The autumnal equinox occurs this Thursday, Sept. 22. The term comes from ancient Latin, meaning “equal night.”

By popular definition and Latin derivation, on that day and on the vernal (spring) equinox in March, day and night will be approximately equal — about 12 hours of daylight and 12 hours of darkness.

It really means that the sun is above the horizon for 12 hours and below it for 12. But that definition turns out to be incorrect.

The equinox also defines the first day of autumn. But that definition also seems false. The temperatures have already started to cool. Autumn has been in the air for a couple of weeks, and the leaves won’t begin to turn in earnest for a couple more.

Why am I questioning these definitions? Because I’m an astronerd, that’s why.

Astronerdingly speaking, the autumnal equinox is not a day or a seasonal change but an exact place and time. The sun’s position in the sky at a particular moment defines the equinox.

Unfortunately, determining the sun’s motion and its location at any given time is far more complicated than it might seem at first blush.

Two motions complicate the sun’s position.

The yearly motion is determined by the Earth’s revolution around the sun. The daily motion is determined by the turning of Earth on its axis to make day and night.

It’s like figuring out where you are in space while you’re on the Teacups ride at the state fair. But let’s give it a go anyway.

As Earth moves around the sun, old Sol seems to move slowly across the sky from day to day, rising and setting at different spots on the horizon each day. Of course, the sun’s motion is purely an illusion. We’re the ones who are moving.

The sun path carries it once a year in front of the fixed background of stars along a great circle called the ecliptic. The ecliptic passes through certain constellations we have come to call the zodiac.

However, Earth also spins around a line from the north to south poles once a day. “Equator” is the term we use for the circle that Earth moves around.

Astronomers measure the sun’s daily motion against the starry background. Of course, we can’t see the stars in the daytime, but we know exactly where they are.

The daily motions of the stars are parallel to a great circle that is Earth’s equator projected on the sky. That imaginary line in the heavens is called the celestial equator.

So now we have two great circles, the ecliptic and the celestial equator.

You can see an image of those circles at www.researchgate.net/figure/The-positoin-of-the-ecliptic-and-celestial-equator-in-geocentric-system-1-7_fig1_338633506.

Sadly, Earth’s axis is tilted 23 1/2 degrees away from its path around the sun. That means that the ecliptic and the celestial equator are also inclined with respect to each other. As a result, they intersect one another in two places.

We officially mark the beginnings of autumn and spring at those two intersections — when the sun crosses the celestial equator as it travels along the ecliptic.

Or, to put it another way, for one magnificent instant on Sept. 22, the sun is at that spot where the celestial equator and the ecliptic meet, a place we call the autumnal equinox.

Not coincidentally, during the day of the equinox, day and night are nearly the same lengths, about 12 hours for each.

Day and night are nearly equal because Earth’s axis is perpendicular to its orbit on that day. You can see an image of that moment in space and time at www.timeanddate.com/calendar/september-equinox.html.

As a result, the shadow line on Earth, called the terminator, that separates night from day, runs through both the north and south poles.

But notice that I said nearly equal. From where we sit on Earth, daylight is, in fact, about eight minutes longer than nighttime.

Our bounty of sunlight is the result of two factors. The sun is not a point of light but a disk that takes a while to set completely.

Also, Earth’s atmosphere acts like a lens and bends the sunlight around the planet a little. The result is that when you see the sun touching the horizon, it is actually below the horizon. (Don’t look at the sun, BTW. It will harm your eyes. You’ll have to take my word for it.)

If you were wondering, the equilux, the day when daylight and nighttime are precisely equal, is always a few days after the autumnal equinox.

(Yes, “equilux” is a legitimate word. If you were looking for proof that astronerds have a term for everything even vaguely astronomical, look no further.)

Unfortunately, the equinox moment doesn’t happen at the same time or on the same date every year. The number of days in a year is not even. Earth takes 365 1/4 days to travel around the sun, and putting 1/4 day on the calendar is tough.

So we add a leap day every four years to make the calendar come out right in the long run. Meanwhile, our clocks and calendars are always wrong if we measure time by the positions of the sun and Earth.

Considering all the above factors, I am pleased to report that the sun’s center will be at the autumnal equinox on Sept. 22, at 9:03 a.m. EDT, and autumn will officially begin.

On March 22, 2023, the vernal equinox will occur. Earth will be on the opposite side of its orbit around the sun. The sun will again cross the celestial equator, spring will spring forth, and day and night will be nearly equal again for one day.

Oh, and by the way, the real equilux always happens a couple of days before the vernal equinox.

Please note that I have used “autumn” to refer to the season, not the more common “fall.”

Personally, I prefer “fall.” However, a couple of years ago, one of my British Facebook friends chastised me for using it. Apparently, some folks in England consider “fall” an American barbarism. The Brits prefer “autumn.” An explanation is in order. To give it, I must now remove my astronerd hat and replace it with my English-major chapeau.

For a long time before the terms “fall” and “autumn” ever existed, people in England referred to the season as “harvest,” which makes perfect sense.

During the 17th century, many English people migrated from rural areas to cities. “Harvest” didn’t make much sense in an urban context.

Two terms emerged, and both became popular. The more hoity-toity “autumn” comes from the Latin term “autumnus,” which comes (through Norman French) from the Latin verb “augere,” which means to “ripen” or “prosper,” more or less.

The Latin term seems to maintain some of the sense of the old word “harvest,” but the word is more likely to be used by the upper, Latin-educated classes.

The more plebeian word “fall” goes back centuries to the Old English word “fiaell,” which means to “fall from a great height.”

Yes, that’s right. “fall” really does refer to the leaves falling from the trees. Or perhaps it refers to the significant change that comes as the glorious heights of summer descend into the depths of winter.

Most of the folks who migrated from England to the New World were more likely to be from the plebeian classes, so “fall” took hold in America.

So “fall” it is, thank you very much. God bless the USA.