Adios, winter. Spring's almost here — or at least it is for some of us. Above the equator, the first day of the season comes in late March during an event locally known as the spring (or "vernal," meaning spring) equinox. On this momentous occasion, the sun will be positioned directly above the equator, something that only happens twice a year, on this date and the autumnal equinox. The 2021 spring equinox arrives on March 20 at 5:37 a.m. EDT. For your astronomical pleasure, here are five quick facts about it.
1. The Term "Equinox" Means "Equal Night"
Time to talk about word origins. Translated from Latin, equinox means "equal night." In that ancient tongue, the words for "even" is "aequi" while "nox" means "night."
On the date of an equinox, daytime and nighttime have similar — but not quite equal — lengths. This has everything to do with the way our planet moves. Earth's axis is tilted at a 23.5-degree angle relative to the plane of its orbit around the sun.
An equinox marks the exact moment when the sun enters an imaginary line in the sky. Picture a giant, invisible ring around the Earth that sits directly above the equator. Scientists call this make-believe halo the celestial equator. It's a line the sun only crosses twice a year — on the spring and autumn equinoxes. Due to that solar angle, every region of our planet will experience close to 12 hours of daylight and 12 hours of darkness on March 20. (Technically, the daytime is going to last a few minutes longer than the nighttime.)
2. Climate Scientists Recognize a Different "First Day of Spring"
By way of tradition, the March Equinox is celebrated as the first day of spring in the Northern Hemisphere (and the first day of autumn in the Southern Hemisphere). But did you know that a competing start date exists?
Astronomers define seasons on the basis of Earth's position around the sun. According to the astronomical calendar, spring and autumn both begin with an equinox. Conversely, summer and winter start with a solstice. Those are biannual dates on which one hemisphere experiences its shortest day of the year.
So, if we go by the astronomical calendar, the spring of 2021 in the Northern Hemisphere will last from March 20 to the summer solstice on June 20. But climate scientists don't define the seasons this way. They use a different calendar — known as the "meteorological calendar." On it, the year is divided into four seasons lasting three months each, with spring beginning on March 1 and then lasting through April and May. Climate scientists base their calendar on the temperature cycle rather than the astronomical position of the sun — it's easier to calculate statistics and forecast trends using full months and the same dates each year.
3. The Spring Equinox Ushers in the Persian New Year
"Navroz Mobarak!" The precise moment of the vernal equinox marks the first day of the first month on the Iranian solar calendar. It's also the start of Nowruz, an awesome 13-day celebration sometimes called the Persian New Year. Observed by 300 million people around the world (mainly in central and western Asia), Nowruz is preceded by a flurry of household chores. After the spring cleaning ends, families enjoy decadent meals and sometimes paint eggs that represent fertility. On the last Tuesday before the equinox, there's a ritual in which people jump over bonfires. This symbolizes the feeling of renewal that the new year offers. Another tradition sees children bang on pots and go door-to-door asking for treats. On the last day of the celebration, the family will go for a picnic as it is unlucky to stay home.
4. And Other Holidays Too, Like Easter
For centuries, practitioners of Shintoism used the solar event as an occasion to honor their forebears. The Japanese government later converted this tradition into a secular, national holiday formally known as Vernal Equinox Day in the year 1948. It's still being observed today.
In most denominations of Christianity, Easter is celebrated on the Sunday after the first full moon that follows March 21. What's so special about this date? Well, it marks the so-called ecclesiastical Spring Equinox. Ask any astronomer and they'll tell you that a spring equinox can fall on March 19, 20 or 21. But for simplicity's sake, many church leaders treat March 21 as though it were the designated spring equinox date every year. Easter can fall as early as March 22 or as late as April 25.
On this day too, a snake-like shadow also appears to crawl down one flank of El Castillo pyramid in Mexico, possibly in tribute to a serpent god (see sidebar for more info).
5. Earth's Wobbling Will Affect Future Equinoxes
Like a spinning top, our home planet wobbles slightly on its axis. Today, the axis points toward the famous "north star," Polaris. But 12,000 years from now, it will shift away from Polaris and aim itself at a different star: Vega. This whole cycle will then repeat itself over the course of 26,000 years.
Gravitational forces exerted by the sun and moon are the main reason why planet Earth wobbles, and as it wobbles, the sun's relationship with the zodiac calendar changes. During the vernal equinox, the sun crosses in front of the constellation Pisces. Or at least that's true these days. Prior to the year 68 B.C.E., the sun used to line up with Aires — another constellation — on the spring equinox. By 2567, the sun will line up with the constellation Aquarius.
Originally Published: Mar 20, 2019