What’s the deal with Leap Day?

Everyone knows every four years February ends on the 29 instead of the usual 28, but do you know why leap day happens? To explain what the deal is with leap day, we’ve assembled a quick guide.

Why do we have Leap Years?

Simply put, leap days are in place for astronomical reasons.

It’s meant to keep our calendar (called the Gregorian calendar, named after the 16th-century pope who introduced it in 1582) in sync with the Earth’s orbit around the sun, usually called a “solar year” or a “tropical year.”

It’s all about a pesky quarter of a day adding up over time

Generally speaking, it takes about 365.25 days for the Earth to complete a rotation around the sun (or more precisely, 365 days, 5 hours, 48 minutes, and 45 seconds).

Where we run into problems with that measurement is that extra quarter of a day (.25). With that .25 in place, a calendar without leap years would “drift” by about six hours each year. If we were to keep going this way, in about 100 years the calendar would be off by a whole 24 days. This messes with the start of the seasons and eventually, February and March would “drift” into the northern hemisphere’s summer period. Imagine going for your annual summer vacation, but it’s “February” instead of “July.”

To fix this problem, we add a day to the calendar every four years to keep everything somewhat aligned. It’s not exactly the most elegant solution though — adding those leap days every four years adds a few extra seconds to each year.

After about 10,000 years, we’ll have three extra days to play within the calendar. Nobody seems overly concerned about this right now though, so we’re essentially kicking this problem down the road for future generations to deal with.

How are leap years calculated?

The general rule of thumb is that a leap year is every four years, but there’s more to it than that. According to the National Research Council of Canada, leap years can be calculated using the following rules:

1. If a year is divisible evenly by 4 with no remainder, it can be a leap year (e.g. 1996 divided by 4 equals 499, so 1996 was a leap year).
2. If a year is divisible evenly by 100, it can’t be a leap year (e.g.1900 divided by 100 equals 19) unless it is also divisible by 400. So a ‘century year’ like 2000 could be a leap year and the year 1800 could not because it is not divisible by 400.

The next leap day will be on Thursday, Feb. 29, 2024, as it satisfies both rules above (2024 divided by four equals 505 with no remainder and it’s not the start of a new century

With files from the Associated Press, National Geographic and The National Research Council of Canada.