developed to address that issue, as it could be measured with mechanical clocks. It involved imagining a virtual sun that moved uniformly throughout the year, rendering those fluctuations null. The discrepancy between true solar time and the mean time indicated by mechanical clocks became known as the “equation of time.”
In the past, the margin of error in establishing time in relation to the virtual sun was indicated on meridian lines and in some well-crafted sundials directly on the true local noon line. That was done for each noon of the year, creating a sort of elongated figure-eight called the “lemniscate” or “equation of time curve.” That figure was formed by tracing two overlapping lines. To read them, one had to consider the line connecting the winter solstice to the summer solstice during winter and spring and the line connecting the summer solstice to the winter solstice during summer and autumn.
The use of true local time remained common as long as travel and communication were slow, comparable to medieval times. However, in the 19th century, with the development of railways and telegraphs speeding up interactions even between distant cities, the difference in local times began to pose problems.
Sundial of the cathedral of Como with a lemniscate line for mean time superimposed on the meridian line.
In Italy, shortly after the country’s unification in 1861, several railway time schedules existed. They were unified in 1866 by adopting Rome Mean Time (using Italy’s prime meridian at 12°27’08.4” east of Greenwich). Rome’s meridian was chosen due to its central position as well as political reasons, despite Rome not yet being part of the kingdom.
Rome Mean Time came into effect on December 12 of that year for railways, post offices, and telegraphs. Additionally, both for patriotic reasons and practical purposes, Rome’s time was adopted by public offices and private citizens, effectively becoming a national time.
The last modification to Italy’s time system happened when the country adhered to the international time zone convention established in 1884, which Italy adopted in 1893. Like most of Europe, Italy took Central European Time as its official time, based on the fifteenth meridian east of Greenwich.
This is the time Italy uses in winter, commonly referred to as “solar time,” because, although it is a conventional time, it remains the closest to true solar time, especially when compared to daylight saving time.
Reading the time
The two dials were designed to facilitate the reading of time, omitting any details that were not essential for their timekeeping function or any astronomical references, such as the equinox and meridian lines.
The hour lines are marked at half-hour intervals, with Roman numerals indicating the full hours and dots for the half hours on the lower part of the dial. Each hour line follows the curve of the equation of time, developing into a sinusoidal curve that stretches and bends according to the projection of the celestial sphere onto the vertical plane of the dial. When lemniscate lines are drawn less than an hour apart, as in this case, it is common practice to divide the sundial into two — one for reading in summer/autumn and one for reading in winter/spring. This division helps split each lemniscate into two parts and prevents them from overlapping, which would cause confusion when reading the time.
The light that filters through the pinhole in the gnomon’s metal disc indicates the time as it moves like a clock hand across the dial throughout the day. Unlike a conventional clock hand, this light spot changes its distance from the gnomon over the days, reaching the upper margin of the hour lines around the winter solstice and the lower margin during the summer solstice. Pinhole gnomons were typically used because the light spot that indicates the time is less blurred and thus more precise than the shadow cast by a traditional gnomon. The entire dial resembles a mathematical mirror of the sky, projecting an inverted image of the ecliptic section in front of the instrument. The light spot indicating the hours represents the sun, its position and its apparent path in the sky.
Comparing the sundial with our current time zone
The map below shows the longitudinal distance between the Rome meridian (12°27’08.4” east of Greenwich) — for which the sundial was designed — and the 15° East meridian, which is the reference for our current time zone. The sun apparently covers this longitudinal distance of 2°32’52” in 10 minutes and 11 seconds. Consequently, the sun rises, reaches its zenith at noon, and sets in Rome 10 minutes and 11 seconds later than at our current reference meridian.
Per poter confrontare la precisione dell’orologio solare, progettato per indicare il tempo medio di Roma, con il nostro attuale sistema orario basato sul 15° meridiano est, si devono aggiungere 10 minuti e 11 secondi al tempo indicato dall’orologio solare. Ovvero, ipotizzando che il punto luminoso cada sulla linea delle ore 11.00, il nostro orologio da polso dovrà (se ben sincronizzato) segnare le 11.10 e 11 secondi; se il punto luminoso indica le ore 11.30′ il nostro orologio da polso dovrà segnare le 11.40 e 11 secondi, e così via anche per tutte le altre linee orarie. Aggiungendo un’ora in più quando è in vigore l’ora legale.
To compare the accuracy of the sundial — designed to indicate Rome Mean Time — with our current system based on the 15° East meridian, you need to add 10 minutes and 11 seconds to the time indicated by the sundial. For example, if the light beam falls on the 11:00 hour line, your wristwatch (if properly synchronized) should read 11:10 and 11 seconds. If the light indicates 11:30, your wristwatch should read 11:40 and 11 seconds, and so on for all other hour lines. An additional hour should be added when daylight saving time is in effect.
Almost side by side, the two Italic Time sundials are simple, unadorned and essential in serving their time-keeping function. These two dials are far removed from our modern conception of time, embodying a very natural and rather approximate way of measuring time that was accepted for what it was. They are not necessarily works of great value but are extremely important for their history and ours. They represent a civilization and entire communities of the past that, for centuries, have marked their lives with the changing of day and night, following the natural rhythm of the seasons.
A short distance away stands another sundial, which is almost the opposite of the first two — a product of the Industrial Revolution. It is also devoid of decoration, and its dial is divided according to the seasons only for practicality, without being tied to astronomy or religion. The dial is in black and white, perfectly flat, stripped of the equinox and meridian lines, and its function is solely to mark the passing of time every thirty minutes with cold precision. A time that, for the first time, conformed to the rest of the Kingdom of Italy and from which emerges the transition to modern times; an artificial form of time that now had a value that could be traded.