The Gregorian Calendar

Why a 16th century Pope decreed that 10 days would not happen

The cover page of the first printed
edition of the calendar, in 1582

The Gregorian Calendar, which is used today by every country in the world with just four exceptions, was introduced by Pope Gregory XIII on this day in 1582.

The calendar replaced the Julian calendar, which had been implemented by Julius Caesar in 46 BC but which was based on a miscalculation of the length of the solar year and had gradually fallen out of sync with the seasons.

The Catholic Church wanted to make the change because the actual spring equinox - one of the two days in each year when the sun appears directly above the equator - was drifting further away from the ecclesiastical date of the equinox, which in turn determines the date of Easter. 

In Christian tradition, Easter marks the resurrection of Jesus three days after his crucifixion, which historical evidence suggests occurred around the time of the spring equinox, nominally dated as March 21. 

The miscalculation in the Julian calendar seems tiny, an assumption that the average solar year was exactly 365.25 days when the reality is 365.2422 days. Yet even after the inclusion of the supposedly corrective leap year every four years, the error meant that over the 1,628 years of the calendar’s use, the gap between the actual equinox and the date of the equinox in the ecclesiastical year had grown to 12.7 days.

Aloysus Lilius, one of a number of Italian scientists invited by Pope Gregory XIII to submit proposals for how the calendar might be reformed, realised that the addition of a leap year on an unvarying four-year cycle still made the calendar slightly too long. 

Lilius - sometimes called Luigi Lilio or Luigi Giglio - came up with a variation that adds leap days in years divisible by four, unless the year is also divisible by 100. If the year is also divisible by 400, a leap day is added regardless. 

A bust of the scientist
Aloysus Lilius

The German mathematician, Christopher Clavius, described as the architect of the Gregorian calendar, made slight modifications but largely adopted the Lilius formula as presented.

The formula did not completely correct the problem, but reduced the drifting apart of the solar equinox and the ecclesiastical equinox to just a few seconds per year, which means it will take until 4909 for the solar year and the Gregorian calendar year to be just one day out of sync. 

The more dramatic part of the change came in correcting the cumulative effect of the Julian calendar’s miscalculation so that re-alignment could happen immediately. To make this happen, 1582 was shortened by 10 days.

Thus when midnight was reached on Thursday, October 4, the date of the next day was changed to Friday, October 15.

The Gregorian calendar also renumbered the leap day as February 29, ending the practice in the Julian Calendar by which every four years February 24 lasted 48 hours rather than 24.

Nowadays, only four countries in the world - Afghanistan, Iran, Ethiopia and Nepal - do not use the Gregorian calendar, although its acceptance when first introduced was by no means universal and it was several hundred years before it became recognised as the world’s calendar.

Pope Gregory XIII had a
reputation as a reformer

The countries and colonies of Spain, Portugal, France, the Polish-Lithuanian Commonwealth, Italy, the Catholic Low Countries and Luxembourg instituted it immediately.

However, the Protestant countries largely rejected the change at first because of its introduction by a Catholic pontiff. It was not until 1700 that Protestant Germany switched over, while Great Britain and its colonies - including at that time much of what would become the United States of America - remained faithful to the Julian calendar until 1752.

Another 121 years had passed before Japan made the change in 1873, China did not come on board until 1912 and Saudi Arabia - the last country to make the switch - not until just seven years ago, in 2016.

Many countries - including most of Western Europe - also agreed to standardise New Year’s Day as January 1 with the introduction of the Gregorian calendar. Until 1752, the start of a new year in Great Britain had been March 25 - the Feast of the Annunciation, also known as Lady Day.

