Because we have leap years every 4 years — but not always

Next week sees an extra day on the 2024 calendar—a leap day. You will 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 day – Thursday, February 29.

About five million “jumps” around the world will celebrate their birthdays once every four years on February 29, according to the BBC. Increase sales of special Leap Day birthday cards.

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

Because we have leap years

Having a leap year every four years means keeping the Gregorian calendar in sync with the seasons. The reason is the Earth’s orbit towards the sun. It completes a full orbit in exactly 365 days, 5 hours, 48 ​​minutes and 45 seconds, so to account for this quarter of a day, every four years is counted as an extra day on the calendar. Since February only has 28 days, that’s where the extra day comes in. Technically, February 29th is called if intermediate day, according to Time and date.

Thus, the 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 is evenly divisible by four, it will be a leap year. But there’s one problem — the year 2100 won’t 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? That’s why the Gregorian calendar replaced the Roman-era Julian calendar in the 16th century.

“In 46 BC, Julius Caesar 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 a predictable correction to the issue of the quarter shift.” said Dr. James McCormack, a researcher at Warwick University’s Astrophysics Group, in an email. “However, this was a slight overcorrection of the problem. As the solar year was not exactly 365.25 days, but was, in fact, slightly shorter at 365.2422 solar days, the Julian Calendar and the solar year were again moving apart, albeit much more slowly, at a rate of 11.2 minutes annually”.

13 Day Drift

The Julian calendar’s oversimplification of the Earth’s orbit around the Sun created a 13-day shift to accumulate by the end of the 15th century. This led Pope Gregory XIII, Pontifex Maximus of the Catholic Church, to create the Gregorian calendar in 1582.

In addition to introducing a leap day every four days, the Gregorian calendar skips three leap days every four centuries. So now, if the year is 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 Smithsonianas it was in the 1700s, 1800s, and 1900s — but not in the year 2000.

I wish you clear skies and open eyes