Friday, February 29, marks leap day, a day that’s added to the month of February almost every four years – that is, every leap year – to keep the modern calendar in line with the celestial cycles that frame it.
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?
An Extra Day for Everyone – Lobbying for Leap Year Status (February 24, 2004)
The short answer is: Yes. Think of leap year as the little trick the world uses to make up for lost time.
The Quarter-Day Conundrum
We observe the modern leap year because Earth orbits the sun every 365.242 days – not an easy number for a calendar to accommodate.
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 – like festivals, agricultural milestones, religious observations, and other important dates – 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. Imagine people in prison for a year.”
The problem was compounded when officials responsible for managing the calendar began to use the practice for political gains, like 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 Reforms Roman Calendar?
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.
So 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.
That 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.
Enter Pope Gregory
The extra quarter of a day that the leap year added was slightly longer than the 0.242 of a day 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.
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Leap year: how the world makes up for lost time