Summer now has the hours counted. At 20:20 today, Monday 22 September, the autumn equinox will start, that is the symbolic moment that marks the end of the summer season and the transition to autumn. In the evening the sun, in its apparent motion among the stars along the ecliptic (i.e. the projection in the sky of the orbit of the earth), will cross the celestial equator from the north to the north, that is, the projection in the sky of the Earth equator.
The autumn equinox 2025: farewell summer
An important milestone, which marks the astronomical arrival of a new season: the autumn for the north hemisphere, which therefore leaves the summer, and spring for the southern hemisphere, which instead leaves the winter behind it. The term derives from the phrase “aequa nox”, that is “the same night (a day)”. In the equinox, the center of the sunscreen remains for the same number of hours both above and under the horizon: a “equitable” division between day and night, net of the twilight, the slow apparent motion of the sun and the atmospheric refraction.
“From this moment – explains the astrophysicist Gianluca Masi, head of the Virtual Telescope Project -, the north hemisphere will see the duration of the day constantly reduced to the advantage of that of the night, with the happiness of the lovers of the stars, up to the culmination of the December solstice, the winter of the southern hemisphere, it will be the night to thin more and more, going to the summer. The long arctic night, on the south pole, on the other hand, the interminable Antarctic day “. At the Equinozi the sun rises exactly east and sets perfectly to the west, also, dawn and sunsets are more “quick”, with reference to the time necessary to see the entire sun album appear (or disappear). Obviously net of the atmospheric effects, the refraction and diffraction of sunlight.
The equinox in popular culture
Popular culture places the Boreal Autumn Equinox on 23 September, but due to the real astronomical mechanisms below, which are glimpsed in the convention of the bisastile years and their calculation (this theme that reminds us of the reform of the calendar made by Pope Gregory XIII and entered into force on October 4, 1582, precisely to realize the calendar and the liturgical needs – in the first place of the Easter – Movements of the sky), we will see it happen this year on day 22. In 2092 and 2096, compared to the standard time of universal time, we will have the autumn equinox even on September 21.
The “real” duration of the seasons
Finally, a technical “curiosity”: due to the ellipperity of the terrestrial orbit, the four astronomical seasons in which the year is divided, they do not all have the same duration. The longest is our summer: 93.65 days, the shortest winter, with 88.99 days. In our hemisphere, the spring + summer couple totals 186.4 days, the autumn + winter instead 178.84.