The Gregorian calendar

Roger Bacon

After the fall of the Western Roman Empire, the Julian calendar remained in force for more than a millennium. Although very approximate, it was not perfect. The duration it assigned to the year was 365.25 days, while its actual duration is 365.2421988... days. Consequently, the error made is 0.0078011... days per year, about 11 minutes and 14 seconds, which may seem small, but over a thousand years, several days are accumulated. The error amounts to approximately one day every 128 years, or about three days every 400 years.

In the 13th century, since the Council of Nicaea, the accumulated error was equal to eight days, so the spring equinox no longer fell on March 21, but took place on the 13th of the same month. The English philosopher and scientist Roger Bacon noticed the error. In 1263, he wrote to Pope Urban VII explaining the problem. However, although Bacon's project had the support of his successor, Pope Clement IV, the time was not conducive to reforms: the Holy Roman-Germanic Empire of the Hohenstaufen had collapsed. The second half of the 13th century is characterized, in central Europe, by factional fights: Guelphs and Ghibellines in Italy. Under these conditions, no reform of the calendar was undertaken. Two centuries later, the attempts of the German scholar Nicholas of Cusa and the German astronomer Regiomontanus were also unsuccessful.

Gregory XIII

In the last quarter of the 16th century, Pope Gregory XIII decided to solve the problem. By then, the accumulated error of the Julian calendar amounted to ten days, and the spring equinox fell on March 11. The interest of the Church was due to the fact that the calculation of the date of Easter, according to the norms of the Council of Nicaea, fixes the equinox on March 21. The pope convened a team of scientists and clergymen, whom he commissioned to prepare a draft reform. The team adopted the solution proposed by the Jesuit astronomer Christopher Clavius, originally formulated by Luigi Lilius, a Verona-born professor of medicine. According to the Wikipedia, the solution adopted was proposed by the University of Salamanca in 1515 (unsuccessfully) and again in 1578 with more success, although the project was attributed to other people. The pope promulgated the new calendar, which was named Gregorian in his honor, in the bull of February 24, 1582.

To reestablish the spring equinox on March 21, ten days were suppressed, jumping from the night of Thursday, October 4, 1582 to the morning of Friday, 15 of the same month. In addition, to correct the error in the Julian calendar, three leap years were deleted every four hundred years, with the following rule: there would be three ordinary 365-day years (those not multiples of four) plus one leap year (those multiples of four). But among those years that with the Julian calendar would have been leap years, those that are multiples of one hundred, but not four hundred, are excluded. Since the Gregorian calendar was established, this has happened three times: the years 1700, 1800 and 1900. The year 2000, however, was a leap year. The next different year will be 2100, which will not be a leap year.

Every four hundred years, 97 additional days are added to the Gregorian calendar, rather than the 100 in the Julian calendar. As a consequence, the average duration of the year is not 365.25 days, but 365.2425. Compare that figure with the true one, 365.2421988... There is still an error, but it is smaller: 0.0003011... days per year (about 26 seconds). Over time, the accumulated error will reach one day, but for that to happen, 3321 years will have to pass, counted from the date of the Council of Nicaea (325), i.e. towards the year 3646. It does not seem that we should be concerned at the moment.

The man in the street was not affected by the reform of the leap year system, but he took to heart the skipping of ten days on the calendar. Some wondered if the reform would disorient migratory birds, which precisely in October had to choose the right time to travel south.

In 1582, as a consequence of Luther's reform, the Church had been divided. Protestant countries, suspicious of the pope's authority, refused to adopt the Gregorian calendar, which was official at first only in the Catholic countries. Little by little, the evidence of the facts prevailed: the Gregorian calendar is more exact than the Julian. The Protestant German states, Holland and Denmark resisted until 1699: the nearness of 1700, in which their calendar would accumulate the eleventh day of error, precipitated the adoption of the Gregorian.

George Washington

Great Britain did it half a century later: the change took place on September 2, 1752, from which it jumped to the 14 of the same month. There were public protests, and the government was accused of stealing eleven days of life from the citizens. This calendar change took place a quarter of a century before the independence of the United States of America, but the birth of its architect, George Washington, had taken place on February 11, 1732 by the Julian calendar, then in force. However, in the United States, Washington's Birthday is celebrated on February 22, the equivalent Gregorian date.

Eastern Europe and the Orthodox Church followed the Julian calendar longer. Albania, Bulgaria, Romania, Estonia, Latvia, Lithuania, Yugoslavia, the Soviet Union and Greece adopted the Gregorian between 1912 and 1923. Some non-European countries were also adopting the Gregorian calendar. Japan was one of the first, in 1873; China did so in 1912. By 1924, Mustapha Kemal Atatürk imposed it on Turkey.

Currently, only some branches of the Orthodox Church (including the Russian) use the Julian calendar for liturgical celebrations. Since the current delay of this calendar amounts to thirteen days, these churches celebrate Christmas on January 7 of the following year.

The same post in Spanish

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Manuel Alfonseca