The ten days Portugal never had: the calendar change of October 1582

Thursday 4 October 1582 was the feast of St Francis of Assisi. The next day was a Friday, as it would have been anyway. But in Portugal its date was 15 October.2 sources

Nothing happened in the sky that night. Ten dates had been struck out of the calendar by order of Pope Gregory XIII, and in Portugal by a law that King Philip I had signed a fortnight earlier, on 20 September. The original of that law is still in the national archive, the Torre do Tombo in Lisbon.1 source

The days of the week ran on without a break. Only the numbering jumped. Spain and most of the Italian states made the same jump that night, and Poland changed in the same month. For the next 170 years a letter sent from Lisbon to London carried a date ten or eleven days ahead of the English one.2 sources

The calendar Portugal had used until then was the Julian calendar, named after Julius Caesar. It counts 365 days a year and adds a leap day every fourth year, so its average year is 365.25 days long. The year of the seasons, which astronomers call the tropical year, is shorter: about 365.2422 days, or 365 days, 5 hours, 48 minutes and 46 seconds.3 sources

The gap is a little over 11 minutes a year. That is 0.0078 of a day, enough to add up to a whole day in about 128 years. Over the 1,257 years from the Council of Nicaea in AD 325 to 1582 it came to almost ten days. Medieval astronomers knew this. A reform worked out at the Council of Basel in 1435, for one, proposed dropping a leap day once every 136 years.3 sources

For daily life the drift barely mattered. For the Church it mattered a great deal, because the date of Easter depended on the spring equinox. By 1580, by the reformers' reckoning, the real equinox came around 11 March, while the tables still assumed 21 March. The church's table of new moons, built on a 19-year cycle, had slipped as well, by about four days.2 sources

The solution came from Luigi Giglio, better known by his Latin name, Aloysius Lilius. He was a physician and astronomer born around 1510 at Cirò, in Calabria, who studied in Naples and settled in Verona. His proposal reached the papal commission in 1575, presented by his brother Antonio, also a physician. Luigi died in 1576, before it was adopted.2 sources

Lilius took the Alfonsine value for the year, which is about 1/134 of a day shorter than the Julian year. Since 1/134 is close to 3/400, the calendar would stay in step if it left out three leap days every 400 years. Here he followed Pietro Pitati of Verona, who had proposed dropping the leap day in century years in 1564.1 source

Lilius's own invention was for the moon. He replaced the old golden numbers with a cycle of epacts, the age of the moon at the start of each year, which could be shifted when it fell behind.2 sources

In 1577 the pope consulted Portugal's foremost mathematician, Pedro Nunes, the kingdom's chief cosmographer and for many years professor at Coimbra. Nunes died in August 1578. The commission, chaired from about 1577 by Cardinal Guglielmo Sirleto, printed a summary of the plan, the Compendium, and on 5 January 1578 the pope sent it to Catholic rulers and universities for comment.3 sources

The commission's best-known member was Christopher Clavius, a German Jesuit from Bamberg who taught mathematics at the Collegio Romano. He took part from at least 1577 and spent the rest of his life defending the calendar. As the last surviving member, he wrote the official explanation of it, published in 1603.1 source

Gregory XIII, then 80, signed the bull on 24 February 1582 at Mondragone, the papal villa near Frascati, and it is dated at Tusculum, the ancient town nearby. On 1 March it was posted on the doors of St Peter's in Rome. It takes its name from its opening words, Inter gravissimas, “among the most serious”. It did two things: it removed ten days once, and it changed the rule for leap years for good.3 sources

The new rule kept a leap day every fourth year but dropped it in century years, unless the year divides by 400. The bull spelled it out: 1600 would be a leap year, 1700, 1800 and 1900 would not, and 2000 would be. That leaves 97 leap years in every 400 instead of 100, an average year of 365.2425 days. Against the modern figure for the tropical year, 365.2422 days, that is about 26 seconds a year too long, or one day in roughly 3,300 years.3 sources

A papal bull did not change the civil calendar by itself. In each kingdom the ruler had to order it. In 1582 Portugal's ruler was Philip II of Spain. King Sebastião had died in 1578 without an heir, and the crown passed to his elderly great-uncle, Cardinal Henrique. When Henrique died too, Philip claimed the throne, and after his army won at Alcântara, outside Lisbon, on 25 August 1580, he ruled Portugal as Philip I. The two crowns stayed joined until 1640, a period known as the Iberian Union.3 sources

