Spring begins today in America. Good.
Perhaps you are mildly surprised to learn that March 19 is the first day of spring. Perhaps you learned as a child that the spring equinox — when day and night are roughly the same length — occurs on either March 20 or March 21.
Indeed, the equinox has historically fallen on one of those dates. This is the first time in 124 years the first day of spring has occurred on March 19 nationwide, irrespective of time zone — even the graphics on the National Weather Service's website have yet to catch up with the new reality.
So, how did we end up with an extra-early spring?
Defining spring: maybe not what you expect
The word "spring" refers to two things: the weather that happens between winter and summer, which is meteorological spring; and one portion of the Earth's oval-shaped journey around the Sun, which is astronomical spring.
They don't necessarily line up with each other, because the climate can change (as it is now, causing spring weather to arrive earlier) but the Earth's orbit basically remains the same.
Encyclopaedia Britannica/Universal Images Group via Getty
On the equinox, day and night are roughly equal everywhere on Earth. The date varies because the 365-day calendar doesn't perfectly line up with the motion of the Earth around the Sun.
Encyclopaedia Britannica/Universal Images Group via Getty
When we talk about the vernal, or spring, equinox, we're talking about astronomical spring.
Astronomical spring begins in the northern hemisphere at the moment around this time of year when the Earth's axis is tilted neither toward nor away from the Sun. The moment is the same everywhere, but the time and even the date when you experience that moment depends on your time zone.
And that means the first calendar day of spring can vary across the U.S.
In 2016, the spring equinox in the Eastern time zone was at 12:30 a.m. on March 20, which means it was at 9:30 p.m. on March 19 in the Pacific time zone.
Some parts of the United States have technically experienced the spring equinox on March 19th multiple times in the last two decades, specifically during leap years (more on that in a moment).
This year people in the Eastern time zone are finally joining the rest of the country and we are celebrating the earliest nationwide March 19 equinox since 1896.
But why isn't the time of the equinox the same each year?
The short answer is that the time and the date are imperfect human constructs that we use to keep track of our planet's movements.
The longer answer involves leap years.
"All of this is caused simply by the fact that the spin of the Earth doesn't divide evenly into one year," says Michelle Thaller, an astrophysicist turned space communications expert at NASA.
One spin of the Earth around its axis is one day. "The problem is we're happily spinning on our axis, and the Earth is going around the Sun, but one year — one complete path around the Sun — isn't an even, exact number of days. In fact, it's 365.24 [days]."
Most years, we drop that extra 0.24 and pretend that the year is simply 365 days long. But the Earth's location in its solar orbit doesn't change just because we rounded, so the time at which we experience the equinox gets about a quarter of a day (or about 6 hours) later each year. For example, the time of the equinox would move from around 3 a.m. to 9 a.m. to 3 p.m. over the course of three years.
Every fourth year, we add a leap day to the calendar — February 29 — which gets us almost back in sync with the Earth's orbit. The leap day turns back the clock on the time of the equinox, nearly resetting the approximately 6-hour annual leap forward from the previous three years.
The curse of the remainder
Here's the wrinkle: the leap day we observe every four years would fix things if the length of a year were exactly 365.25 days. But remember, the actual length is closer to 365.24 days.
So the leap day intended to get us back in sync with the Earth's solar orbit doesn't quite do it. And that discrepancy shows up in the time of the equinox, which gets about 45 minutes earlier every leap year.
All those little rounding errors add up over time. If we blithely added a leap day every four years forever, our whole calendar would eventually be totally out of whack with the actual orbit of the Earth.
To make the calendar more accurate, we don't add a leap day when the year is a multiple of 100 (e.g. 1700, 1800, 1900) unless that year is also a multiple of 400 (e.g. 2000). This was the big innovation of a 16th century astronomer working for Pope Gregory XIII who designed the Gregorian calendar we still use. Withholding some leap days helps keep us in sync.
As a result, there was no leap day in the year 1900, so we didn't reset the clock at all and the time of the equinox remained later than it would have been. People who lived most of their lives in the 20th century became accustomed to observing the equinox on March 20 or 21.
But all that time, the equinox was slowly getting earlier by about 45 minutes every four years. And, unlike in 1900, we observed a leap day in the year 2000, which turned the clock back even more. From there, it was just a matter of time before the whole country found itself celebrating the equinox on March 19 regardless of time zone, as we do this year.
Will spring always be early?
If this all makes you a little queasy, do not fear.
Next year, the spring equinox will return to March 20 in much of the country (remember, it moves 6 hours later each year). But spring will begin on March 19 every leap year for the rest of this century, and the time of the spring equinox in non-leap years will move earlier and earlier. By the end of the century, the spring equinox will fall on March 19 most years.
Eventually, in 2100, we'll skip a leap year as we did in 1900 and the time of the equinox will move later again.
NASA's Michelle Thaller says the messiness of the calendar is a reminder that the movements of the Earth are not designed for the pleasure and ease of humans. "We all sort of think that the universe was made for us," she says. "The day and night cycle does not fit into the year. Why isn't it perfect? Because why should it be?"
But, she says, as an astrophysicist she finds meaning in things that are beyond the control of humans. "You know, we're all on this little rotating rock together and we're tiny," she says. "The only thing we have is each other. The only thing that's going to help the loneliness is each other."