This year, the vernal equinox falls on Wednesday, March 20. Are day and night equal on this day? Does the Sun rise exactly east and set exactly west? Learn about the March equinox from Bob Berman, Almanac astronomer.
Why Does the Spring Equinox Fall on Different Dates?
If you thought that the Spring Equinox was only on March 21, you may be dating yourself.
Spring arrived on the 21st of March during most of the 20th century, but the event slides earlier and earlier during the 400-year Gregorian calendar cycle. The final March 21 equinox was in 2007—even if we use Greenwich Time as many almanacs do.
Now the 21st is gone for the rest of our lives, unless you believe in reincarnation and want to check back in during the 22nd century. In a few more decades, the vernal equinox will sometimes start landing on the 19th.
Are Day and Night Equal at the Equinox?
The word equinox is derived from two Latin words—aequus (equal) and nox (night).
The equinox is famously the time of balance, with theoretically 12 hours of sunshine and 12 hours of non-Sun.
In practice, it’s not exactly equal. There’s actually more day than night on the day of an equinox. Why? Earth’s atmosphere bends (refracts) sunlight upward. Also, the Sun isn’t a single point of light but a large disk. Together, these factors add more daylight to the equinox. The real date of sunlight equality is three or four days ahead of the equinox.
The difference depends on where you live (your latitude). See the sunrise, sunset, and day length for your zip code.
Does the Sun Rise Exactly East and Set Exactly Due West?
Yes, it really does. The equinox is also when every place on earth rotates perpendicularly into our planet’s “terminator”—its day-night shadow line. As a result, on the day of the equinox, the sun will rise precisely due east and set exactly in the west, and this is true everywhere.
It’s the best time to observe the cardinal compass directions. (Well, not quite everywhere. From both poles, you’d see the equinoctial Sun hovering fully above the horizon, never setting, but moving horizontally. At the north pole, the Sun moves rightward, and it chugs along leftward for the folks at the South Pole research station, rolling atop the horizon like a red rubber ball.)
How Does Sunlight Change?
This is the week when sunlight changes at its maximum annual rate!
There are three extra minutes of daily Sun from typical US cities, but nearly seven daily minutes for the folks in Fairbanks, Alaska. That may be the equinox’s greatest gift, and worthy of an early morning Sun Salutation.
Check it! See your day length times on the Almanac sunrise/set calculator.
What Else Happens at the Equinox?
In the Northern Hemisphere, plants and animals start feeling the Sun’s energy, though its intensity depends on our latitude.
The equinoctial Sun always misses your zenith (or straight overhead point) by the same number of degrees as your latitude. In Bennington, Vermont, latitude 43°, the midday Sun stands 43° from precisely overhead on the day of the equinox. Essentially, it’s halfway up the sky. It’s a gratifying change from just a month ago, and a dramatic shift since December, when the midday Sun only climbed an anemic one-fifth of the way up the sky. Since solar rays are stronger the higher up it is, you can now palpably FEEL the Sun’s growing intensity.
And it’s not finished: Watch the sky at 1:00 PM each day, and you’ll see that the Sun manages to climb four of its own diameters higher each week. This rapidly ratchets up its intensity. This is the year’s greatest solar-energy boost for those who live north of the equator.
Wherever you are, take a few minutes to enjoy this year’s vernal equinox—and all the good things that come with it.
This article was first published in 2017.
Image Credit: Josephine Wall. Enjoy more art on her Web site.