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Here’s Why Wednesday Is the First Day of Fall

2 minute read

There are plenty of annual events that mark the beginning of fall: the Labor Day holiday, the arrival of Pumpkin Spice Lattes at Starbucks, the changing colors of the leaves, the first overwhelming urge to wear a sweater since the end of spring. But there’s only one official start of the season, and that’s the autumnal equinox, which arrives this Wednesday, Sept. 23., according to the U.S. Naval Observatory.

As TIME has explained before: “While that might sound like a name of a place where hunks sculpt their biceps, the word comes from the Latin term aequinoctium, meaning equality between day and night (aequi = equal and ­noct = night). That is what [the equinox] is after all: one of the two periods of the year when the sun crosses the equator and the days and nights are in equal length all over the earth.”

If you want to get very technical, day and night aren’t actually perfectly equal on the fall equinox. That’s because the equinox is officially marked by the day the center of the sun sets exactly 12 hours after it rises everywhere in the world—while the length of a day is measured differently, based on the time the very top of the sun rises and sets, according to the U.S. Naval Observatory. But the measurements are close enough that day and night are approximately equal for several days around the equinox.

While it’s technically still summer until Sept. 23, that doesn’t mean it hasn’t already started feeling like fall—just look at this TIME interactive graphic that tracks the arrival of temperatures we most associate with autumn.

PHOTOS: Breathtaking Aerial Views of Fall Foliage in Poland

Autumn in Pomerania, north side of Poland. Forest near Gdynia.
A clearing in a forest near Gdynia with trees changing colour in autumn.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland. Forest located in Region of Kashubia.
A patch of forest in the region of Kashubia.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland. Forest located in Region of Kashubia.
A view over a lake in a forest with trees changing colour in autumn in Kashubia.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland. Forest located in Region of Kashubia.
An aerial view of a forest with the trees changing colour in autumn in Kashubia.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland. Forest located in Region of Kashubia.
A view of a house in a forest with trees changing colour in autumn in Kashubia.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland. Forest located in Region of Kashubia.
A clearing surrounded by trees changing colour in autumn in a forest in Kashubia.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland. Forest located in Region of Kashubia.
A view of a clearing amongst trees changing colour in autumn in a forest in Kashubia.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland.
A view over a forest with trees changing colour in autumn.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland.
A view over a forest with trees changing colour in autumn.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland. Forest located in Region of Kashubia.
A view over a forest with trees changing colour in autumn in Kashubia.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland. Forest near Gdynia.
A view over a forest with trees changing colour in autumn near Gdynia.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland. Forest located in Region of Kashubia.
A view of a forest in the edge of a lake with the trees changing colour in autumn in Kashubia.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland. Brda river valley near Tuchola.
The Brda River runs through a valley near Tuchola.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland. Forest located in Region of Kashubia.
A river runs through farmland in Kashubia.Kacper Kowalski—PANOS
Autumn in Pomerania, north side of Poland.
A view over a forest with trees changing colour in autumn.Kacper Kowalski—PANOS

 

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Write to Nolan Feeney at nolan.feeney@time.com