Leap year 2024: why is one day every four years recovered in February?

This year has 366 days instead of the normal 365. While it may seem a little strange to ‘randomly’ add another day onto February every four years, there is actually some serious science behind the decision.

A leap year, also referred to as an intercalary or bissextile year, occurs because the Earth takes slightly less than 365 1/4 days (365.242190 days, or precisely 365 days, 5 hours, 48 minutes, and 56 seconds) to complete its orbit around the sun.

Why is the extra day added onto February?

While the fractional difference may appear insignificant, accumulating those decimals over a few centuries would disrupt the alignment of seasons, potentially resulting in an unusual scenario like a sunny Christmas in the northern hemisphere—something deeply undesirable and a problem that even the Romans found tricky to resolve.

To address this concern, an extra day was incorporated into the already short-changed February to recover the time lost.

The decision to put the extra day in February dates back to Roman times, the first people to realise that their calendar was sliding too much ever so often. The Romans had already added 2 new months to the calendar, Ianuarius (January) and Februarius (February), to stop things going astray, but it wasn’t enough.

So about every two years, Roman consuls would add a 27- or 28-day 13th month (Mercedonius) to realign their measure of time with the sun. This extra month would be inserted after February 23 and chopping it down by five days. Aren’t you glad somebody did all this calendar rejigging for you.

However, the problems persisted and it was the work of Julius Caesar and Greek astronomer Sosigenes (an adviser to Cleopatra) who came after and worked on the aptly named Julian calendar, which had new maths and a 365 days and 6 hours-long year. Caesar’s calendar added a day after February 23 by extending February 24 to 48 hours.

The Julian Calendar, despite the science, was still not enough and ended up accidentally overshooting the solar year by 11 minutes annually, a discrepancy that accumulated to cause a 10-day difference with the solar cycle by the 1570s. This was when Pope Gregory XIII introduced a new calendar - the Gregorian calendar - which we use today.

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Why don’t we have leap years every four years?

Now, back to the modern day phenomenon of knowing a year is actually 365.242190 days long. That’s still not right.

As we’ve been looking at unexpectedly complicated history of the leap year, you have probably noticed the error and are shouting at the screen: multiplying 5 hours, 48 minutes, and 56 seconds (as a decimal, 5.815) by 4 yields 23.262222, not exactly 24, or what would be a full day. This means that not every 4 years can certainly be a leap year without some more mathematical wiggling.

The rule says that if a year is divisible by 100 but not divisible by 400, the leap year is omitted. For instance, the year 2000 was a leap year, while 1700, 1800, and 1900 were not. The next instance of skipping a leap year will be in 2100.

The next leap year will be in 2028 as they come around every four years, except in the situation previously mentioned. If you’re already in 2028 and want the answer to the above question, then you should probably look for a more updated article.