Wednesday, February 29, marks leap day—an extra 24 hours that gets added to the month of February roughly every four years—that is, every leap year—to keep the modern calendar in line with celestial cycles.
But where did leap year come from? How does it work? And have other cultures with their own systems for tracking time needed to use it, too?
Leap Year Needed to Correct Calendar Drift
We observe the modern leap year because Earth orbits the sun every 365.242 days—not an easy number for a calendar to accommodate. (See an interactive map of the solar system.)
As a result, many cultures since ancient times have taken on the practice of adding extra days, or even months, to round out the calendar year.
Early calendars were often based on lunar months, which average 29.5 days. But a year of such months totals only about 354 days.
This discrepancy resulted in annual events—festivals, agricultural milestones, religious observations—drifting out of alignment with their intended seasons as the years passed.
"Civilizations like Rome would add months to try to correct the drift of the lunar calendar," said David Ewing Duncan, author of the book Calendar: Humanity's Epic Struggle to Determine a True and Accurate Year.
But Duncan describes the Roman solution as "sloppy."
"It played havoc with everything from religious holidays to market times," he said.
"Remember, this is a sophisticated society. You had rents due, interest accruing on loans, all kinds of things that would be moved out of shape."
The problem was compounded when officials responsible for managing the calendar began to use the practice for political gains, such as extending the terms of their allies.
"The calendar in Rome had drifted so much that it was months off, and you might have the harvest holiday as seeds are being planted, things like that," Duncan explained.
"People didn't think of their calendar as solid and stable [as we do], but this was clearly way out of hand."
Love Is Root of Roman Leap Year?
Reform came to Rome (and later to the Western world) via the Egyptians, who—along with the Maya and possibly the Babylonians—were among the first to determine the true length of the solar year.
Egypt adopted a leap-year system, with an extra day every four years, during the Greek rule of the Ptolemaic Dynasty (305 to 30 B.C.).
The last Ptolemaic ruler, Cleopatra, was at least indirectly responsible for introducing the concept to her visiting lover, Julius Caesar. (Also see "Headless Egypt King Statue Found; Link to Cleopatra's Tomb?")
In 46 B.C., Julius instituted a single year some 445 days long—later known as the Year of Confusion—to correct years of drift in one fell swoop and prepare for the start of a reformed calendar.
The so-called Julian calendar reorganized the 12 Roman months into a 365-day year with a leap year every four years.
It was a tremendous improvement—but with a lingering flaw: The extra quarter of a day that the leap year added was slightly longer than the 0.242 of a day left over in the actual solar year.
This seemingly small difference made the solar year about 11 minutes too long, resulting in an entire day of discrepancy every 128 years.
Because of this glitch, the Julian calendar had drifted ten days by the late 16th century.
"Finally it became so ridiculous that Pope Gregory XIII was convinced by his astronomers that basically all the Christian holidays were being celebrated on the wrong days," Duncan said.
The pope introduced his Gregorian calendar in 1582, which determined that only one out of every four "century years" would observe a leap year. Thus while the years 2000 and 2400 are leap years, 2100, 2200, and 2300 are not.
The Gregorian calendar was gradually, and sometimes grudgingly, adopted by much of the world and remains in common use.
China's Leap Year Solution
In China the Gregorian calendar is commonly used, but the traditional lunar-solar calendar is still observed to determine the dates of festivals such as Chinese New Year.
As with other ancient calendars, the Chinese traditional calendar corresponds to the phases of the moon.
But the Chinese system also includes a solar calendar and introduces an entire leap month about every third year to keep the calendar in synch with the seasons.
Chinese leap years of 13 months have 383, 384, or 385 days.
Such a system preserves a monthly cycle that begins with the new moon and centers on lunar events, which are important for the timing of religious and cultural milestones.
Maya's Missing Leap Year
The ancient Maya, famed for their elaborate and accurate calendar systems, observed two calendar years, but neither seemed to have bothered with a leap year.
"As far as we know, the people of Mesoamerica—the Maya included—didn't care about leap years," said Anthony Aveni, an expert in ancient Mesoamerican astronomy at Colgate University.
The Maya solar year of 365 days was central to the agricultural cycle, while their ritual year of 260 days was critical for determining auspicious dates.
These calendars were carefully designed to synchronize in 52-year cycles, but no effort was made to prevent "drifting" dates.
"They didn't care if they didn't have a white Christmas, or if their Fourth of July wasn't in the summer, to put it in our terms," Aveni explained.
The Maya instead placed priority on marking the passage of time through additional calendar systems such as the Long Count, which unfolds on a cycle more than 5,000 years long.
"Our philosophy about leap year is a complicated scheme to make the seasons jibe with the calendar," Aveni said.
The Maya "were more concerned that time should be unbroken, not interfered with, and that the count of time should have continuity," he said.
"To break continuity would be to break order."