FALL 2023. When the autumnal equinox occurs, the chapter of summer is closed and we enter the season of autumn. How exactly does this astronomical phenomenon take place? What date and time?
[Updated September 11, 2023 at 2:46 p.m.] In 2023, the astronomical event of the autumnal equinox will officially begin on Saturday, September 23, 2023 at 8:49 a.m. and 56 seconds French time precisely according to the Institute of Celestial Mechanics and Computation IMCCE ephemeris (UTC 2). Autumn will then continue as every year for three months until the winter solstice.
Among the first effects of the autumnal equinox, an astronomical event similar to the spring equinox, is the fact that the days become shorter than the nights again. With fewer hours of sunshine, temperatures immediately drop... Remember that on average, during the fall, we lose 4 minutes of sunshine every day in France and temperate regions! A considerable reduction which is not without consequences on the body... We will have to wait until December 22, the date of the winter solstice, to see this cycle (finally) stop and for the days to start getting longer again. ..
If the leaves color in the fall, turning yellow or orange, before leaving the branches, it is to prepare for winter in the best possible conditions. The trees anticipate the cold to come and concentrate their energy - the sap - in the channels that must above all be preserved and insulate the leaves, which are destined to freeze. The tree is able to create a small "plug" at the base of each petiole, with a little cork. The leaves use their own resources and end up dehydrating, hardening, then falling... (Learn more in our paragraph below).
The equinox is when the Sun crosses the plane of our Earth's equator. In summary: this is the instant the sun crosses the zenith of the Equator. At that time, we change seasons. The astronomical event therefore closes the summer in the blink of an eye. In the northern hemisphere, it usually occurs between September 21 and 24. But why does fall 2023 fall a "23" and not a "22" or a "21"? While many of us once again identify the date “21st” of September as being that of the autumnal equinox, it is not so simple. The date of the transition to autumn actually corresponds to a very precise moment: that when the Sun crosses the plane of the equator at its zenith. Scholarly calculations have determined this, carried out by the Institute of Celestial Mechanics and Ephemeris Calculation (IMCCE) hosted by the Paris Observatory since 1998. And as the Earth does not revolve around the sun in exactly 365 days, the D-Day can change from year to year.
In 2023, the date of the autumnal equinox falls on Saturday, September 23. The date of the autumnal equinox precedes the change to standard time by approximately one month. The first day of autumn also occurs halfway between the summer solstice (June 20-21) and the winter solstice (December 20-21). But each year, the autumnal equinox takes place on a different date. This is because the Earth revolves around the sun in 365 days, 5 hours and 46 minutes, not exactly 365 days. Most of this shift is corrected by adding February 29 in leap years. Mathematically, the autumnal equinoxes can only occur between September 21 (next time in 2092) and September 24 (next time in 2303).
What is the equinox? The word comes from the Latin æquinoctium (equal night). Indeed, for us, the most obvious phenomenon of the equinox is that night and day have the same duration at this time of the year. At the autumnal equinox, the days, which lasted about 16 hours on June 21 in France, greatly reduced in length. They will thus shorten until the winter solstice. For what ? Due to geometry. This is because the earth's axis of rotation is tilted 23.4° relative to the plane of its orbit: our planet "leans" relative to the sun. The star therefore does not illuminate us in the same way depending on the time of the year. In winter, France (for example) only receives light eight hours a day, compared to double that in summer. This determines the behavior of air masses and gives rise to the seasons as we know them in temperate zones.
But don't confuse "equinox" and "solstice". In astronomy, the solstices correspond to the two times of the year when the sun is furthest from the equator. This corresponds to the maximum (for the summer solstice) or minimum (for the winter solstice) length of the day. In other words, this phenomenon is very different from that of the equinoxes and their day and night of equivalent duration!
What is the specific autumn landscape based on, with its "discolored" leaves and bare trees? The leaves actually say goodbye to their sunny green color by momentarily losing the chlorophyll in their plant cells. This “green” activating pigment disappears from its hosts in parallel with drops in temperature and light. The leaves then turn red, orange, yellow.
As the Huffington Post explains in an entire article dedicated to the subject, the trees are already preparing their new look several weeks before the arrival of astronomical autumn (end of September). And if no one seems to be surprised that every year they undress and then regain their green habit, the German forester Peter Wohlleben writes, in his fascinating and successful work "The Secret Life of Trees": "the fall and the regrowth annual leaves are a small miracle, because the process implies that the trees have a sense of time. Science explains the phenomenon thus, as relayed by Sciences et vie: during winter, trees are in a state of "dormancy", but before that, until around the end of October, they first go through a state of transition called "paradormance" during which its growth slows down, before stopping completely. The tree also stores sunlight during sunny days, as fuel for its new shoots in spring. A bit like a bear anticipating its hibernation of several months.
But that's not all: according to the work presented above, "a majority of species [of trees] undertake to gradually reduce their water content, and therefore their activity, from the month of July." Goal: Avoid ingesting too much water too soon before winter, as excess water, when freezing, could cause tree tissue to crack. In addition to relying on falling temperatures to detect the beginnings of cold seasons, trees also "count" the days, thanks to sensors on their buds.
In 2023, the Autumnal Equinox, as an astronomical event, will officially begin on Saturday, September 23, 2022. Here are the dates and times of the Autumnal Equinoxes following the 2023 Autumnal Equinox, estimated through 2025:
The autumnal equinox of September 23, 2023 took place precisely at 8 hours 49 minutes and 56 seconds French time (6:49 in universal time) according to the Institute of Celestial Mechanics and Calculation of Ephemerides (IMCCE). Autumn then continues as every year for three months until the winter solstice.
Like the spring equinox, the autumn equinox is each year synonymous with rituals in certain cultures or ancient beliefs (followers of Druidism and shamanism). Called "Mabon" in Celtic, the astronomical phenomenon designating this moment when day and night have the same duration also corresponds to the time, on the autumn side, when a good part of agricultural products are harvested. The pagan festivals then symbolize thanks for these famous harvests. In ancient times, after exhausting work in the orchards and fields, this festival was also a welcome break.