Leap Day 2024: Why We Have Leap Years Every 4 Years—But Not Always

Next week sees an extra day in the 2024 calendar—a leap day. You’ll remember the last leap year. It was 2020, and a certain global pandemic was dominating everything. Fast forward four years, and once again, it’s time for another lucky leap day—Thursday, February 29.

About five million “leaplings” around the world will celebrate their once-every-four-year birthday on February 29, according to the BBC. Cue a spike in sales of specialist Leap Day birthday cards.

But there’s no better way to spend 2024’s extra day than by learning why we have a leap day and a leap year—and why we occasionally don’t have them when you might expect.

Why We Have Leap Years

Having a leap year every four years is to keep the Gregorian calendar in sync with the seasons. The cause is Earth’s orbit of the sun. It completes one entire orbit in precisely 365 days, 5 hours, 48 minutes and 45 seconds, so to account for that quarter of a day, it’s tallied every four years into an extra day in the calendar. Since February only has 28 days, that’s where the extra day is inserted. Technically, February 29 is called an intercalary day, according to Timeanddate.

So, previous leap years of this century include 2020, 2016, 2012, 2008, 2004 and 2000. The next leap years are 2028, 2032, 2036 and 2040, and so on. The rule seems very simple—if the year can be evenly divisible by four, it will be a leap year. But there’s a problem—the year 2100 will not be a leap year. Why not?

Julian To Gregorian

Unfortunately, 5 hours, 48 minutes, and 45 seconds is not the same as six hours. What about those 11 minutes and 15 seconds? They are why the Gregorian calendar replaced the Roman-era Julian calendar in the 16th century.

“In 46 BC, Julius Cesar proposed the new Julian Calendar, which would add a day to the shortest month of the year (February) every four years in an attempt to allow for a predictable correction to the issue of the quarter day drift,” said Dr James McCormac, researcher in Warwick University’s Astrophysics Group, in an email. “However, this was a slight overcorrection to the problem. As the solar year was not exactly 365.25 days but was, in fact, slightly less at 365.2422 solar days, the Julian Calendar and the solar year were now drifting apart again, albeit much more slowly, at a rate of 11.2 minutes per year.”

13 Day Drift

The oversimplification of the Julian calendar concerning Earth’s orbit of the sun created a drift of 13 days to accumulate by the late 1500s. This caused Pope Gregory XIII, the Pontifex Maximus of the Catholic church, to create the Gregorian calendar in 1582.

As well as inserting a leap day every four days, the Gregorian calendar skips three leap days every four centuries. So now, if the year can be evenly divisible by four, it will be a leap year unless it is also divisible by 100 and not divisible by 400. So, in the year 2100, a leap year will be skipped, according to the Smithsonian, as it was in 1700, 1800 and 1900—but not in the year 2000.

Wishing you clear skies and wide eyes