How can you tell what time it is?
You would obviously take a look at your watch. But if your watch was wrong, or had it stopped, you would need a reference to reset it.
In the past centuries, most of Rome's commoners did not have to worry about these problems, because they simply did not have a watch, nor a clock, as these were still considered luxury items: they relied on the bells that rung from the many churches in Rome. But instead of marking the exact time, often the bells rung too early or too late, as the time was still measured rather inaccurately.

An official time reference for all the city was therefore an important need for Rome: pope Clement XI must have thought of this when, at the beginning of the 18th century, he gave commission to Francesco Bianchini to build an important and complicated sun-dial in the church of Santa Maria degli Angeli. Clement was also the pope who reformed the religious calendar, by introducing a standard parameter to determine the date of Easter day, every year: the first Sunday following the full moon after the spring equinox.

Michelangelo turned the remains of the ancient Baths of Diocletian into the church of Santa Maria degli Angeli

	the church's right transept; a red circle indicates the sun-dial	So Clement XI wanted the sun-dial to act both as a precise time reference for the city's benefit, and as a religious calendar for calculating each year's Easter day. The church of Santa Maria degli Angeli was chosen as a proper site for the sun-dial, because the building was strong and steady: it would have granted the absence of vibrations, which might have caused the floor notches to shift, thus making the time measurement no longer reliable.	the sun-dial in detail; the marble frieze below the hole was cut so that the light beam could reach the floor

This church is famous because it stands on the site of the great baths built by emperor Diocletian between AD 298 and 306, during the late years of the roman empire; their surface was over 360 x 370 metres wide (about 400 x 410 yards), and the church, despite being one of the largest in Rome, now covers only a small part of this area. When in 1565 Michelangelo drew the project of Santa Maria degli Angeli, he used the surviving central halls of the ancient complex, with their original columns. The same façade of the church is in fact a tall niche, or exedra, belonging to the roman structure.

some of the lines and notches on the church's floor Further lines indicate the Terminus Pascae (i.e. the date limits Easter day must fall within), and minor lines and notches indicate the position of the North Star, and other astronomical observations.		Clement's sun-dial was unveiled in 1703, and still today it stands on its original site. It consists of a large papal crest, that hangs above the right transept's wall, at a height of about 20 metres (65 feet), with a hole in its lower part: at mid-day, a ray of light passes through the hole, reaching a very precise scheme on the floor, made of lines and notches inlaid in coloured marble. The main line (known as "Clement's Line") shows the spot touched by the beam of light, each day of the year; by making some calculations, it is also possible to use the line and find the date. Along both its sides are marble panels featuring the zodiac constellations. two of the zodiac constellations

Clement XI's sun-dial acted as Rome's official time reference for about one century and a half. Meanwhile, other sun-dials had been built in Rome: one of them was housed in a small tower in piazza del Collegio Romano, behind the church of Sant'Ignazio di Loyola. However, by those days "mid-day" was still merely considered an astronomical time reference: in fact, the people in Rome used to count the hours of the day from the last religious function which all churches in the city held at a given time. The function, called by locals Avemmarìa (after the Ave Maria prayer), acted as a time reference for the rest of the evening, and for the following day; expressions such as "at twenty-one hours" actually meant twenty-one hours after the function (i.e. in the afternoon of the following day). Things were complicated by the fact that the evening prayer was held at about 7:15 PM, but during winter it was brought forward at 6:15 PM: therefore, "at fifteen hours" meant a quarter past ten in June, but a quarter past nine in January.

conversion table of the timing system in Rome until the mid 19th century

the astronomical tower in piazza del Collegio Romano

Only by the second half of the 19th century, in Rome too mid-day became the main reference from which the hours of the day were officially counted, replacing the religious time method. Despite the sun-dial, though, the bells kept ringing at different times. To avoid confusion, pope Pius IX introduced the custom of publicly announcing mid-day throughout the city in a rather curious way: as of December 1st 1846, every day at 12 o'clock, a cannon was fired from the top of Sant'Angelo Castle, striking the official time. The actual mid-day was measured from the above-mentioned tower behind Sant'Ignazio's church: as soon as the beam of light reached the sun-dial's line, a large sphere was lowered from the dome of the nearby church; this signal was seen from the castle, where the cannon was promptly fired.

This tradition continued in time, although mid-day was no longer measured from the small tower behind Sant'Ignazio. In 1903 the location of the gun shooting ceremony was moved to Monte Mario, a hill on the north-western side of the city, and then, one year later, to the top of the Janiculum Hill, the highest spot in Rome. The daily gun-shot was stopped during World War II, but in 1959, on April 21st (the day traditionally believed to be "Rome's birthday"), the ceremony was resumed. Since then, it has never been stopped again: still today, in the age of precision quartz watches, the firing of the Janiculum's gun is a popular daily event for roman people.

the cannon ceremony at mid-day; note the crowd, leaning from the balcony above

the cannon ceremony is a must for every child	Especially on weekends, crowds of children and tourists gather by the famous balcony on top of the hill, excitedly awaiting for the cannon to be rolled out: on a small terrace below, just in front of the breath-taking view over the city, three soldiers ritually load a howitzer, and after a short countdown they fire a blank shot, at 12 o'clock sharp. The blast is so loud that it can be clearly heard throughout the centre of Rome (despite nowadays the heavy traffic may sometimes cover it): many romans still check their watches to the sound of the peculiar "time alarm" from the Janiculum.