The Piazza Maggiore, pictured at dusk, is the
heart of the city of Bologna

Travel tip:

Pope Gregory XIII was born Ugo Boncompagni in Bologna in 1502. Bologna is one of Italy's oldest cities, with a history that can be traced back to 1,000BC or possibly earlier, with a settlement that was developed into an urban area by the Etruscans, the Celts and the Romans.  The University of Bologna, the oldest in the world, was founded in 1088.  Bologna's city centre, which has undergone substantial restoration since the 1970s, is one of the largest and best preserved historical centres in Italy, characterised by 38km (24 miles) of walkways protected by porticoes.  At the heart of the city is the beautiful Piazza Maggiore, dominated by the Gothic Basilica of San Petronio, which at 132m long, 66m wide and with a facade that touches 51m at its tallest, is the 10th largest church in the world and the largest built in brick.

The rugged hill-top town of Cirò in Calabria.
the birthplace of Aloysius Lilius

Travel tip:

Aloysius Lilius, a doctor, astronomer, philosopher and chronologist, was born in Cirò, in Calabria, a rugged hill town about 40km (25 miles) north of the port city of Crotone, on the Ionian Sea coast, of which it offers commanding views. The site of a settlement since the Bronze Age, the town became an important regional centre between about 1300 and 1500, with a castle that has now fallen into disrepair. The town’s economy is based on agriculture, with the production of oil, wine, cereals and citruses as well as cattle breeding. Cirò is famous for the production of Calabria's most important wine, marketed simply as Cirò, a red wine made from Gaglioppo grapes, sometimes described as ‘Calabria’s Barolo’. On the coast below, Cirò Marina is a town of 14,000 inhabitants that has become a popular resort which has been awarded Blue Flag status for the quality of its sea water. Archaeological finds unearthed locally provide evidence of the area’s importance at the time of Magna Graecia.

Also on this day:

1633: The birth of physician Bernardino Ramazzini

1657: The birth of painter Francesco Solimena 

1720: The birth of printmaker Giovanni Battista Piranesi

1994: The birth of tenor Ignazio Boschetto

The Feast Day of Saint Francis of Assisi 

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Pope Gregory XIII

Pontiff used his power to change the date overnight

Pope Gregory XIII took his papal name in
honour of another reformer, Gregory I

Pope Gregory XIII was born Ugo Boncompagni on this day in 1502 in Bologna.

Gregory XIII is chiefly remembered for bringing in the Gregorian calendar, which is still the internationally accepted calendar today.

As Ugo Boncompagni, he studied law in Bologna and graduated in 1530. He later taught jurisprudence and among his students were the Cardinals Alessandro Farnese and Carlo Borromeo.

Before he took holy orders, Ugo had an affair with Maddalena Fulchini, who gave birth to his illegitimate son, Giacomo Boncompagni.

Pope Paul III summoned Ugo to Rome in 1538 to work for him in a judicial capacity. He went on to work for Pope Paul IV and Pope Pius IV. Ugo was made Cardinal Priest of San Sisto Vecchio and sent to the Council of Trent by Pius IV.

He was also sent to be legate to Phillip II of Spain and formed a close relationship with the Spanish King.

In 1572, after the death of Pope Pius V, the 70-year-old Cardinal Boncompagni was chosen to be the next pope and assumed the name of Gregory XIII, in homage to Pope Gregory I, who is remembered as a great church reformer.

Mathematician and astronomer Christopher
Clavius was co-writer of the calendar

Following in his namesake’s footsteps, Gregory XIII dedicated himself to reforming the Catholic Church and putting into practice the recommendations of the Council of Trent.

The Roman College of the Jesuits grew under his direction and became an important centre of learning. It is now known as the Pontifical Gregorian University.

Gregory XIII is best known for replacing the Julian calendar, which had been in use since 45 BC, with the calendar produced by astronomer Luigi Giglio and the German Jesuit priest, mathematician and astronomer Christopher Clavius, making the year slightly shorter.

In the Julian calendar, each year was too long, meaning that the March equinox had slipped back to an earlier date over the centuries.

The Pope decreed in 1582 that the day after Thursday, 4 October would be Friday, 15 October. The new calendar became known as the Gregorian calendar and is now used universally.

