Aleaping we will go

It’s February 29th today, so I thought, like 90% of bloggers on the planet, I would do a post about leap years.

Those silly people who think our planet is less than 6000 years old also sometimes think that the leap year thing is because science somehow screwed up and we have to fudge the numbers occasionally or that it’s because the Earth’s orbit is speeding up.

In fact, the leap year was refined over many years, starting with the romans who added an extra month every couple of years, basically whenever they felt like it, to keep the seasons in sync. Then Julius came along and realised the seasons were still all messed up, so abolished the extra month concept, replacing it with a slightly longer year of 365 days (it was 355 before) with extra days added to various months. There was also an extra day added every three years to try to keep the seasons lined up, but it wasn’t enough and they drifted again. A few years later the period was changed to four years, and things were better, but not perfect.

It wasn’t until the 1500s that someone realised that things weren’t quite right. It happened to be a Pope, Pope Gregory XIII to be precise. He noticed that if things carried on, Easter would eventually bump into Christmas, and we obviously couldn’t have Jesus being nailed to the cross on the day he was born, so he changed the rules, with the help of Kepler’s astronomical observations.

The Gregorian Calendar has been used ever since, and the leap year calculation remains unchanged. The calculation Gregory implemented is one I’ve used many times in computer programs. A year is a leap year if it’s divisible by 4 but not divisible by 100 except when it’s divisible by 400 (that’s why 2000 was a leap year).

Our year now averages out to be 365.2425 days long which is accurate enough that we’ll only be out by one day after 4000 years.

We need more Popes like Gregory XIII, even though he was a bit of a bastard to the English and Irish.

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