For centuries, humans struggled to sync civil, religious, and agricultural calendars with the solar year. Adding a ‘leap year’ solved the problem—though just for the next 3,300 years.
It’s that time again: Saturday, February 29, is a leap day, the calendar oddity that occurs (almost) every four years.
For centuries, attempts to sync calendars with the length of the natural year have sowed chaos—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 connected in any way to how long it takes for the Earth to get around the sun,” says John Lowe, who led the National Institute of Standards and Technology (NIST)’s Time & Frequency Division until his retirement.
The solar 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.
Humans have long organized our lives in accordance with what we’ve observed in the skies. Ancient Egyptians planted their crops each year on the night when the brightest night star disappeared, while historians in ancient Greece and Rome also relied on the positions of the stars to anchor events in time. Religious leaders expected feast days to align with certain seasons and lunar phases.
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. “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 strategies
Efforts to make nature’s schedule fit our own have been imperfect from the start.
Early Egyptians (prior to about 3100 B.C.) and other societies from China to Rome once used lunar calendars to track time.
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.
Other 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.
The practice of adding extra days to the year is at least as old as these systems.
“When the Egyptians adopted this calendar they were aware that there was a problem,” says Lowe. “They just added an extra five days of festivals, of partying, at the end of the year.”