ORIGIN: Starting the Conversation

Leap Year

02.29.16 by Tom Hendrick
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When I was a fourth-grade student at Garfield Elementary in Lawton, Oklahoma, one of my classmates used to tell me she was older than her mom. Of course, we all know that is not possible. What she really meant to say was she had celebrated more “birthdays” than her mom. How is that possible? As you may have figured out already, it was only possible because my classmate’s mother was born on February 29th, a leap year. As I recall, my classmate turned nine in fourth-grade and her mom had celebrated eight “birthdays” at that time.

Most everyone knows leap year occurs February 29th. And most people would probably say they know it occurs every four years. But actually that isn’t true. So do you really know when we have leap year? Do you know why we have it? Do you know when it started and who is responsible for it? If you don’t, you are in luck! Keep reading.

Julius Caesar replaced the previously used Lunar Calendar with a 365-day calendar (Julian calendar) in the first century and added leap years in order to keep the calendar working properly. The typical 365 day calendar year is meant to match up with the solar year, or the time it takes the earth to complete its orbit around the Sun – about one year. However, the actual time it takes the Earth to travel around the Sun is in fact longer than a year, so the calendar year and the solar year don’t completely match. The actual time it takes the Earth to orbit the Sun is about 365 ¼ days (365 days, 5 hours, 48 minutes and 46 seconds to be precise). So Julius Caesar added a day to the calendar every four years to account for this “1/4” day). However, because the solar year is not actual 365 and 6 hours (1/4 day), Julius Caesar’s addition of a leap year every four years turned out to be too much time. By the late 1500’s, more than ten days had been added to the calendar than should have been. So in 1582 Pope Gregory XIII, along with a German priest named Christopher Clavius set out to change this once and for all. That year, if your birthday fell between October 5 and October 14 inclusive, you might as well have been born on February 30th because those ten dates for that year were eliminated. Of course the problem would have arisen again, except they also decided to also omit leap years three times every four hundred years. So in addition to the rule that 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. Hence the reason we had leap year in the year 2000, but did not have it in 1700, 1800, or 1900. This ingenious correction worked beautifully and the calendar became known in 1582 as the Gregorian calendar in honor of Pope Gregory. However, for a long time many Protestant nations resisted adopting a “catholic” calendar. That finally changed for England and her colonies in 1752 (another year of vanishing dates) and the now the Gregorian calendar is pretty much used universally throughout most of the world.

Now the calendar year and the solar year are just about a half-minute apart. At that rate, it will take 3,300 years for the calendar year and the solar year to diverge by a day.

So now you know the rest of the story.

Happy Leap Year!

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Tom Hendrick, PE


Tom received his Bachelor and Master of Science in Civil Engineering degrees from the University of Oklahoma and is a licensed professional engineer in eleven…

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