Spring arrives today: Here’s the science behind the vernal equinox

The Equinox

After the bizarre winter of 2022-2023, the most eagerly anticipated words in southeast Michigan have been “spring begins.”

On this, the first day of spring, March 20, 2023, it is important to note that astronomical spring officially begins this evening at 5:24 p.m.

So what does that mean?

Well, it begins with something you were taught back in elementary school: Earth is tilted. In other words, the north pole doesn’t point straight up. Rather, if you drew an imaginary line from the north pole to the south pole, that line would be tilted 23.5 degrees from vertical, and that tilt does not change during our planet’s annual trip around the sun.

As you know, we live in the northern hemisphere -- the northern half of the planet. In our summer, the northern hemisphere is tilted toward the sun. Conversely, in the winter, we’re tilted away from the sun. Those moments where that imaginary line is pointed farthest away from or closest toward the sun are called solstices.

However, on a certain day every year in March and September, the sun’s rays are shining on the “side” of that imaginary line -- directly over the equator. That moment halfway between the solstices are called equinoxes, and tonight’s equinox is called the vernal equinox (the one in September is called the autumnal equinox).

What happens on the equinox?

First of all, the sun rises and sets due east and west all across the planet, except right at the north and south poles, where there is no east and west!

Second, contrary to popular belief, we do NOT have an equal amount of daylight and night on the equinox. I’ve been asked about this countless times over the years, and here’s the explanation: First, sunset is defined as the moment that the top of the sun, not the center of the sun, passes below the horizon. So that gives us some extra time for daylight. Second, Earth’s atmosphere refracts the sun’s light, and makes it appear a little higher in the sky, so that gives us a little extra time before it passes below the horizon. All together, these factors give us about eight more minutes of daylight than night on the equinox. Our sunrise today was at 7:36 a.m., and our sunset is at 7:44 p.m.

Finally, and this is something I didn’t know that I just learned from the fine folks at EarthSky.org, the sun actually sets faster on the equinoxes than on the solstices! At first, this didn’t make sense to me, but now it does, and it’s all simple geometry. On the solstices, the sun is setting at a shallower angle, whereas the sun sets at a sharper angle on the equinoxes -- that’s why the top of the sun drops to the horizon faster on the equinoxes.

Close to our latitude, the sun takes about two minutes forty-five seconds (00:02:45) to set on the equinox, and about three minutes fifteen seconds on the solstice. That’s why a beautiful sunset over the lake or ocean mid-summer or mid-winter seems to last a little longer than sunset does in mid-March or mid-September: Because it really does!

Finally, no, don’t try to balance an egg at 5:24 p.m. – that’s a common myth that surfaces every year at this time. The vernal equinox has nothing to do with your ability to balance an egg. I’m asked this every year, and every year I have to disappoint parents and teachers who want to make the equinox that kind of a teaching moment -- sorry.

About the Author:

Local 4 meteorologist Paul Gross was born in Detroit and has spent his entire life and career right here in southeast Michigan. Paul has researched, written and produced eight half-hour documentaries for WDIV, as well as many science, historical and environmental stories.