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What is the Julian calendar?

We explain what the Julian calendar is and to whom its name is due. In addition, how it was composed and the causes of its replacement.

  1. What is the Julian calendar?

The calendar model introduced by the Roman military and political leader Julio César in 46 BC (708 AUC, that is,  ab Urbe  condita , “since the founding of Rome”) is known as the Julian calendar . This calendar model came into force since the Roman conquest of Egypt, and was the predominant in Europe and its colonies until 1582, when it was gradually replaced by the Gregorian calendar, more accurately at 0.002%.

The emergence of this unified calendar replaced the lunar calendars of many ancient cultures , such as traditional Etruscan and Latin calendars, unifying the Roman world and its colonies around the same model, heir to the Egyptian solar calendar, the first of which we have news , developed in ancient antiquity to try to predict the floods of the Nile.

In fact, the Roman Empire was governed based on a pagan calendar of 304 days distributed in 10 months, whose irregularities and lags were corrected according to economic and political needs (such as the time to pay workers or the delay of votes of the Republic), adding the month of  mercedonius  biennially.

The Julian calendar  introduced a regular year of 365.25 days over 12 months , with a leap day introduced between February 23 and 24 every 4 years. For this, a year of 445 days, called the “last year of confusion”, had to be counted during the year prior to its implementation. The idea of ​​numbering the days arose later, inheritance of the Visigoths, and was implemented by decision of Charlemagne.

The Julian calendar also took the beginning of the year on January 1 instead of March 1, as usual. Later the months of quintilius  and  sextilius were renamed   July and August , in honor of the Roman emperors Julio César and César Augusto, respectively. Other emperors tried to rename months at will, but failed in the attempt: Caligula wanted to call September  Germanicus , Nero wanted to call  Neronian  to April and Domitian wanted to call  Domitianus  to October.

The mathematical considerations of the Julian calendar were taken even though it was already known, from the ancient Greek astronomers, that the tropic year was slightly shorter than 365.25 days. Thus, this calendar model lost almost three days every four centuries . This motivated, during the Council of Trent (1545-1563), the need for a new model that corrected the accumulated offset from the Council of Nicea (325).

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