The Vernal Equinox

Open the windows, cue In Bloom, and rejoice at the end of winter! Spring is here again!

Today is the first day of the new season! The 2023 vernal equinox transpires (transpired) at 5:24 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). Vernal comes from the Latin word for spring (“ver”). So, the vernal equinox is the one that transpires in March in the Northern Hemisphere. Conversely, our brethren in the Southern Hemisphere just experienced the autumnal equinox, entering the season of fall.

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

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 inclination. But on the equinox, for a brief moment, we’re “standing straight up.”  Check out the NASA video below to watch the effect of tilt on sunlight. The two moments when the dark-light line is straight up are the equinoxes.

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 Summer Solstice, the moment of the maximum tilt toward the sun and the longest day of the year.

Looking at the above image, one might wonder how the equinox is not the day 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.

An image of the March 2019 equinox by a NOAA satellite

The dates when day and night are actually equal to each other has been coined the equiluxIn spring this date happens a few days before the equinox, but the date of an equilux changes with your latitude. Here in Ohio, the equilux was on St. Patrick’s Day, three days ago. In autumn, the equilux happens after the equinox.

I like to look at the chart below and see my latitude creeping toward that yellow band of 14-16 hours of daylight. I love winter, snow, and don’t mind cold like most people, but the lack of sunlight is a bummer. The amount of daylight has been increasing since late December, but we’ve finally crossed that threshold where it outlasts the nighttime!

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

Further Reading and Exploration

March equinox 2023: All you need to know – EarthSky

Spring Equinox 2023: When Is It, And What Is It? – Farmers’ Almanac

Find your equilux based on latitude –

Equinoxes – video by National Geographic

A New Beginning: Celebrating the Spring Equinox – kid’s book by Wendy Pfeffer

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