Gregorian Calendar – Concept, history and composition

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

Gregorian calendar
The Gregorian calendar name comes from Pope Gregory XIII.

What is the Gregorian calendar?

The Gregorian calendar is known as the calendar originating in Europe, currently accepted 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 very serious (1582).

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

There was already a lag in calendar matters since the Council of Nicea (325), due to an inaccurate count of the days of the year, which produced a variation of 10 days per year. To solve this, 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 solved the temporal accuracy problems by proposing that the year have 365 days (actually 365.2425 days), and that instead of having a leap year every four years, that is, 366 days, as proposed by the Julian calendar, we would have leap years when their 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 brings problems since the speed of rotation of the Earth tends to slow down over time, due to to the lunar force of attraction. This has been determined from sustained measurement by atomic clocks, but it is not too problematic in practical terms.

Instead, the month-to-month discrepancy of the number of business days (work), between 24 and 27 depending on the number of weeks in 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, far into the future, further adjustments to regain its accuracy.