The Autumnal Equinox

Pumpkin spice permeates the air. Color creeps into the leaves. A favorite season for many people begins tomorrow – autumn. With all the fun of fall, unfortunately, comes the notion that, indeed, winter is coming. Daylight is slipping away. For those of us who loathe the lack of daytime in the winters, the autumnal equinox brings only one good feature: we’re already halfway to the point where days start to get longer!

If you think it sounds like I’m trying to psych myself up, you’re correct. We still have three full months of shortening daylight. I love the colors of fall, the cooler temperatures, and the lack of allergens in the air after the great die-off, but the shorter sessions we have with our star are depressing.

To keep my mind occupied, let’s explore the science behind the equinox!

Friday is the first full day of fall, though the autumnal equinox technically occurs Thursday at 9:03 PM EDT. From the Latin word aequinoctium, equinox means “equal night.” Occurring twice every year, equinoxes mark a change in astronomical season and happen when the length of day and night are very close to equal (more on this in a bit). Etymologists actually don’t have a hard origin for autumn, but, obviously, the word intersects with the traditional time of the harvest in the Northern Hemisphere. Conversely, our brethren in the Southern Hemisphere will experience the vernal equinox, entering the season of spring.

Illumination of Earth by the Sun at the equinox - image by Przemyslaw Idzkiewicz

Unlike the difference between the summer and winter solstices, where the amount of sunlight is diametrically opposed, the equinoxes are kind of like the same day, just with the “movement” of the sun going in a different direction. A person looking at a snapshot of the Earth at the equinoxes might have a hard time determining whether the date is March or September.

While we tend to think of the equinox as the day when light and darkness arrive in equal measure, that’s technically not correct. The equinox is more precisely the time when the Earth is not tilted in relation to the sun’s path. As we learned in a previous episode on The Tilting of Uranus, seasons are caused by our planet’s tilt. But on the equinox, for a brief moment, we’re “standing straight up.”  On the equator, at the equinox, the sun will be directly overhead. It is also the day that the sun rises exactly in the east.

In the Northern Hemisphere, we’re now racing toward the Winter Solstice, the moment of the minimum tilt toward the sun and the shortest day of the year.

Looking at the above image, one might wonder how the equinox is not the date when day and night are exactly even. The answer stems from the fact that our atmosphere refracts light and that we see the sun as a disk, not a single point of light. So when the sun is rising, we can actually see light before the disk ascends over the horizon; conversely, after the sun has dipped below the horizon at night, we can still see some light from the disk. The result is actually a few minutes more daylight on the equinox than night.

Contour plot of the hours of daylight as a function of latitude and day of the year, showing approximately 12 hours of daylight at all latitudes during the equinoxes - image by Sebastian Helm

The date when day and night are actually equal to each other has been coined the equilux. In the spring, the equilux occurs before the equinox. In the fall, we still have not yet reached the equilux. In Ohio, where this missive originated, that date is September 26. Check the Further Reading section below for an article you can read to find the date of the equilux where you live.

Unfortunately, looking at the chart above, at 40 degrees north latitude we are fast approaching the 8-10 hour band, meaning nearly two-thirds of each day will be sun-less. I dig winter and all its attributes, other than the lack of daylight.

So, now, issue three of four on the astronomical seasons in 2022 is complete. Seems just yesterday we published the spring article! Hopefully, time moves just as quickly to the upcoming solstice, so we can start adding daylight to our lives.

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