Why Do We Have Leap Years?

2024 is a leap year, meaning February will have an extra day. We know leap years occur every four years, but why?

It takes 365.24 days for the Earth to orbit the Sun, however we usually round down the days in a calendar year to 365. We can’t forget about that extra quarter day each year though – subtracting 5 hours, 46 minutes and 48 seconds off of a year might not seem like a big deal, but if you keep subtracting almost 6 hours every year for many years, it can hugely disrupt our seasons. If we never had leap years, all those missing hours would add up to days, weeks and eventually months – in a few hundred years, July would actually take place in the cold winter months!

To correct this, leap years began more than two millennia ago, when the scientific advisers to Roman emperor Julius Caesar noticed that the years were not properly aligning with the seasons. This meant in 45 BC, Caesar decreed there would be an extra day every four years to compensate.

The issue was, however, the Julian calendar didn’t fully take into account the slow drift that happened because that quarter of a day wasn’t exactly a quarter of a day. This wasn’t noticed for centuries, until 1582 when Pope Gregory XIII was advised that the alignment of the seasons had become out of sync by 10 days.

To compensate for this, the pope’s astronomers worked out from then on, every year dividable by four would stay as a leap year in addition to every end of century year divisible by 400 to correct the shift. Despite this, it was agreed immediate action still had to be taken, resulting in the pope deciding to drop 10 days from that year in October. This meant the day after the 4th October 1582 became the 15th October.

Since then, the Gregorian calendar has been working well, however as a non-Catholic country, Britain didn’t adopt the Gregorian calendar until 1752, by which time 11 days had to be dropped to catch up.