Leap years

Feb. 29 comes around once every four years—most of the time. If your birthday happens to fall on the day, you’ve likely been treated to many jokes about being of elementary-school age, even if you are actually in your thirties.

This one day helps the entire world stay in order. Without that extra 24 hours, the calendar would fall out of sync with Earth’s rotation of the sun. Seasons would be mixed around, spring in the Northern Hemisphere would soon start in June and then July and August, and winter in Australia would steadily creep toward December.

Other calendars have taken note of the significance of an extra day as well. The Bahá’í and Hebrew calendars include their own leap days. So how did this important intermittent day find its place? Let’s mark our calendars.

>11,000: Members in the Honor Society of Leap Year Day Babies around the world

365.242189 days: Length of Earth’s revolution around the sun, or 365 days, 5 hours, 48 minutes, and 45 seconds

1 in 1,461: Chances of being born on Feb. 29

4: Divisibility the year must have to fall on a leap year; it must also be not divisible by 100 unless it is divisible by 400

11 minutes: Annual surplus from including “century years” in the every-four-years calendar model

5 million: Approximate number of living humans who were born on Feb. 29

0.53 seconds: Time the tropical year—which the Gregorian calendar is based on—shortens every century, due to the increase in the Earth’s precession rate

Not every calendar considers a leap year. The Islamic calendar is based on a lunar system and adds up to 354 days, shifting around 11 days from the Gregorian calendar each year—that’s why Ramadan dates change depending on when the ninth month falls. A single leap day is sometimes added to the calendar, though.

The Chinese lunar-solar calendar isn’t the official calendar of China, Vietnam, or Taiwan, but it’s still heavily used in many traditions and in everyday life. The calendar is based off the phases of the moon and will include a leap month about once every three years.

While neither of these calendars are considered official in terms of coordinating dates around the globe, they are just as valid in tracking a year and passage of time.

For some ridiculous reason, to which, however, I’ve no desire to be disloyal,

Some person in authority, I don’t know who, very likely the Astronomer Royal,

Has decided that, although for such a beastly month as February, twenty-eight days as a rule are plenty,

One year in every four his days shall be reckoned as nine and twenty.

—From Pirates of Penzance, in which a leap year birthday is a pivotal plot point

~3000 BCE: Sumerians have a 360-day calendar of 30-day months, plus an extra five days of partying and festivals to stay in line with the Earth’s rotation around the sun.

46 BCE: Julius Caesar and Sosigenes of Alexandria develop the Julian calendar, which includes an extra day in February every four years. The original calendar had too many leap years.

1577: The Julian calendar falls 10 days out of alignment.

1582: Pope Gregory XIII creates the Gregorian calendar to upgrade the leap year format, which includes skipping “Century Years” (e.g. 1700, 1800) unless the century is divisible by 4 (e.g. 2000). The calendar is still in use today.

1997: The Honor Society of Leap Year Day Babies is formed. One of its main missions is to get Leap Year Day capitalized in dictionaries and noted on calendars.

2012: Parts of Microsoft Azure’s cloud service go down due to a leap year bug.

One of the most famous Leap Year Day traditions is women proposing to men in Ireland and Great Britain. The exact beginning of the practice is unknown. There’s the theory of St. Brigid of Kildare complaining to St. Patrick in 5th-century Ireland that women had to wait too long for their suitors to propose, convincing St. Patrick to allow women one day to ask the question. But there’s no proof the tradition existed before the 19th century.

Greeks believe leap years to be bad luck, and, apparently, one in five couples will actively avoid setting a wedding date during a leap year. Some traditions are on the less serious side. France has a satirical newspaper, La Bougie du Sapeur, that only publishes on Feb. 29. It has been published every leap year since 1980 and covers four years of news, making it the world’s least frequently published newspaper.

The Guinness World Record for consecutive generations of leap year babies in a family is three. Peter Anthony was born in Ireland in 1940, his son Peter Eric was born in the UK in 1964, and his granddaughter Bethany Wealth was born in the UK in 1996. 

The current Gregorian calendar has an average length of 365.2425 days, which is about half a minute longer than the solar year. Dozens of generations will pass before the calendar moves a day out of the seasonal cycle.

“So 3,000 years from now, people may decide to tweak it,” John Lowe, leader of the National Institute of Standards and Technology (NIST)’s Time and Frequency Division, told National Geographic. “We’ll just have to wait and see.”

In the meantime, Feb. 29 isn’t going anywhere.

At Slate, Phil Plait explains the lengthy, surprisingly effective math behind the Gregorian leap year (and how it could be a little better).