What is the autumnal equinox, this astronomical phenomenon that takes place this Saturday, September 23?

It's already the end of summer. This Saturday, September 23 marks the official beginning of autumn, an entry into a new calendar season guided by the autumnal equinox. This phenomenon, which occurs twice a year, corresponds to the day when the duration of sunshine is equal to the duration of the night anywhere in the world, twelve hours. Astronomically, this is the time when the sun will rise precisely in the east and set precisely in the west.

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The Earth's axis of rotation is inclined about 23.5° relative to the plane of its orbit, which is why the northern hemisphere is oriented towards the Sun half of the year, and the southern hemisphere is oriented towards the Sun for the other half. During an equinox, both hemispheres receive the same amount of sun, because the celestial body is right vertically above the equator. Etymologically, the word equinox is also formed from two Latin terms, "aequus" which means "equal", and "nox" which means "night", meaning "equal night".

Why doesn't it fall every year on the same date?

This astronomical phenomenon corresponds to the moment when our celestial body passes above the Zenith, the point of intersection between the vertical of the equator and the celestial sphere, which is the case at the beginning of spring and autumn, that is to say around March 21 and September 21.

If the day of the autumnal equinox can vary from one year to another, this is no coincidence. This is simply because the rotation time around the Sun is 365 days and less than 6 hours, which is longer than a calendar year of 365 days. For spring to always fall on the same date, our year in the Gregorian calendar would have to last exactly 365 days, 5 hours and 46 minutes and not three times 365 days and 366 days once every four years.

In 2023, the autumnal equinox will be held precisely on September 23 at 08:50 and 1 second French time, according to the Institute of Celestial Mechanics and Calculation of Ephemeris (IMCCE) and the season then continues for three months, until the winter solstice on December 22.