Spring is coming: at 10.24 pm Italian time on March 20, the equinox takes place, the astronomical event during which the sun's rays are perpendicular to the Earth's rotation axis and the duration of the hours of light equals that of the hours of darkness.
In the Northern Hemisphere, it's also the time when daylight hours begin to gradually increase, until the summer solstice.
In fact, the word
derives from the Latin
and indicates that in a given day the duration of the diurnal and nocturnal periods are equal.
Graphic representation of the illumination of the Earth during the equinox (source: Lillorizzo, from Wikipedia)
The alternation of the seasons and the variable duration of day and night are a direct consequence of the inclination of 23°27′ of the earth's axis with respect to the planet's orbital plane.
This implies that the sun's rays fall on the surface of the earth at different angles and only twice a year are perpendicular to it, ie during the equinoxes.
The change of seasons in the northern hemisphere (source: Tauʻolunga, from Wikipedia)
The vernal equinox usually falls between March 19 and 21.
The reason why it takes place on a different day every year is due to the fact that the Earth completes a complete revolution around the Sun in 365 days and 6 hours, the duration of the so-called sidereal year, which does not correspond to that of our calendar.
It is precisely for this reason that the leap year was introduced, which makes it possible to recover 24 hours every four years.
Despite this, due to the accumulated delay, the equinoxes can occur at a different time every year.
Although according to tradition spring arrives on March 21, this has not happened since 2007 and will continue like this until at least 2030. From 2044, on the other hand, the vernal equinox will be able to further anticipate its arrival to March 19: this again due to the between calendar and sidereal year, which will continue until 2100, the year that will bring the arrival of spring back to March 21st.