Most of the modern world has adopted the Gregorian calendar and its leap year system to allow days and months to stay in step with the seasons.
Photograph by Abraham Nowitz, National Geographic
It's that time again: This Monday, February 29, is a leap day, the calendar oddity that occurs (almost) every four years.
For centuries, trying to sync calendars with the length of the natural year caused confusion—until the concept of leap year provided a way to make up for lost time.
“It all comes down to the fact that the number of Earth's revolutions about its own axis, or days, is not equal to or connected in any way to how long it takes for the Earth to get around the sun,” says John Lowe, leader of the National Institute of Standards and Technology (NIST)'s Time & Frequency Division.
The solar, or tropical, year is approximately 365.2422 days long. No calendar comprised of whole days can match that number, and simply ignoring the seemingly small fraction creates a much bigger problem than one might suspect.
The evidence lies in a long history of wildly shifting dates and accompanying civil, agricultural, and religious chaos.
That's why most of the modern world has adopted the Gregorian calendar and its leap year system to allow days and months to stay in step with the seasons. (Also see "World Will Gain a Leap Second on Tuesday: Here's Why.")
“We've made a calendar that comes close,” Lowe says, “but to make it work you have to do these leap day tricks that have some quirky rules.”
Ancient Timekeeping
Efforts to make nature's schedule fit our own have been imperfect from the start. Some ancient calendars, dating to the Sumerians 5,000 years ago, simply divided the year into 12 months of 30 days each. Their 360-day year was nearly a week shorter than our annual journey around the sun. (Also see "Where Our Fear of Friday the 13th Came From.")
The practice of adding extra days to the year is at least as old as these 360-day systems.
“When the Egyptians adopted this calendar they were aware that there was a problem, but they didn't add any more days to the calendar,” says Lowe. “They just added an extra five days of festivals, of partying, at the end of the year.”
Earlier Egyptians (prior to about 3100 B.C.) and other societies from China to Rome once used lunar calendars to track time. (See National Geographic's moon facts.)
But lunar months average 29.5 days and years only about 354. So societies that kept lunar time quickly drifted well out of sync with the seasons due to the 11-day lag.
The Romans regularly tried to tweak this calendar by irregularly adding days or months, but those patchy efforts only highlighted the need for reform.
"Year of Confusion"
By the time Julius Caesar enjoyed his famed affair with Cleopatra, Rome's calendar had diverged from the seasons by some three months. But Egypt was observing a 365-day year, and as early as the third-century B.C. had even established the utility of a leap-year system to correct the calendar every four years.
Julius adopted the system by decreeing a single, 445-day-long Year of Confusion (46 B.C.) to correct the long years of drift in one go. He then mandated a 365.25 day-year that simply added a leap day every fourth year.
But even this system was flawed, because the 0.25 day leap year adds annually is a bit longer than the solar year's leftover 0.242 day. That made the calendar year some 11 minutes shorter than its solar counterpart, so the two diverged by an entire day every 128 years.
“As it turns out, if you stick in one every four years, that's a few too many,” says James Evans, a physicist at the University of Puget Sound and an editor of the Journal or the History of Astronomy.
Between the time Julius Caesar introduced the system in 46 B.C. and the 16th century B.C., this small discrepancy had caused important dates, including the Christian holidays, to drift by some ten days. Pope Gregory XIII found the situation untenable, so his Gregorian calendar was unveiled in 1582—after another drastic adoption of time-warp tactics.
“Gregory reformed the calendar and they dropped ten days from the month of October that year,” Evans says. “Then they changed the leap day rules to correct the problem.”
Now leap years divisible by 100, like the year 1900, are skipped unless they're also divisible by 400, like the year 2000, in which case they're observed. Nobody alive remembers the last lost leap day, but dropping those three leap days every 400 years keeps the calendar on time.
Alternative Calendars
Even today, some calendars discount the leap year meant to keep us in time with our orbit, or ignore the sun altogether.
The Islamic calendar is a lunar system that adds up to only 354 days and shifts some 11 days from the Gregorian calendar each year—though a single leap day is sometimes added.
And while China uses the Gregorian calendar for official purposes, a traditional lunar-solar calendar is still popular in everyday life. It follows the phases of the moon and implements an entire leap month about once every three years.
“There's nothing sacrosanct about locking a calendar to the solar year the way ours is,” says Evans. “People can get used to any calendar system. But once they are used to it what really seems to rile them up is when something is changed.” (Related: "The Politics of Daylight Saving Time.")
Future Decisions
The current Gregorian calendar system makes the fractional days of the solar year and leap year calendar nearly equal by occasionally skipping a leap day.
This system produces an average year length of 365.2425 days, just half a minute longer than the solar year. At such a rate it will take 3,300 years before the Gregorian calendar moves even a day from our seasonal cycle.
That means future generations will have a decision to make on leap year, though not for a long time.
“So 3,000 years from now, people may decide to tweak it," Lowe says. "We'll just have to wait and see.”