Monday September 22, 2025 to 20:19 Italian (18:19 UTC) will officially end the summer and therefore autumn will begin: yes, the moment when the change of season takes place is not a whole day but a precise moment, called autumn equinox (or rather Equinox of September), in which the sun crosses the celestial equator from the north to the south. During that day, every point on the planet will have 12 hours of light and 12 hours of darkness: this is literally the meaning of the term “equinox”, which derives from the Latin aequa nox (“Equal Night (a day)”).
What happens on the day of the equinox and what it means from an astronomical point of view
By definition, the day of the equinox is marked – almost exactly – from 12 hours of light and 12 hours of darkness. For us in the boreal hemisphere, the equinox takes place during a period of shortening of the day: The hours of light are in fact decreasing from June 21, the day of the summer solstice, and will do so until the winter solstice that this year will fall on December 21st. It is natural therefore to take this day as the border between the summer and the autumn seasonin which the duration of the day exceeds that of the night.
The reverse naturally takes place in the southern hemisphere: there the hours of light gradually increase after the summer solstice, and the September equinox represents the moment when the day exceeds the night in duration, thus starting spring.
But how does all this work from an astronomical point of view? The reason is the fact that the terrestrial axis is inclined with respect to the plan on which the earth orbit. To be precise, the inclination of the Earth’s axis is of 23 ° 27 ‘. Now, while our planet moves along its orbit around the sun, this inclination means that during the year it also changes the height of the sun in the sky and consequently the duration of the day: it is the same reason why on our planet the seasons.
In this apparent motion of the sun on the celestial vault during the year, the equinoxes represent two important moments. If we project the terrestrial equator on the celestial vault we get the so -called celestial equatorwhich divides the sky into two equal parts. In our spring and summer the sun is located above the celestial equator: it goes up to the solstice and begins to go down immediately afterwards. The autumn equinox is the moment when The sun is exactly on the celestial equator To trespass in the southern hemisphere of the celestial vault.
On earth this translates into the fact that the equator, at noon of the day of equinox, The sun is perfectly to zenit: at that moment the objects do not project shadows! This means that the line that divides the day from the night (into technical jargon lighting club or terminator) passes through the earth’s poles: geometrically means that every point on the earth’s surface, over the 24 hours, takes half the route in the part enlightened by the sun and halfway in the dark part of the earth. In other words, everywhere in our planet the day is divided into equal duration of day and night. In the other days of the year this is true only to the equator.
Because this year autumn does not start on September 21st but on 22
Many are amazed by the fact that the seasons do not begin on March 21, June, September and December. In fact, once it was taught that the seasons began on day 21. But it is not necessarily so, especially as regards the so -called “half seasons”. In fact, the seasons are the beginning of the seasons solstices (summer and winter) and equinox (spring and autumn), whose arrival varies slightly from year to year. This because of the fact that The motion of the earth around the sun does not last exactly 365 days but about 365.25 days: This gap between the duration of the calendar year and that of the Gregorian calendar makes the moment in which the sun reaches these four “critical” moments in its apparent motion in the sky.
Specifically, the autumn equinox generally falls the 22 or 23 Septembereven if it can go from September 21 to 24 September. Ironically, between 2000 and 2030 no summer solstice has fallen or will fall on September 21!
