Like an immutable ritual, the days get shorter. The trees take on red and orange hues. The leaves litter the soils of cities and countryside. After the hot summer weather, the first freshness invites you to change your wardrobe. No doubt, autumn is here. It begins this Saturday, September 23 at 6:49 am and 56 seconds UTC precisely, or 9:09 am and 56 seconds in France, according to the Institute of Celestial Mechanics and Calculation of Ephemeris (IMCCE), the French institution dependent on the Sorbonne in charge of making the ephemerides. If it is the same date as last year, nothing is set in stone. In 2021, for example, painters' favorite season began on September 21. In 2024 and 2025, it will be September 22.
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Why such a date differential? The answer lies in an astronomical phenomenon: the autumnal equinox, which traditionally marks the kick-off of this new season. It refers to the time of year when the length of the day is equal to that of the night, i.e. 12 hours each. To return to the etymology, the word equinox is formed by two Latin terms: "aequus" which means "equal" and "nox, noctis" which refers to "night".
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Two equinoxes per year
Two occur a year: for the transition from summer to autumn, then from winter to spring. In scientific terms, the beginning of autumn coincides with the Sun's transit at the zenith of Earth's equator, explains IMCEE.
It remains to explain this date shift. To do this, we must return to some fundamentals. The Earth takes an average of 365 days, 5 hours and 46 minutes to circle the Sun. A year legally counts 365 days, so you have to make up for the difference of a few hours. Hence the need for leap years, every four years, which last 366 days and partially restore the balance. These particular years also affect the dates of the equinoxes. In fact, the beginning of autumn can vary between September 21 and 24, while the spring equinox takes place between March 19 and 21.
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In any case, this transition to a new season has a direct impact on our lives: the nights begin to last longer than the days. Throughout autumn, the France is expected to lose between three and four minutes of daily sunshine until the winter solstice, described as the day of year when the night is longest. The days will then begin to lengthen again. Whew!