8 Amazing Facts About Leap Years

Leap Years Exist Because a Year Isn’t Exactly 365 Days Long

When it comes to the human construction of time and the movement of our planet in the solar system, nothing lines up perfectly, and leap years are evidence of this. It doesn’t take the Earth exactly 365 days to revolve around the sun (nor does it take 366 days, for that matter) — the exact number that mathematicians, popes, and politicians have spent millennia trying to reflect in a calendar is 365.24219 days. Notably, the number after the decimal is very close to 0.25 or simply one-fourth. Because it’s impossible to experience one-quarter of a day every year, humans invented a leap year that occurs nearly every four years (more on that in a minute).

Julius Caesar Didn’t Invent the Leap Year

Although the precursor to today’s modern calendar bears his name, Julius Caesar wasn’t a great astronomer or a great mathematician. To devise a new calendar that would do away with Rome’s extremely confusing system, Caesar called in the big brains: Sosigenes of Alexandria. Caesar met this great Greek thinker while in Egypt pursuing a political rival around 48 BCE (and while having a fling with Cleopatra).

Before instituting the new calendar on January 1, 45 BCE, Caesar added 67 days to the previous year, making it the longest year on record. Following this “year of confusion,” the Julian calendar got off to a rocky start because a misunderstanding of Sosigenes’ instructions led to a leap year being inserted every three years instead of every four. Luckily, Caesar’s nephew and heir Octavian (aka Augustus) corrected the mistake during his reign as emperor.

Technically, Leap Years Don’t Happen Every Four Years

If Western society still ran on the Julian calendar, a leap year would occur every four years like clockwork, but it doesn’t. While Sosigenes’ creation was a major calendrical leap forward, the Julian calendar was oh-so-slightly longer than the actual solar year of 365.24219 days. Although a miniscule difference, this led to the calendar drifting from solar reality by 11 minutes every year. By the 16th century, the Julian calendar was a full 10 days behind where it had been in 325 CE, when the Council of Nicaea fixed the date of Easter in relation to the spring equinox. The situation seriously irked Pope Gregory XIII.

Yet much like Caesar, Gregory XIII wasn’t an expert with numbers, so he relied on the work of Italian mathematician Aloysius Lilius, as well as Christopher Clavius, a German mathematician, to finally settle the whole calendar thing once and for all. Their innovation essentially boiled down to this: If a year is divisible by 100 but not by 400, the leap year is skipped. This means that while the year 2000 was a leap year, being divisible by both 100 and 400, the years 1700, 1800, and 1900 were not leap years (and leap year will also be skipped in the year 2100). This edged the number south of 365.25 days just enough that this newly minted Gregorian calendar was much more accurate than its Roman predecessor.

The Modern Leap Year Calendar Still Isn’t Perfect

Even after 2,000 years of tweaking, today’s calendar isn’t perfect. It takes the Earth 365.24219 days to travel around the sun, as noted, but the Gregorian calendar is 365.2425 days long — much closer to solar reality than previous calendars, but not 100% accurate. This means that the Gregorian calendar will still drift one day every 3,236 years. So in the year 4818, humans, human-AI hybrids, world-conquering aliens, or whoever happens to be living on the planet at the time will have a tough calendrical decision to make.

Leap Seconds Also Exist

In the past 50 years, 27 leap seconds have been added to time because the world doesn’t rotate at a uniform rate. Earthquakes, volcanoes, tidal forces, and even wind patterns can temporarily affect the length of a day, and as these oddball rotations add up, scientists eventually have to add an extra second by having the clock strike 23:59:60 before turning over to a new day. These extra seconds can wreak havoc on satellites, airlines, tech companies, and financial systems that rely on extremely precise timekeeping. In 2015, for example, global markets halted trading to avoid any weirdness when a leap second was added on June 30. Because of this temporal disruption, the scientific community agreed to end the practice of adding (or in extreme cases possibly subtracting) seconds by 2035. It remains to be seen if a new kind of time-altering practice will take its place.

Only Around 5 Million People in the World Have a Leap Day Birthday

The odds of being born on a leap day are pretty small — 1 in 1,461. However, those odds aren’t nil, and about 5 million people have the distinction of having a birthday that only exists every four years. Known as “leapers,” “leaplings,” and “leapsters,” these people usually celebrate their birthdays on either February 28 or March 1 in non-leap years.

One of the most curious cases of leap year birthdays concerns the Irish British Keogh family. The grandfather, Peter Anthony, was born on leap day in 1940, his son Peter was born on leap day 1964, and Peter’s daughter Bethany was born on February 29, 1996. Luckily, the Keogh family doesn’t hail from Scotland, a region that traditionally believes that someone born on leap day “will live a life of untold suffering.”

Sexist Traditions Surround Leap Years Throughout History

Every leap year, Ireland observes a strange tradition known as “Bachelor’s Day,” in which women are encouraged to propose to men. While this practice is mostly lighthearted and harmless, for years the U.S. celebrated a similar gender-swap tradition that only laid bare the inherent inequalities between men and women in America. On February 29, women held jobs usually reserved for men (such as posts on normally all-male municipal town boards), and advertisements painted leap day as a moment when overly aggressive, lovesick women could use their 24 hours of power to entrap unmarried men. (This was in stark contrast to other countries, such as Greece, that considered marriages held on leap day to be bad luck.) Although this gender-swapped tradition dates back to at least the late 1700s, the practice slowly faded as marriages became more egalitarian in the 1970s and onward.

A Town in Texas Calls Itself the “Leap Year Capital of the World”

Not all leap year traditions are mired in the bad ideas of a bygone era. Boldly declaring themselves the “Leap Year Capital of the World,” Anthony, Texas, celebrates leap day with a three-day festival that begins with an “exclusive leap day celebration for leap day babies” on February 29. That’s followed up with two days of music, food, and fun for the whole family at Anthony Municipal Park. The west Texas tradition began back in 1988 when Mary Ann Brown and her neighbor Birdie Lewis — both born on leap day — took the idea for a festival to the small town’s Chamber of Commerce, which eventually gave the go-ahead and initially sponsored the event as well. Today, people from around the world travel to the town to celebrate with fellow leaplings.