Here we are: the September 22 summer will officially be a thing of the past. At 20:21 (Italian daylight saving time) in the northern hemisphere the astronomical autumn will begin. It will be as usual to sanction the way the autumn equinox, a recurring phenomenon this time of year, but that does not have a fixed date. The next year (as happened in 2019) will fall for example the September 23, among other things in further delay compared to what is traditionally considered the canonical date of early autumn, that is 21 September.
These fluctuations depend on the difference between the our calendar (said Gregorian), which lasts 365 days, and the time that the Earth it really takes to complete its circle around the Sun, equal to 365 days, 6 hours, 9 minutes and 6 seconds. To be even more precise, the actual discrepancy is slightly less: ie 5 hours, 48 minutes and 46 seconds due to the so-called precession of the equinoxes, a very slow movement of the Earth similar to a spinning top, which subtracts another few minutes from those 6 hours.
To avoid the progressive detachment between the Gregorian calendar and the cycle of the seasons was invented leap year, which every four years fixes things by adding one day to the month of February (rounding: 6 hours x 4 = 24 hours). A further trick, useful to better balance the accounts, also provides that they can be leap only the secular years (i.e. marking the turn of the century) divisible by 400; reason why the year 2000 was for example a leap, while 2100 will not be.
The consequence of all this is that the autumn equinox (as well as that of springs and the two solstices) can change date. Most of the time the 22 or 23 September, but in 1931 it happened, for example, that the September 24; event that will repeat itself as far back as 2303.
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