Saturday, July 23, 2022


It is well known to all that the length of the year is 365 days. Every four years, one day is added, the 29th of February. This is the leap year, the explanation of which is linked to astronomical reasons and in particular to the movement of the Earth around the Sun.

The calendar year is the year that is used in everyday life and therefore obviously must have a whole number of days.

The solar year, linked to the cycle of the seasons, is the time that the Earth takes to complete an orbit and return to the same initial position with respect to the Sun. Its duration is 365,2422 days.

To ensure that the seasons always fall in the same months and that there is no change over time, the duration of the calendar year must be as similar as possible to that of the calendar year. Since it must consist of an integer number of days, it has been established in the past that the calendar year would last 365 days.

The difference between the two years was however very significant and was therefore decided in 46 B.C. with the introduction of the Julian calendar that each year divisible for four was 366 days instead of 365. Thus was born the leap year.

The four-year average calendar year was 365.25 days.

The difference between the two years as small as it still existed and spread over time. By 1582 the delay had become 10 days. Pope Gregory XIII took an interest in the problem and, with the introduction of the Gregorian calendar (still in use today), it was decreed that it would pass directly from October 4 to October 15, 1582. Eliminating these ten days the accumulated difference between the two years was canceled. In addition, to make the calendar year more precise, a new rule was introduced, which has not been amended since. It was decreed that each year divisible by four trenches bisestile, with the exception of the secular years (ie those divisible by 100) not divisible by 400. So the year 2000 was leap because it was divisible by 400, while 1900 and 2100 are not leap because they are divisible by 100 but not by 400.

In this way, the average duration of the calendar year is 365.2425 days, just 26 seconds longer than the calendar year. This difference is so slight that the backlog since 1582 is insignificant and no correction had to be taken into account.


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