March 1 will be a little delayed this year. February is sticking around for an extra day.
That’s because 2024 is a leap year. The glitch-in-time keeps our calendars aligned with the Earth’s revolutions around the sun by adding a rare Feb. 29 every four years—give or take.
It’s a custom dating back to the Roman Empire and Julius Caesar himself. And while it seems bizarre and antiquated, without it our seasons could drift so wildly that we might celebrate the Fourth of July in a snowstorm.
“When we look up in the sky, we see the sun rise and set—and set and rise again. And we call that a day,” said George Washington University Associate Professor of Astrophysics Sylvain Guiriec. “But time is much more complicated.”
Indeed, while our calendars mark a year as 365 days, that’s not entirely correct. The Earth takes about 365.25 days to orbit the sun—more like 365.242189 days, actually. Those leftover quarters add up to an extra day roughly every four years, equaling a leap year.
Without the occasional Feb. 29, we would lose almost six hours every year—increasingly falling out of sync with our seasons to the point where we would be off by about 24 calendar days in 100 years. “At some point, January 1 would be in the summer season,” Guiriec said. “You would be planting your crops in the winter and not in the spring.”
History’s ‘hodgepodge’
Leap years have had consequences for centuries—and not just for the roughly one-in-1,460 people today who are born on Feb. 29. Ancient civilizations depended on the cosmos to guide their decisions, charting the motion of celestial bodies to determine when to harvest crops or graze animals.
But while the first calendars helped prepare for changing seasons and weather, they tended to be a “hodgepodge,” noted Andrew M. Smith II, associate professor of history and classical and ancient near eastern studies at GW’s Columbian College of Arts and Sciences (CCAS).
They often overlapped between lunar- and solar-based and had almost no continuity across borders. “Virtually every ancient city had their own calendar and their own way of measuring time,” Smith said.
The Romans established a 12-month, 355-day version around 500 BC—which almost immediately fell out of line with the sun’s place in the sky. From time to time, a politically appointed high priest called the Pontifex Maximus would add temporary, short-term months to the calendar, in order to, for example, hold festivals honoring Roman gods or extend consulship terms. But those arbitrary additions sowed even more confusion across the empire. “If you had, say, a day of work in Rome, you could go south to Naples and celebrate a festival instead,” Smith said.
Even after the Julian calendar introduced the extra day every four years in about 45 BC—and became the standard across the western world until the 16th century—the adjustments weren’t quite precise enough. The Julian leap days added too much time. Every four years, the calendar gained about 45 extra minutes. In 1582, the Gregorian calendar, which we still use today, corrected Caesar’s rounding errors by skipping leap years every 400 years. If the year was divisible by 100 and not divisible by 400, a leap year was skipped. The year 2000, for example, was a leap year, but not 1700, 1800 and 1900.
But the formula gets even more complicated: From time to time we add leap seconds in order to sync our atomic clock even more precisely to the Earth’s rotation.
The speed at which the planet rotates around its axis fluctuates daily, Guiriec explained. Natural phenomena like earthquakes and storms can accelerate or slow down the Earth’s spin, while the tidal effect of the moon is constantly affecting our rotation.
To compensate, one second is occasionally added—either on June 30 or on Dec. 31—creating a minute with 61 seconds that ticks off just before midnight. A leap second is unpredictable, however, and is typically announced just a few months before it’s applied. The last leap second struck on Dec. 31, 2016.
Leap year, doom year
Other calendars—including the Islamic calendar, Chinese calendar and Ethiopian calendars—also have versions of leap years, some adding multiple leap days or even shortened leap months. The Hebrew calendar adds an entire month called Second Adar every few years to keep pace with its lunar cycle, noted Christopher Rollston, chair of the CCAS Department of Classical and Near Eastern Languages and Civilizations.
In some corners, leap years can be a cause for celebration—like in the twin cities of Anthony, Texas, and Anthony, New Mexico, the self-proclaimed “Leap Year Capitals of the World,” which both hold four-day leap year festivals. In Scotland, however, leap year is thought to be bad for livestock, leading to the Scottish saying, “Leap year was ne’er a good sheep year.” And an Italian proverb warns “Anno bisesto, anno funesto”—leap year, doom year.
Meanwhile, leap years may be most challenging for the world’s roughly 5 million “leapers”—people born on a Feb. 29 who must decide whether to blow out the candles on Feb. 28 or March 1.
“There are big consequences to leap years,” Guiriec said. “And they are mostly for legally defining birthday celebrations!”