Gregory XIII encouraged Phillip II of Spain in his plans to dethrone Elizabeth I of England, causing English Protestants to regard all Catholics as potential traitors.

Detail of the monument to Pope Gregory VIII in the
Basilica of St Peter in Rome

He equipped an expedition to Ireland to help the Catholics in their struggle with the Protestants, but all the soldiers, sailors and women and children on board the boat were either beheaded or hanged on landing in Kerry, during what became known as the Smerwick Massacre.

In Rome, Gregory XIII had work completed on the magnificent Gregorian Chapel in the Basilica of Saint Peter and extended the Quirinale palace. He appointed his illegitimate son, Giacomo, as Castellan of Sant’Angelo and Gonfalonier of the Church. Venice enrolled Giacomo among its nobles and Phillip II made him one of his army generals. Gregory also helped his son acquire the Duchy of Sora on the border between the Papal States and the Kingdom of Naples.

To fund these projects, Gregory XIII confiscated houses and properties belonging to the church.

Pope Gregory XIII became ill with a fever in 1585 and died on 10 April, aged 83. He was succeeded by Pope Sixtus V, who found the papacy had been left considerably impoverished.

The campanile of the Basilica of San Sisto
Vecchia, where Gregory XIII was priest

Travel tip:

The Basilica of San Sisto Vecchia in Piazzale Numa Pompilo in Rome, where Pope Gregory XIII was Cardinal Priest for seven years, is one of 60 minor basilicas in the city. The basilica was built near the Baths of Caracalla in the fourth century and is dedicated to Pope Sixtus II, who was martyred in 258. His relics were transferred to the church from the Catacomb of Callixtus in the sixth century. San Sisto was rebuilt in the 13th century and restored in the 18th century, preserving only the bell tower, apse and a 13th century fresco cycle from the medieval church.

The dome of the Gregorian Chapel, finished by Giacomo
della Porta, in St Peter's Basilica

Travel tip:

Pope Gregory XIII commissioned the architect Giacomo della Porta to complete the work started by Michelangelo on the chapel in St Peter’s Basilica that was to be named after the pontiff. It has been described as ‘the most beautiful chapel in the world’ because of its marbles, mother-of-pearl, precious stones, gilded bronze, multi-coloured mosaics and stucco ornamentation. The monument to Pope Gregory XIII in white marble, executed by Milanese sculptor Camillo Rusconi, is in the Basilica near the entrance to the Gregorian Chapel. The Pope is portrayed giving his blessing on top of an urn bearing a relief showing the promulgation of the Gregorian calendar in 1852.

Also on this day:

1625: The death of religious composer Ruggiero Giovanelli

1655: The death of Pope Innocent X

1797: The tricolore flag is hoisted for the first time

1920: The birth of actor Vincent Gardenia


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Pope Sixtus V

Pontiff who cleaned up and rebuilt Rome and reformed church

Pope Sixtus V introduced a programme of
measures to tackle Rome's problems

Pope Sixtus V, whose five-year reign was one of the most effective of any pontiff in history, was born Felice Peretti on this day in 1521 in Grottammare, a coastal resort in the Marche region that was then part of the Papal States.

Succeeding Pope Gregory XIII in 1585, Sixtus V inherited an administration that was riddled with corruption and a city of Rome that to a large extent fallen into the hands of thieves and criminal gangs.

He responded with a series of measures that brought about profound change with far-reaching consequences for the city and the wider country, making his mark on a scale that few pontiffs had matched before or since.

As well as tackling crime with brutal ruthlessness, he introduced significant reforms in the administration of the Catholic Church and commissioned lavish building projects that changed Rome from a medieval city to a one of Baroque grandeur.

The son of a poor farm hand in Grottammare, the future pope entered a monastery when he was nine years old and joined the Order of Friars Minor three years later. His familiarity with adversity made him resourceful and strong.

