How do leap years work?

2024 is a leap year, which means that the month of February has an extra day at the end: February 29. Although this is a relatively common phenomenon, occurring every four years, many people don't really understand why it happens.

In a world where people didn't keep track of time with names and numbers, there wouldn't be much of a reason to add an extra date here and there. It became increasingly important as people began to write down holidays and histories according to when they took place.

Strictly speaking, there aren't really any "leap years," as that would imply that an entire year was being skipped. However, that is the colloquial term that is generally used to describe a year that contains a leap day. The reasoning behind the addition of leap days has a surpring amount to do with astronomical forces and the Christian liturgical schedule.

The science behind leap days

When we look back on the creation of regimented time, most of the lengths of time we're accustomed to are created due to astronomical forces. A day is one full rotation of the Earth. A month is a revolution of the Moon around the Earth (sort of). And a year is a full revolution of the Earth around the sun.

However, these astronomical events don't always fit perfectly into our rigid calendars. While we usually think of a year being 365 days long, it's actually closer to 365.25 days long. Because of this, we have to adjust the calendar to avoid time slips.

The process is known as intercalating. We artificially add time into the standard calendars and clocks to make sure that the date and time stay relatively consistent from year to year. While the additional day in leap years is the most commonly discussed version of this, there are also leap seconds and even leap months when necessary.

For those of us using the Gregorian calendar, we get an added day at the end of February in nearly every year that is a multiple of 4. However, on years that are multiples of 100 (but not 400), the leap year is skipped. This is because the length of an astronomical year is actually calculated as 365.2425 days long. But since this is relatively rare, the four year rule is all most people need to know.

What would happen if there weren't leap years?

Since there is an extra quarter of a day in every astronomical year, that has to be accounted for somewhere, or else the things we associate with certain months or seasons will start to move.

Over the course of 100 years, there would be nearly a month's worth of days left unaccounted for. That would drastically change things like seasons, which agricultural communities plan their lives around. In addition, it would make it hard for communities that plan their holidays around seasonal factors to keep track of when to celebrate.

The Gregorian calendar specifically adds this extra day in February in order to keep track of the start of Spring. This is directly tied to the Christian liturgical season. Easter is set to take place on the first Sunday after the first full moon after the Vernal Equinox. By adding the extra day in the end of February, the Vernal Equinox is consistently on or around March 21, making Easter easier to track.

Are leap days and years lucky or unlucky?

While leap years only exist because of the complexities of calculating astronomical revolutions, they have historically held a lot of spiritual weight. In some cultures, the extra days are treated as particularly lucky, while others believe that they are inherently unlucky.

In Ireland, leap days were considered to be lucky days for love, as they were associated with increased female agency. According to legend, St. Patrick mandated that leap days were the only times that women could propose to men, rather than the other way around. This tradition extended to the UK and some parts of mainland Europe, along with the belief that a man rejecting a leap day proposal should provide the woman with a gift.

In Greece, however, it's believed that getting married at any time during a leap year is bad luck for the couple's future. Those who do are considered more likely to get divorced, and those that get divorced during a leap year will supposedly remain loveless.

Taiwan takes it a step beyond romantic prospects, believing that leap years are bad luck for the elderly altogether. In order to protect their parents, adult children are expected to cook pig trotters and deliver it to their parents on the leap day.

Those born on the leap day, called "leaplings," are considered to have bad luck in Scotland. While it is considered a fun quirk in most countries, the Scottish see it as a bad omen for the person's life, which will supposedly be filled with hardship.

At the end of the day, it really just depends on your culture whether leap days and leap years are good luck or bad. Either way, they're an interesting way to manage a scientific problem, ensuring that seasons and holidays never get too far off track.