29 Feb 2012

Why the Extra Day?

I think most of us would have known that this year is different from most years because it's declared as a leap year. As far as we know, that means we get the extra day in February. But why exactly do we need to do this every four years?

Certainly it isn't meant to confuse people all over the world using this system, nor is it deliberately meant to short-change people in various ways, for instance having to work an extra day without pay in Australia! The short answer is that the standard year, or the time it takes for Earth to complete one revolution around the Sun, is roughly 365.25 days. If we don't add in the extra day, the calendar ends up losing one day every four years, which is bothersome.

However, the truth is we don't always add the extra day every four years! And why is that?
Simply put, the real length of a year is slightly less than 365.25 days. If we did add the extra day every four years we'll end up overshooting after a while! So years that are divisible by 100 end up being exempted from the "leap day" tradition... UNLESS that particular year is divisible by 400. That means years 1600 and 2000 and 2400 are leap years, but years 1500, 1700, 1800, 1900 etc are not. Sometimes to adjust things further, extra time durations (intercalations as their fancy term) of finer divisions are inserted, maybe an extra hour, minute, or even a leap second!

And the standard Gregorian calendar isn't the only one that deals with leap years: the Chinese calendar also adds a "leap month" (闰月) in such a way that the 11th month always contains the northern winter solstice. Hindu calendars also have their extra month (adhika maasa)every 2 to 3 years such that it compensates for the missing 11 days per year.

There are a lot of traditions linked to the leap year, usually involving marriages. Some say it to be lucky to wed on a leap year, others advise against it. Some countries even say a leap year is the only year where the woman gets to propose to her man, and that refusal of her proposal will lead to some compensation to her!

Sometimes people are lucky (or unlucky, depending on the point of view) to be born on exactly the leap day itself, which means their anniversary only comes mostly every four years. These people are usually called "leapings" or "leapers" for obvious reasons. Of course it would be impractical for somebody who looks about 20 years old to claim that he's only about 3 or 4 years of age! So for convenience most countries decided that leapers' birthdays would fall on either February 28th or March 1st.

So don't think of the leap year as just a way to prolong your agonising days at work or school; without it our calendar system would go really out of date real quick! And remember to think of all those leapers who only get to celebrate their "true" birthday today ;)