After being ordained, he impressed many with his preaching and was a popular choice as pope, attaining office at the age of 64 following Gregory XIII’s death.

Pope Gregory XIII left Rome's finances in a
parlous state, while crime gripped the city

He was determined from the start to be remembered as a pope who left Rome and the Church in a much better state than the one he inherited.

The brigands and criminal gangs who held the city in their grip were his first targets, and he used the full strength of his papal forces to crush them, beginning a clampdown that would eventually see as many as 27,000 criminals and their mob bosses rounded up an executed.

As a deterrent to others, Sixtus V would regularly have the heads of executed brigands placed on top of stakes around the city.

The result was that within two years Rome was the safest city in Europe, the countryside was free of bandits and the economy prospered.

Sixtus tackled his reforms of the central administration of the church with similar zeal. By a papal bull - edict - issued in 1586,  he redefined the Sacred College of Cardinals by setting the number of cardinals at no more than 70, a limit that stayed in place until the pontificate of John XXIII (1958–63).

In 1588 he overhauled the Curia - the government of the Catholic Church - in its entirety, establishing 15 congregations - departments - in a structure that also remained substantially unchanged until the 1960s.

Pope Sixtus V had an estimated 27,000
criminals put to death in a ruthless purge

In his crackdown on waste and corruption, he introduced harsh financial policies, which included the sale of offices, the creation of new loans, regulation of prices and the imposition of new taxes.

Sixtus V was determined to refill the treasury, although he could hardly be accused of sitting on the new wealth he created.

Turning his attention to the urban plan of Rome, he ordered the creation of broad new streets that were the beginnings of how the map of the city looks today. The Porta del Popolo, where he placed an obelisk in the piazza, the Via Sistina and Via delle Quattro Fontane all owe their existence to Sixtus V.

The Cupola of St. Peter’s Basilica was completed after he ordered the architect Giacomo della Porta to finish what had been a ten-year plan of works inside 24 months.  Sixtus V also built the loggia of Sixtus in the Basilica di San Giovanni in Laterano and the chapel of the Praesepe in Santa Maria Maggiore, as well as commissioning repairs to the Quirinal, Lateran and Vatican palaces.

He restored Rome’s four Egyptian obelisks, including that in St Peter's Square.

Sixtus V restored the aqueduct of Septimus Severus - called the Acqua Felice after his baptismal name - in addition to the Acqua Vergine, which flows into the Trevi Fountain, with the purpose of bringing clean water into a less inhabited area and encouraging people to live there. The Palazzo Barberini and the Triton fountain were later made possible thanks to Sixtus V’s aqueduct.

Sixtus V placed a statue of St Paul atop the
Column of Marcus Aurelius, built in 193AD

Not everything he did was popular. He displaced many residents by razing buildings to make way for his new streets and showed little appreciation for antiquities. He envisioned converting the Colosseum into a wool factory with homes for its workers, and turned the Column of Marcus Aurelius and Trajan’s column into pedestals for statues of St. Peter and Paul.

In his foreign policies, Sixtus V proposed the conquest of Egypt and supported King Philip II of Spain in his planned invasion of England, with ambitions to excommunicate Elizabeth I, although in the event none of those things happened.

Sixtus V died on August 27, 1590, the last pope to have used the name Sixtus.  He was disliked by many of his subjects but history has recognized him as a significant figure in the Counter Reformation, and a pope who took on great enterprises and made significant achievements. He also left five million crowns in the coffers of what had been a bankrupt treasury.

