CONCEPTS

# What is the Gregorian calendar?

We explain what the Gregorian calendar is and the origin of its name. Also, how it was composed and a bit of its history.

1. ### What is the Gregorian calendar?

The Gregorian calendar is known as the calendar originating in Europe, of current acceptance throughout the world since it replaced the Julian calendar in the year 1582. Its name comes from Pope Gregory XIII (1505-1585) , who was its most important promoter, especially through the papal bull  Inter  gravissimas  (1582).

This calendar was adopted by Spain, Italy and Portugal in the same year, but found resistance in the Anglo-Saxon world, especially in Britain and its colonies, until 1752. It is currently the world calendar , and was the product of studies sent to the Holy See by academics of the University of Salamanca, as a way to carry out the agreements of the Council of Trent (1545-1563) to improve and complete the existing Julian calendar since 46 BC

There was already a lag in terms of calendar since the Council of Nicea (325), due to an inaccurate calculation of the days of the year, which produced a variation of 10 annual days. To solve it, a Calendar Commission was appointed headed by astronomers Cristóbal Clavio and Luis Lilio, the first a Jesuit mathematician and the second one of the main authors of the Gregorian calendar.

The new calendar resolved the problems of temporal accuracy by proposing that the year be 365 days (actually 365.2425 days), and instead of having a leap year every four years, that is, 366 days, as the calendar proposed Julian, you would have leap years when your last two figures are divisible by 4 or 400, except for multiples of 100.

This, mathematically, implies an advance of ½ minute every 3300 years, which in human life time does not seem to mean much, but in the long run it also causes problems since the speed of rotation of the Earth tends to slow down over time, because to the force of lunar attraction. This has been determined from sustained measurement thanks to atomic clocks, but it is not too problematic in practical terms.

Instead, the month-to-month discrepancy of the number of business (work) days, between 24 and 27 depending on the number of weeks of each month (depending on whether you have 30 or 31 days, or 28 or 29 for February). This implies that, in the long term, our calendar will require, very in the future, new adjustments to recover its accuracy.