Happy Leap Year… here are the little known rules!

February 29, 2008

Random Thoughts

EarthSun [Blog] Happy leap day! It’s a special day that only comes every four years… or does it? I’ve always believed that every four years there is a February 29th, but that is not exactly true. I came across this interesting fact this week.

In case you don’t know why there is a leap year every… er, almost every four years, I’ll tell you why. Most of the world uses the Gregorian calendar which has 365 days in a year. However, a solar year (the time it takes for the Earth to circle the sun) is 365 days and 6 hours. So every 4 years we have to add an extra day to make up for the extra 6 hours (6 hours over four years equals 24 hours… one day).

Oh, but that doesn’t explain why we don’t always celebrate leap year (something I didn’t know). I wasn’t honest above. A solar year is not 365 days and 6 hours. It is actually 365 days, 5 hours, 49 minutes and 12 seconds. “Why don’t we round up?” you may ask. Well, the same reason we don’t just round up now with the extra 6 hours every year. If we didn’t, we’d eventually be celebrating Christmas in the summer (I’m not talking to those of you who live in the southern hemisphere).

So, to make up for this very small difference, here is the rule. If a year is divisible by 100, it is not a leap year. However, if it is divisible by 400, it is a leap year. That means at the turn of every 4th century, where there should be a leap year, there is not. For example, there was a leap year in 1892, 1896 and 1904, but not in 1900. Because 1900 is divisible by 100, it is not a leap year. Most recently, the year 2000 was a leap year because it is divisible by 400. So, the next time this “skipping of a leap year” will happen is 2100, only 92 years from today. I’m really looking forward to it, I’ll only be 123 years old. A man can have a dream, right?

If any of this confused you, read more detailed explanations here or here!


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