The remains of the 16th century fortress stand over the
coastal resort of Grottammare, where Pope Sixtus V was born

Travel tip:

Grottammare is one of the beach resorts that make up the Marche region’s Riviera delle Palme, a stretch of coastline around the larger town of San Benedetto del Tronto. It is notable for a fine, sandy beach but also for the well preserved remains of a fortress overlooking the town that was built following the sacking of Grottammare by the Montenegran Princes of Dulcigno in 1525.  The centre of the older part of the town is Piazza Peretti, a square enclosed by the Church of San Giovanni Battista, the Town Hall, Municipal Tower and Teatro dell'Arancio.  Grottammare takes pleasure in celebrating its geographical position on the 43º parallel, the line of latitude that also passes through the cities of Assisi (Italy), Santiago de Compostela (Spain), Lourdes (France), Medjugorje (Bosnia), Vladivostok (Russia), Sapporo (Japan), Buffalo and Milwaukee (United States).

The Fountain of Moses at the end of the Aqua
Felice aqueduct into Rome

Travel tip:

The Aqua Felice was the first aqueduct built during the Roman Renaissance, more than 1,000 years after the ancient aqueducts in Rome had been cut by invading Goths. Sixtus V commissioned his favourite architect, Domenico Fontana, who came up with the Fountain of Moses which also serves as the terminus of the Aqua Felice. Although the fountain, sometimes known as ‘Fat Moses’, has never been particularly well-liked, it is seen as representative of the great urban renewal that took place in Rome during the Renaissance, particularly under Sixtus V.


More reading:

Urban VIII, the pope whose extravagance led to disgrace

Pius V, the pope for excommunicated Queen Elizabeth I of England for heresy

The kidnapping of Pope Boniface VIII

Also on this day:

The Festa di Santa Lucia

1466: The death of Renaissance sculptor Donatello

1720: The birth of playwright Carlo Gozzi


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Ulisse Aldrovandi – naturalist

Professor became fascinated with plants while under house arrest

Ulisse Aldrovandi: a portrait housed at the Accademia
Carrara in Bergamo, attributed to Agostino Carracci

Ulisse Aldrovandi, who is considered to be the father of natural history studies, was born on this day in 1522 in Bologna.

He became renowned for his systematic and accurate observations of animals, plants and minerals and he established the first botanical garden in Bologna, now known as the Orto Botanico dell’Università di Bologna.

Aldrovandi’s gardens were in the grounds of Palazzo Pubblico in Bologna but in 1803 they were moved to their present location in Via Imerio, where they are run by the University of Bologna but are open to the public every day except Sunday.

The professor was also the first person to extensively document neurofibromatosis disease, which is a type of skin tumour.

Aldrovandi, who is sometimes referred to as Aldrovandus or Aldroandi, was born into a noble family. He studied humanities and law at the universities of Bologna and Padua and became a notary. He then became interested in studying philosophy and logic, which he combined with the study of medicine.

Aldrovandi commissioned artists to
illustrate his catalogues of wildlife

He was charged with heresy in 1549, accused of supporting theories doubting the Holy Trinity, and kept under house arrest in Rome until he was absolved in 1550.

During this time he developed an interest in botany, zoology and geology and, as soon as he was free to travel, he organised expeditions to the mountains, countryside, islands and coastal regions of Italy to collect and catalogue plants.

After obtaining a degree in medicine and philosophy in 1553, Aldrovandi started teaching logic and philosophy at the University of Bologna.

He rose to become Professor of Philosophy and then in 1561 became the first Professor of Natural Sciences at the university.

Aldrovandi assembled a collection of curiosities, about 7000 nature specimens, for which he wrote detailed descriptions. He collected plants for an herbarium and preserved more than 4,000 sheets of dried specimens in 16 volumes. He also commissioned artists to make illustrations of his specimens.

After a dispute about the composition of a popular medicine with pharmacists and doctors, Aldrovandi was suspended from all public positions for five years.

Aldrovandi painstakingly dried and
preserved the leaves of plant species

In 1577 he sought the aid of Pope Gregory XIII, who was a cousin of his mother. The Pope wrote to the authorities in Bologna asking for Aldrovandi to be reinstated and also gave him financial aid to help him publish his books.

