While pumpkin spice season seems to arrive earlier each year, the autumnal equinox reliably takes place every third week of September, marking the approach of crisp fall weather and shortening days.  

Equinox comes from the Latin aequus (equal) and nox (night), for the 24-hour period when day and night are of equal length. It is the moment when the center of the sun passes Earth’s equator on its way toward the southern hemisphere; this year, that moment is 9:21 a.m. central time on Sep. 22.  (Because a year is actually 365.25 days long, which we adjust with each leap year, the autumnal equinox date can vary from the 22nd to the 23rd.)

A Tilted Planet on the Move
Of course, the sun is not actually moving south: equinoxes, solstices and seasons occur because of Earth’s movement around the sun and because our planet is permanently tilted on its axis – the imaginary line between both poles which is the center of Earth’s rotation. Instead of pointing up and down, Earth’s axis is tilted 23.5 degrees – a position mirrored by the tilt of standard classroom globes. The relationship of that tilt to the sun determines seasonal and daylight shifts throughout the year.

“The tilt of the earth is constant as it revolves around the sun,” explains Andy Rivers, Ph.D., Associate Professor of Instruction at Northwestern University’s Department of Physics and Astronomy, “so there are times when the axis of the earth’s rotation is pointed toward the sun – that would be summer in the northern hemisphere, with the greatest magnitude toward the sun during summer solstice.” The northern hemisphere axis points away from the sun during our winter, he says, with the axis being farthest from the sun at December’s winter solstice.   

During equinoxes, the axis is pointing neither toward or away from the sun – it is basically sideways, which results in equal day and night across the entire planet for about 24 hours. At the north and south poles, which spend half of each year in daylight or darkness, the sun is uniquely positioned during equinoxes.

“The sun basically skirts the horizon,” explains Dr. Rivers, “so it’s kind of half above and half below for the whole day.”  You could actually see that above-below phenomenon, he says, except for one important feature:  the earth’s atmosphere bends or refracts sunlight so the sun appears higher above the horizon than it actually is. After September’s equinox, the sun sinks below the horizon and the North Pole won’t see it again for the next six months. (Watch how daylight and darkness shift on Earth over an entire year: tinyurl.com/year-of-sunlight)

So Close, and Yet So Cold
Ironically for us northerners, Earth is closest to the sun in January (about 91 million miles) and farthest from it during our summer (about 95 million miles).  Dr. Rivers explains that the tilt rather than our distance from the sun determines temperature around the planet.  

“The tilt has a greater effect in summer when we have these much longer days,” he explains, “and because the sun is higher in the sky, the light coming down on the surface of the earth is on a more direct line.”  More sunlight hours combined with more direct sunlight overcomes the fact that we are farther from the sun. In winter the opposite is true: the sun is much lower in the sky, coming in at an oblique angle and over a longer area, he says, so we don’t feel its heating effect in the northern hemisphere even though we are closer to it.

So, When Do Seasons Really Begin?
We think of equinoxes as marking the start of spring and autumn, but weather scientists are more likely to use calendar months to differentiate the seasons.

“While the four equinox and solstice events are interesting, they are not really the best way to define the start of seasons in Illinois,” explains Illinois state climatologist Jim Angel on the Climate Illinois blog.  Starting dates of March 1 for spring, June 1 for summer, Sept. 1 for fall, and December 1 for winter are better aligned with the typical or average seasonal temperature patterns for Illinois. The “climatological” summer dates of June 1 to Aug. 31 “nicely covers the warmest part of the year,” writes Dr. Angel; similarly, climatological winter dates of Dec. 1 to Feb. 28 better represent the coldest (on average) time of year.  

Equinoctial Myths and Mysteries
Besides a change of season, look out for a few other events related to the equinox.  In Chicago’s east-west oriented cityscape, the equinox sunrise and sunset are almost perfectly framed between downtown high-rises. The phenomenon is dubbed “Chicagohenge” for its similarity to the framing of solstice sunrises and sunsets at Stonehenge in the United Kingdom.

The possibility of seeing Northern Lights increases around the equinoxes, according to spaceweather.com.  For reasons not fully understood, but possibly related to an alignment of Earth’s magnetic field with the Sun’s and a periodic wobble of Earth’s axis, auroras appear more frequently around the north and south poles during spring and fall.  Occasionally, they are even strong enough to be seen from the Chicago area.

While equinoxes (and solstices) are commonly associated with mystical rituals and energies, Dr. Rivers gently dispels the myth that something special about equinoxes makes it easier to stand an egg on its end.

“There’s no gravitational effect that makes this possible,” he says.  “You can do it any time, if you’re very careful.”