Google dedicates today’s doodle, Wednesday 22 September 2021 to the first day of Autumn with the arrival of the autumn equinox.
After the doodle of last March 20 dedicated toSpring Eqiunoziosummer has passed and we say goodbye to welcome autumn.
In the original Julian calendar, promulgated by Julius Caesar, the spring equinox fell on March 25, while according to what was established by the Council of Nicaea in 325 AD and then confirmed in 1582 by Pope Gregory XIII, the day should be March 21. However, as has been happening for some years, this year the spring equinox fell on March 20. Once the Spring Eqiunox has been filed, the Autumn Equinox has arrived, that of September with which the summer ends.
The equinoxes occur about six months apart from each other, usually falling in the months of March (Spring Equinox) and September (Autumn Equinox) of the civil calendar; just like the solstices (in summer and in winter), they are conventionally recognized as a moment of alternation of the astronomical seasons on Earth.
Interestingly, while in our northern hemisphere the September equinox ends summer and introduces autumn, in the southern hemisphere exactly the opposite happens: autumn begins with the March equinox and spring begins with the equinox of September.
At the equinoxes, the earth’s rotation axis is perpendicular to the direction of the sun’s rays and therefore in every point of the planet, where the Sun exceeds the horizon, the diurnal duration is equal to that of the night (except for the terrestrial peculiarities due to the ‘atmosphere).
The autumn equinox, in September, was once correct to also call it the point of Libra (omega), but this term of astrological derivation is no longer valid, as, due to the precession of the equinoxes (the eccentric motion of the of rotation on itself), these points are no longer found in the constellation from which they take their name: currently in September it is in the sign of Virgo.
The September equinox is slightly later (usually 22 or 23 September) than that of March, since the revolutionary motion of the sun around the Earth (the so-called sidereal year), is slightly slower in summer, due to of Kepler’s second law.
This year, the autumn equinox falls exactly on September 22nd at 19:21 (UTC, 21:21 in Italy), and will continue until March 20, 2022 at 15:33 (17:33 in Italy), when the spring equinox will occur. The astronomical date of the March equinox is also anticipating more and more (often it falls on March 20) and from around 2044 it will fall on March 19. The reason why the spring equinox day is always brought forward over the years is because of the organization of leap days in the civil calendar, which involved maintaining the four-year alternation for the year 2000 and therefore caused a progressive shift by one day of all celestial events until the next realignment expected in the year 2100.
It should be noted that, while in the northern hemisphere – where Italy is located – today is the first day of autumn, in the southern hemisphere it is the first day of spring.
The doodle that Google dedicates on September 22, 2021 at the autumn equinox is an image with a cute hedgehog with typical autumn colored leaves instead of the quills that walks.