## Why do we have Leap Years?

2016 is a leap year, which means that the year will have 366 days rather than 365. This extra day is placed at the end of February, meaning that February 2016 will have 29 days, rather than its usual 28.

Leap Years occur every 4 years. They are needed to keep the calendar we use (the Gregorian Calendar) in alignment with the Earth’s revolutions around the sun. It takes the Earth approximately 365.242199 days to circle around the sun once. That is the same as 365 days, 5 hours, 48 minutes, and 46 seconds. As the Gregorian calendar only has 365 days in a year, if there wasn’t an additional day on February 29 every 4 years, we would lose almost six hours off our calendar each year. That means that, after only 100 years, our calendar would be wrong by approximately 24 days; almost an entire month. On each occasion, the leap year date has to be exactly divisible by 4. However, if the year is also divisible by 100, it is not a leap year- unless it can be divided by 400 as well! For example, this means that the years 2000 and 2400 are leap years, while 1800, 1900, 2100, 2200, 2300 and 2500 are not leap years (Still keeping up?).

The idea of a leap year was first introduced by Julius Caesar in the Roman empire over 2000 years ago. The Julian calendar was different to our more modern Gregorian calendar in that leap years were worked out more simply, with the only rule being that the year had to be divisible by 4. However, this ended up producing too many leap years! The situation still wasn’t corrected until the introduction of the Gregorian Calendar by Aloysius Lilius (an Italian physician who named it after Pope Gregory) in 1582, though, and it wasn’t until 1752 that the Gregorian calendar was adopted in Britain and America, and our own leap year calendar was corrected.

As for the future, well, if you’ve been paying attention you’ll know that the next leap years will be 2020, 2024… and so on.

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Dr Kathryn Bates is a graduate of archaeology and history. She has excavated across the world as an archaeologist, and tutored medieval history at Leicester University. She joined the administrative team at Oxford Open Learning twelve years ago. Alongside her distance learning work, Dr Bates is a bestselling novelist, and an itinerant creative writing tutor for primary school children.