2023-12-18 14:11:37
It seems completely natural to us today to have seven-day weeks and for each year to be 365 days, with the exception of the anomaly of leap years. It seems just as normal to us that each year is made up of 12 months with one anomaly: there are months of 28 days, 30 days and 31 days. All this does not come from nothing, it is the result of two things: on the one hand the historical heritage, on the other the phases of the Moon.
And if today, this functioning is completely integrated, it has not always been so. Moreover, in Roman Antiquity, the seven-day week did not exist, it only appeared in the first century following Jesus Christ. Previously, a year lasted 355 days and the following year 377. Julius Caesar will decide to reorganize all this and create the Julian calendar.
From now on, the year will begin on January 1, the months will be fixed as currently in 30 and 31 days with February which only has 28 except 29 in leap years. This calendar will be in force until the 16th century. But, there is a catch around leap years. In the Julian calendar, with a leap year every 4 years, this means that the average length of a year is 365 days and 6 hours. The problem is that the time interval for the sun to return to a position similar to what it was, is not 365 days and 6 hours but 365 days 5 hours and 48 minutes… Which makes a difference of 12 minutes per year. That is 20 hours of difference in a century and 8 days in a millennium.
In the 16th century, this discrepancy posed above all a religious problem. The biggest Christian festival, Easter, whose date in spring fluctuates depending on the phases of the Moon, is increasingly shifted towards summer. For Pope Gregory 13, a reform of the Julian calendar is necessary. He will ask astronomers to propose a new version of it: it is the Gregorian calendar. Adopted in 1582, this will slightly change the calculation of leap years.
But above all, to correct the errors of the past, the transition to the Gregorian calendar requires a radical reform: purely and simply removing ten days from the calendar. The contemporaries of this reform therefore fell asleep on the evening of December 9 and woke up on the morning of December 20. Creating a gaping hole, nothingness in the timeline.
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