AUTUMN. The chapter of summer is almost closed and we are about to enter the harvest season. How exactly does the astronomical phenomenon of the equinox take place? And why do the leaves change color and then fall?
This Sunday, September 23, 2024, the autumn equinox will cause the Northern Hemisphere to switch from summer to autumn, very precisely at at 12:43 (French time) according to theInstitute of Celestial Mechanics and Ephemeris Calculation located within the Paris Observatory. The autumn equinox marks the beginning of cooler temperatures, landscapes adorned with purple and ochre, falling leaves, and shorter days in the Northern Hemisphere and, conversely, the beginning of high temperatures in the Southern Hemisphere.
First immediate effect of this astronomical event, cousin of the spring equinox, whose date therefore corresponds to the first day of autumn: the length of the day is generally the same as the length of the night (12 hours/12 hours) and the sunshine begins to shorten in the northern hemisphere and therefore in France, with a loss of luminosity of 4 minutes per day. Scientifically speaking, this is the time of year when the Earth, which tilts (i.e. its axis of rotation is inclined by 23.4° relative to its orbit), passes from one sunshine to another. The sun will indeed come to cross the Earth’s equatorial plane. Go behind the scenes of the equinox by browsing our special page using the summary above.
What is the specific autumn landscape based on, with its “discoloured” leaves and bare trees? The leaves actually say goodbye to their green colour of the beautiful days by temporarily losing the chlorophyll in their plant cells. This “green” activating pigment actually disappears from its hosts in parallel with the drop in temperature and light. The leaves then become red, orange, yellow.
As the Huffington Post explains in an entire article dedicated to the subject, trees are already preparing their new look several weeks before the arrival of the astronomical autumn (end of September). And if no one seems surprised that every year, they undress and then find their green clothes again, the German forester Peter Wohlleben writes, in his fascinating best-selling book “The Secret Life of Trees”: “The annual fall and regrowth of leaves is a small miracle, because the process implies that trees have a notion of time”. Science explains the phenomenon in this way, as Sciences et vie reports: during thewintertrees are in a state of “dormancy”, but before that, until about the end of October, they first go through a transitional state called “paradormance” during which their growth slows down, before stopping completely. The tree also stores sunlight during the sunny days, like fuel for its new shoots in the spring. A bit like a bear anticipating its hibernation by several months.
But that’s not all: still according to the work presented above, “a majority of species [d’arbres] undertake to gradually reduce their water content, and therefore their activity, from July.” Objective: to avoid ingesting too much water too soon before winter, because excess water, when freezing, could cause the tissues of the trees to crack. In addition to relying on falling temperatures to detect the onset of cold weather, trees also “count” the days, using sensors on their buds.
The equinox is the moment when the Sun crosses the plane of our Earth’s equator. In summary: This is the moment when the sun passes the zenith of the Equator. At that time, we change seasons. The astronomical event therefore ends the summer in the blink of an eye. In the northern hemisphere, it generally occurs between September 21 and 24. But why will autumn 2026 fall on a “23” and not a “22” or a “21”? If many of us once again identify the date of the “21” of September as the autumn equinox, it is not so simple. The date of the transition to autumn actually corresponds to a very precise moment: the moment when the Sun crosses the plane of the equator as it passes at its zenith. Clever calculations have determined it, carried out by the Institute of Celestial Mechanics and Ephemeris Calculation (IMCCE) hosted by the Paris Observatory since 1998. And since the Earth does not revolve around the sun in exactly 365 days, D-Day can change from one year to the next.
Thus, the date of the autumnal equinox precedes the change to standard time by about a month. The first day of autumn also falls halfway between the summer solstice (June 20-21) and the winter solstice (December 20-21). But Every year the autumn equinox occurs on a different date. This is because the Earth orbits the Sun in 365 days, 5 hours and 46 minutes, not exactly 365 days. Most of this discrepancy is corrected by the addition of February 29th during the leap yearsMathematically, the autumnal equinoxes can only occur between September 21 (next in 2092) and September 24 (next in 2303).
The autumn equinox of September 22, 2024 occurred precisely at 12:00 p.m. 43 minutes and 36 seconds French time according to the Institute of Celestial Mechanics and Ephemeris Calculation (IMCCE). Autumn then continues as every year for three months until winter solstice.
What is the equinox? The word comes from Latin aequinoctium (equal night). Indeedfor us, the most obvious phenomenon of the equinox is that night and day are the same length at this time of year. At the autumn equinox, the days, which lasted about 16 hours on June 21 in France, have largely lost length. They will thus shorten until the winter solstice. Why? Because of geometry. Indeed, The Earth’s axis of rotation is inclined 23.4° relative to the plane of its orbit : our planet “leans” in relation to the sun. The star therefore does not illuminate us in the same way depending on the time of year. In winter, France (for example) receives light for only eight hours per 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” with “solstice”. In astronomy, the solstices correspond to the two moments of the year when the sun is furthest from the equator. This corresponds to the maximum duration (for the summer solstice) or minimum (for the winter solstice) of the day. In other words, this phenomenon is very different from that of the equinoxes and their day and night of equivalent duration!
Here are the dates and times of the autumn equinoxes for the 2024 autumn equinox, estimated through 2026:
- 2024 : September 22 at 12:43 p.m.
- 2025 : September 22 at 6:19 p.m.
- 2026 : September 23 at 00:05