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Leap Years - Why We Have Them

March 12th, 2008 11:38 AM by Lehel Szucs

 This is fun.  Enjoy!

2008 is a leap year, which means that it has 366 days instead of the usual 365 days that an ordinary year has. An extra day is added in a leap year - February 29 - which is called an "intercalary day" or a "leap day." What follows is a summary of a piece entitled "Leap Year 101" that I found at www.infoplease.com on why we have leap years.

You probably know this, but leap years are added to the calendar to keep it working properly. The 365 days of the annual calendar are meant to match up with the solar year. A solar year is the time it takes the Earth to complete its orbit around the Sun - about one year. But the actual time it takes for the Earth to travel around the Sun is in fact a little longer than that - about 365 1/4 days (365 days, 5 hours, 48 minutes, and 46 seconds, to be precise). So the calendar and the solar year don't exactly match; the calendar year is a touch shorter than the solar year.

It may not seem like much of a difference, but after a few years those extra quarter days in the solar year begin to add up. After four years, for example, the four extra quarter days would make the calendar fall behind the solar year by about a day. Over the course of a century, the difference between the solar year and the calendar year would become 25 days. In this example, instead of summer beginning in June, it wouldn't start until nearly a month later, in July. So every four years a leap day is added to the calendar to allow it to catch up to the solar year.

FYI, the Egyptians were the first to come up with the idea of adding a leap day once every four years to keep the calendar in sync with the solar year. Later, the Romans adopted this solution for their calendar, and they became the first to designate February 29 as the leap day.

But Wait, It's Not Quite that Simple

Since the solar year is apprx. 365 1/4 days long, the math seems to work out just fine by adding an extra day to the calendar every four years to compensate for the extra quarter of a day in the solar year - but not exactly. This is because the exact length of a solar year is actually 11 minutes and 14 seconds less than 365 1/4 days.

That means that even if you add a leap day every four years, the calendar would still overshoot the solar year by a little bit - 11 minutes and 14 seconds per year. These minutes and seconds really start to add up over long periods of time: each 128 years, for example, the calendar would gain an entire extra day. So, the leap year rule, "add a leap year every four years" was a good rule, but not good enough.

To rectify the situation, the creators of our calendar (the Gregorian calendar, introduced in 1582, see link below) decided to omit leap years three times every four hundred years. This would shorten the calendar every so often and rid it of the annual excess of 11 minutes and 14 seconds.

So in addition to the rule that a leap year occurs every four years, a new rule was added: a "century year" is not a leap year unless it is evenly divisible by 400. Thus 1700, 1800, and 1900 were not leap years, but 1600, 2000, and 2400 are leap years. This rule manages to eliminate three leap years every few hundred years.

This ingenious correction worked beautifully in bringing the calendar year and the solar year into harmony, pretty much eliminating those pesky extra 11 minutes and 14 seconds. Now the calendar year and the solar year are just about a half a minute off. At that rate, it takes 3,300 years for the calendar year and the solar year to diverge by a day.

What If You Are Born On Leap Day?

For those fortunate, or unfortunate, people who were born on February 29, there are some interesting challenges as you might imagine. But first, what are the odds of being born on February 29? According to the experts, the odds of being born on February 29 are about 1 in 1500 worldwide. These same experts estimate that there are about 185,000-200,000 people now alive in the US who were born on Leap Day, and apprx. four million worldwide.

For those who were born on February 29, this means that you have a legitimate birthday only once every four years (or less). Isn't that interesting? And it has led to many leap day slogans over the years, such as leap day babies tend to live four times as long as the rest of us, which of course is not true. There is even a Society of Leap Day born folks. Typically, those born on February 29 celebrate their yearly birthdays on either February 28 or March 1 in non-leap years.

In the modern day world of computers and the Internet, those born on leap day face many unusual challenges. Most specifically, many software programs do not accept February 29 as a legitimate calendar date. As you can imagine, this leap day birthdate phenomenon has even more far-reaching implications. Since the February 29 leap day is not immediately recognized in many software programs, it can also cause problems in receiving one's Social Security benefits, insurance settlements, various calendar-based payments, etc., etc.

Well enough for being born on leap day, but the implications for those born on February 29 are interesting. And there are even leap day implications for the rest of us not born on February 29, especially for investors.

Posted in:General
Posted by Lehel Szucs on March 12th, 2008 11:38 AM

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