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

By Paul Gross - Meteorologist

The Equinox

After the bizarre winter of 2018-2019, the most eagerly anticipated words in southeast Michigan have been “Spring begins.”  

Even though it comes with the caveat that Mother Nature typically doesn’t keep to a calendar and lags behind a bit, it’s still psychologically important to know that astronomical spring officially begins Wednesday at 5:58 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 Wednesday’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 of 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 Wednesday is at 7:37 a.m., and our sunset is at 7:45 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 5:58 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. 

Copyright 2019 by WDIV ClickOnDetroit - All rights reserved.