# Gregorian calendar

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

### What is the Gregorian calendar?

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

This calendar was adopted by Spain, Italy and Portugal in that same year, but found resistance in the Anglo-Saxon world, especially in Great 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 of carrying out the agreements of the Council of Trent (1545-1563) to improve and complete the existing Julian calendar from the year 46 BC

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

The new calendar **solved the problems of temporal accuracy by proposing that the year be 365 days** (actually 365, 2425 days), and instead of having every four years one year Leap, that is, 366 days, as proposed by the Julian calendar, leap years will be available when their last two figures are divisible by 4 or 400, except for the multiples of 100.

This, mathematically, implies an advance of 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 Earth tends to slow down over time, due 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 days (working), 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.

It can serve you: Julian Calendar.