On September 22, goodbye summer: it is the day of the autumn equinox

In the astronomical sense of the term on 22 September we will see the official transition from summer to autumn. The change will take place at 21.21 Italian time (19.21 UTC London). Generally the autumn equinox occurs between 21 and 24 September, always at different times. The variation of the moment in which it happens is caused by the different duration of the solar year and that of the calendar: the planet takes 365.25 days to make an orbit around the Sun. For this reason autumn can also “delay”.

What is the autumn equinox

The term equinox comes from the Latin “aequa-nox”, literally “equal night”. On the day that marks the transition from summer to autumn, in fact, the hours of light and those of darkness, on a theoretical level, should be equivalent. This happens because the Sun is at the zenith of the equator, i.e. the sun’s rays are perpendicular to the earth’s axis. The length of the day, however, also depends on the distance from the equator and is the reason why the hours of daylight are not the same on the whole planet. This astronomical phenomenon occurs only twice a year: in September, in fact, and in March. For the northern hemisphere, the two dates respectively mark the beginning of autumn and that of spring. Conversely, for the southern one, the September equinox coincides with the beginning of spring and that of March, with autumn. The position of the Sun relative to the equator depends on the inclination of the rotation axis of the Earth. This determines from time to time the lesser or greater exposure to light of the hemispheres of our planet.

The change of season

In addition to the astronomical phenomenon of the equinoxes, there is also another current of thought that defines the changes of seasons. This is phenology, a branch of biology that studies the relationship between climatic factors and seasonal manifestations of flora and fauna. According to this science, autumn would not begin on a definite date, but would be characterized by gradual changes in the natural world, such as the colors of plants or the ripening of autumn fruits. What is certain is that the astronomical phenomenon of the equinox unites the Earth to all the planets of the Solar System, in which there are both equinoxes and solstices, with the exception of Mercury, which has an orbit perpendicular to the Sun.