The chances you are born on February 29th are one in every 1,461 births, making a birthday on leap day rather special.  But, why do we have leap year’s, leap days and leap seconds anyway?

The reason for the leap day is actually fairly straight forward.  The earth takes exactly 365.2422 days to complete its orbit around the sun, but the Gregorian calendar uses 365 days. That disconnect of a quarter day means our calendars would eventually not match the seasons.

orbit of earth

If we stopped leap years, after 4 years the calendar would be about a day short.  In 400 years you’d be celebrating Christmas around the time the sun was crossing the autumnal equinox.  So leap years – and leap seconds- are added as means of keeping our calendars (and clocks) in sync with the Earth and its seasons.

Here’s a fun piece of trivia to use this week.  A leap year occurs every 4 years, except when it doesn’t.  If the year isn’t divisible by 400 and 4 there isn’t a leap year.  So 1900, didn’t have a leap year, neither did 1800, or 1700.  In about 10,000 years we will have to rethink the system again as it still isn’t perfect.

Leap Seconds Are Different

The Earth’s rotation is slowing down  by around two thousandths of a second per day.  This means we need to periodically add a second to the clocks.  Not doing so would mean eventually the sun would be rising when your clock said midnight.

Why Do We Add The Day in February?

Presently we use the Gregorian calendar introduced by Pope Gregory in 1582.  Prior to this, the western/Christian word used the Julian calendar was introduced by Julius Caesar in 45 BCE (Before Common Era)


Emperors Ego’s Playing With The Calendars

At the time of Julius Caesar February had 30 days and the month named after him July had 31, but August only had 29 days.  When Caesar Augustus became Emperor he took two days from February and added them to August, his month.  This is why both July and August are 31 days.   Imagine if he had taken two days from July, our summers might be shorter! Maybe he didn’t want to lose out on his vacation time on the Amalfi Coast.

Colonial Time warp 

The United States didn’t switch from the Julian calendar until 1752.  This is one of the reasons some of the Founding Fathers have two birthday dates.  George Washington was born in Virginia on February 11, 1731, according to the then-used Julian calendar. In 1752, when, Britain and all its colonies started using the Gregorian calendar  it moved Washington’s birthday a year and 11 days to February 22, 1732.

If you are wondering why his birthday moved a full year, it’s because the change from the Julian to the Gregorian calendar occurred in three steps beginning at the end of 1750 and finally reaching completion in September of 1752. You can see the steps below. And you thought daylight saving time was confusing.

1752 september leap year

  • December 31, 1750 was followed by January 1, 1750 (under the “Old Style” calendar, December was the 10th month and January the 11th)
  • March 24, 1750 was followed by March 25, 1751 (March 25 was the first day of the “Old Style” year)
  • December 31, 1751 was followed by January 1, 1752 (the switch from March 25 to January 1 as the first day of the year)
  • September 2, 1752 was followed by September 14, 1752 (drop of 11 days to conform to the Gregorian calendar)

Final Factoid-Alaska did not change from the Julian calendar to the Gregorian calendar until 1867 because, up to that point, it was part of Russia.

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