Aldrovandi left his vast collection of works on botany and zoology to the Senate of Bologna and after his death at the age of 82 in 1605 they were conserved in Palazzo Pubblico and then in Palazzo Poggi.

They were distributed among other libraries in the 19th century but in 1907 some of the collection was brought back to Palazzo Poggi. In 2005 an exhibition to mark the 400th anniversary of Aldrovandi’s death was held there.

A ridge of the moon is named Dorsa Aldrovandi and the plant genus Aldrovanda is also named after him.

The Ulisse Aldrovandi Museum at the Palazzo Poggi

Travel tip:

Palazzo Poggi in Via Zamboni in Bologna is now the headquarters of the University of Bologna and contains the Ulisse Aldrovandi Museum. The palace was built between 1549 and 1560 for the Poggi family and became the Home of the Institute of Sciences in 1714. During the Napoleonic period the various collections became scattered between different museums but at the end of the last century the University set up a project to bring the dispersed collections back to their historic home.

The Aldrovandi Botanical Gardens in Bologna

Travel tip:

One of the main sights in the small town of San Giovanni in Persiceto near Bologna is the Civico Orto Botanico Ulisse Aldrovandi, a municipal botanical garden established in 1985 and named in honour of the naturalist.

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Pope Urban VII

Pope for just 12 days but introduced world's first smoking ban

Pope Urban VII banned not only smoking
but chewing tobacco and snuff 

Pope Urban VII was born Giovanni Battista Castagna on this day in 1521 in Rome.

Although his 12-day papacy was the shortest in history, he is remembered as being the first person in the world to declare a ban on smoking.

He was against the use of tobacco generally, threatening to excommunicate anyone who ‘took tobacco in the porchway of, or inside a church, whether it be by chewing it, smoking it with a pipe, or sniffing it in powdered form through the nose’.

The ban is thought to have been upheld for the most part until 1724, when Pope Benedict XIII, himself a smoker, repealed it.

Castagna was the son of a nobleman of Genovese origin and studied in universities all over Italy. He obtained a doctorate in civil law and canon law from the University of Bologna.

He served as a constitutional lawyer to Pope Julius III and was then ordained a priest.

He took part in the Council of Trent and then served as an apostolic nuncio in Spain for four years.

Castagna was also Governor of Bologna, apostolic nuncio to Venice and then Papal Legate to Flanders and Cologne.

The courtyard of the Archiginnasio, part of
Bologna's historic university

He is remembered for his charity to the poor, for subsidising public works throughout the papal states and for being against nepotism.

Pope Gregory XIII appointed him Cardinal-Priest of San Marcello in 1583.

After the death of Pope Sixtus V, he was elected Pope on 15 September 1590 and chose the pontifical name of Urban VII.

But his papacy ended after just 12 days when he died of malaria on 27 September 1590 in Rome. He was 69 years old.

He was buried in the Vatican but his remains were later transferred to the Basilica of Santa Maria Sopra Minerva in Rome.

Travel tip:

Bologna University, where Castagna studied Law, was founded in 1088 and is the oldest in the world. The university’s oldest surviving building in the centre of the city, the Archiginnasio, is now a library and is open Monday to Friday from 9 am to 7 pm, and on Saturdays from 9 am to 2 pm. It is just a short walk from Piazza Maggiore and the Basilica di San Petronio.

The tomb of Saint Catherine of Siena can be seen inside
the Basilica of Santa Maria Sopra Minerva in Rome

Travel tip:

Pope Urban VII is one of several popes buried in the Basilica of Santa Maria Sopra Minerva, which is in Piazza Minerva, close to the Pantheon in Rome. The Basilica is the only surviving Gothic Church structure left in Rome and has the original arched vaulting inside. A sarcophagus containing the remains of Saint Catherine of Siena can be seen behind the high altar.

(Photo of tomb of Catherine of Siena by Medol CC BY-SA 4.0)

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