Let’s leap

Leap year is back. Why do we add a day at the end of February every four years? Why is there no leap year on a century year, except for sometimes? There are rules. They are old and complicated, but they work pretty well.

Most of us know a year is a leap year if it is divisible by four. But, if the year is also divisible by 100, then it is not a leap year unless it is also divisible by 400. That’s why some may remember the year 2000 was a leap year. In fact, it was the first century leap year since 1600.

To thank for all this we have none other than Julius Caesar and Pope Gregory.

Millennia ago, it was recognized that something had to be done to keep the seasons from drifting into different months. The exact length of a year —  how long it actually takes the earth to revolve around the sun — is 365.24219 days. That means if the calendar year were just 365 days, the seasons, which are determined by the position of the earth as it orbits the sun, would shift a quarter of a day every year. Not such a big deal, until 100 years go by and autumn is beginning in mid-August, 25 days earlier than it used to. Another 100 years, and it’s fall in July.

So, in 45 BCE, Caesar decided to add a day at the end of February (back then, it was the last day of the year) every four years.

This was an improvement, because now the average calendar year was 365.25 days, which was closer to the actual year, but it still didn’t fix the problem. At 365.25 days the calendar year was too long, and the seasons would eventually drift the other way, falling one day later every 128 years.

In 1582, Pope Gregory took the matter in hand and changed the end of the calendar year to Dec. 31. He left 10 days out of October and added the “no leap year on a year divisible by 100 unless it’s also divisible by 400” part of the rule.

Now the seasons were back in phase with the calendar, and the average calendar year was 365.2425 days, much closer to a true year.

The Gregorian calendar, with all of its leaping intricacies, is what we still use today in order to keep the seasons where they are supposed to be. But it’s not perfect. Even with the complicated leap year rules, there will still be a drift in the calendar year of one day every 3,300 years.

Think we can live with that?

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