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Joseph Kaminski

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June 29, 2017

Quick History: Leap Year

Happy Leap Year! Our modern day Gregorian calendar is far from perfect. None of the calendar systems in use perfectly reflect the length of a tropical year, which happens to be approximately 365.242189 days long on average. There are better calendars than the one currently in use, the Gregorian calendar, which inherited its name from Pope Gregory XIII, who introduced it in October 1582. Just for the record, here are some of the world’s calendars in order from most accurate to least accurate.

Calendar Introduced Average Year Length Approximate Error
Solar Hijri calendar 2nd millennium BCE 365.2421986 days < 1 sec/year (1 day in 110,000 years)
Revised Julian calendar 1923 CE 365.242222 days 2 sec/year (1 day in 31,250 years)
Mayan calendar ~2000 BCE 365.242036 days 13 sec/year (1 day in 6500 years)
Gregorian calendar 1582 CE 365.2425 days 27 sec/year (1 day in 3236 years)
Jewish calendar 9th century CE 365.246822 days 7 min/year (1 day in 216 years)
Julian calendar 45 BCE 365.25 days 11 min/year (1 day in 128 years)
365-day calendar (no leap years) 365 days 6 hours/year (1 day in 4 years)

It takes the Earth approximately 365.242189 days – or 365 days, 5 hours, 48 minutes, and 45 seconds – to complete a single circle around the sun. So, we need a calendar that can keep up the pace with the world it belongs to, or else everything might eventually fall behind. Sure, it takes some time before those days add up. But, one must realize that time adds up quickly.

The Gregorian calendar has 365 days in a year, so if we didn’t add a leap day on February 29 every four years, we would lose six hours off our calendar every year. See what I mean by time adding up quickly? After only a single century, the calendar most widely used would be off by twenty-four full days! How do we fix this? With a leap year, of course.

Without Leap Year...

There are some weird stipulations when it comes to how the Gregorian calendar is set up. For example, lets go back in time to the year 2000, a leap year. If 1800 and 1900 weren’t leap years, and 2100 and 2200 won’t be either, how was 2000 able to add an extra day onto the calendar in February that year? Since 1752, years exactly divisible by 100 are only considered leap years when they are also divisible by 400. Here’s a FAQ link for you, because I’m unable to explain it in ways I wish I could.

So, Happy Leap Day if you’re reading this in 2016.

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