On 20 September 1582 Philip I issued the law by which Portugal adopted the new calendar. The Torre do Tombo describes the new calendar as the result of “the correction introduced by the mathematician Lilius” (in translation) to the calendar of Julius Caesar. It is kept in the archive's collection of laws, Leis e Ordenações, under the reference Leis, maço 3, nº 36.1 source

The bull itself set the day. After 4 October, the feast of St Francis, the next day was to be called 15 October and kept as the feast of St Denis and his companions. The Sunday that followed, 17 October, counted as the 18th Sunday after Pentecost. So that nobody lost money, the bull told judges, in any dispute over an annual or monthly payment, to add ten days to the due date.2 sources

The historian August Ziggelaar counts Italy, Spain and Portugal as the places where the reform went through most easily. In Spain and Portugal the king was the same man, and he had a stake in the details: the commission kept the equinox at 21 March partly because that meant no change to Mass books and breviaries, which Ziggelaar notes was a wish of the King of Spain. Poland, then joined with Lithuania, also switched in October 1582. Some Catholic countries took weeks or months longer.2 sources

News travelled at the speed of ships, and the bull allowed for it: places too far away to hear in time could make the change in October 1583, or later, once the letter reached them.1 source

The Torre do Tombo summary of Philip's law does not say how it was sent to Brazil, to Goa in India or to Macau in China, and no switch date for those places turned up in the sources consulted for this article. The historian Owen Gingerich noted that working out when the reform took effect in the Latin American colonies would need specialised research.2 sources

The Spanish side of the empire gives an idea of the delay. Philip signed a separate order for his American territories at Aranjuez in 1583; the printed copy is dated 14 May, though Gingerich gives 14 March. It reached Lima, capital of the Viceroyalty of Peru, on 19 April 1584, and the Jesuit José de Acosta wrote that the change there was set for October 1584. The Lima printer Antonio Ricardo printed the order that July. It is the earliest known printed work from a South American press.3 sources

In Europe, France followed in December 1582 after an edict of Henry III. In Flanders the switch came at the end of December, and Christmas that year never happened. An English merchant in Bruges, Thomas Stokes, wrote to Elizabeth I's secretary of state, Sir Francis Walsingham, on 23 December, by the English reckoning, that New Year's Day had just been proclaimed there, “so they have lost Christmas Day here for this year”.1 source

Protestant and Orthodox countries refused a reform issued by the pope. Protestant Germany and Denmark switched in 1700, Sweden in 1753. Britain, with its American colonies, moved in 1752 under the Calendar (New Style) Act, which also moved the start of the English legal year from 25 March to 1 January. Russia changed in 1918, under the Bolsheviks, and Greece in 1923.2 sources

For anyone reading Portuguese history, the rule is simple. Dates in Portuguese documents up to 4 October 1582 are Julian dates. To match a Julian date between the reform and the end of February 1700 with its Gregorian equivalent, you add ten days; after that, eleven.1 source

The trouble starts with countries that switched at different times. Between 1582 and 1752 an English letter and a Portuguese one written on the same day carry different dates. Writers of the time sometimes gave both, as in 23/13 December 1688.2 sources

In English the two reckonings came to be called Old Style and New Style, and English dates before 1752 have a second trap: the legal year in England began on 25 March, although Scotland already started it on 1 January, so a document from February 1750 belongs to what we would call 1751.3 sources

The switch also produced one of history's odd death dates. Teresa of Ávila, the Carmelite reformer and mystic, died at Alba de Tormes during the night of 4 to 15 October 1582, the very night Spain changed calendars. Depending on the source, her death is dated 4 or 15 October: the Catholic Encyclopedia of 1912 gives 4 October, while the Holy See speaks of the night of 15 October. The Church keeps her feast on 15 October.3 sources

The calendar that Philip I put into Portuguese law in 1582 is the one on every phone today. Since 1924, Gingerich noted, the whole world has used Gregorian dates for civil life, although religious calendars such as the Jewish and Islamic ones carry on beside it.2 sources

Easter is where the split survives. The Orthodox churches never accepted the papal reform as a whole. Some follow the Julian calendar and the old Easter rules, some the Gregorian rule and some a mixture. Since 1900 the Julian calendar has run 13 days behind, so churches that keep it celebrate Christmas on what is 7 January in the Gregorian calendar.2 sources

The arithmetic will need attention again, but not soon. With an average year of 365.2425 days, the Gregorian calendar drifts from the seasons by about a day in 3,300 years. Estimates vary with the figure used for the year: the University of Nottingham's guide to dating documents gives 3,700. The next scheduled change comes sooner, in 2100, which will not be a leap year.3 sources