The September 21stalways, in the collective imagination coincides with the beginning of autumn. At school we learned it like this, and even many agendas continue to mark it as a symbolic date. But the truth is different: autumn does not always begin that day. Indeed, this year the equinox will not be 21, But on September 22 at 8.19 pm, And there is a precise explanation behind this shift.
The equinox: when the day and night are equivalent (or almost)
To understand why, you have to start from a simple question: what is autumn from an astronomical point of view? It is not a decisive period at the table, but the instant in which the equinox occurs. During the equinox, the sun is exactly above the terrestrial equator and the sun’s rays affect this imaginary line perpendicularly. The result? The duration of the day and night is almost identical at every point on the planet.
Twice a year this “perfect tie” takes place: in March (spring) and September (autumn). There are no tricks: it is the position of the earth around the sun that will establish the change of season.
Because the equinox does not always fall on September 21st
If you are wondering why as children they taught us that autumn comes on September 21, the answer is that it is an educational simplification. Actually, The equinox can fall on 21, 22 or even 23 September.
The fault lies in the “fence” between the civil calendar and the astronomical time: the calendar year lasts 365 days, 5 hours, 48 minutes and 46 seconds; Our calendar, for convenience, rounds it at 365 days (366 in the bisastiles) and this accumulated waste causes the date of the equinox to move slightly every year. That’s why we cannot take it for granted on September 21 as the beginning of autumn.
This year the equinox will fall on September 22 at 20: .19 (Italian time). It is at that precise moment that, officially, the summer will give way to autumn. From that moment on, the days will begin to shorten in an increasingly evident way. The nights will become longer and will accompany us to the winter solstice of December, when the difference between light and darkness will be maximum.
Ready to greet the summer?
