The Tortured History of Leap Year

This Thursday, Feb. 29, the calendar gets an extra day and in at least 1904, it had its own card. Leap Year happens every four years and is easy to remember if you’re an American. Leap year falls on the year of a Presidential election. But leap year has a much longer story than does the United States.

The year is 46 BC. Julius Caesar is on the Roman throne of the seeming center of the then universe. Julius decided the Roman calendar needed an update.

Before his decision, the Roman calendar was 10 days shorter and divided into lunar months. Occasionally, a month was added to keep the year in sync with the seasons. The added month, known as Mercedonius, was of uncertain length and was added by the wisdom of the high priest.

At some point, Caesar noticed that the Egyptians based their calendar on the sun rather than the moon. Conferring with the astronomer Sosigenes, he came up with the Julian calendar. Each year would be 365.25 days long. He added January and February to address the difference between his lunar calendar and the solar year. The adjusted months were the lengths we would recognize today. The shortest month would get an extra day every four years. Problem solved!

Although it took decades for the new calendar to be observed, it was soon apparent that it didn’t solve the problem. Caesar was off in his calculations. It was too long by 11 minutes and 14 seconds.

This miscalculation interfered with the liturgical (public worship) date setting, such as Easter, that were set by the moon. In a convoluted design, the Gregorian calendar (named after its creator Pope Gregory) was adopted. Besides adding an extra day every four years, it needed more fine tuning. It added the skipping of three leap years every 400 years. So 2000 was a leap year, but 1700, 1800, and 1900 weren’t and 2100 won’t be. This keeps the calendar at least closer to solar alignment.

The Gregorian calendar was introduced in England in 1582. The colonies (that includes us) didn’t adopt it until 1752. By its adoption in the American colonies 170 years later, the error was a whooping 11 days. People went to sleep on Sept. 2 and woke up on September 14. They made the adjustment, probably reluctantly.

Some holdouts didn’t adopt the Gregorian calendar until the 20th century and beyond. Russia adopted it in 1918 and Saudi Arabia in 2016. But today it seems to be the calendar that is universally used. As James Barnes of the National Bureau of Standards quipped, “It takes time to agree on time.”

So why is it called leap year? The technical term of adding time to the calendar is intercalation. But “leap year” is a less forgettable term. A leap year is every four years when the date on the calendar leaps ahead two days of the week instead of one.

So that leaves us with the issue of those born on February 29. The odds of being a “leaper” are 1 in 1,461 according to the Honor Society of Leap Year Babies. It seems the most popular day to celebrate a Feb. 29 birthday on non-leap years is February 28.

That brings us back to the little leap year card. It is postmarked on February 1904. It seems to have a little belated Valentine sentiment attached, or the card was chosen to reassure someone that their valentine status was still intact some 15 days later … you decide. Regardless, Happy Leap Year Day!