Why do we have leap years?

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 years, 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. If we stopped leap years, after four 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.

orbit of earth.jpg

Here’s a fun piece of trivia to use this week. A leap year occurs every four years, except when it doesn’t. If the year isn’t divisible by 400 and four, 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 a bit different
A leap second is added every few years because the Earth’s rotation is slowing down on the order of about two thousandths of a second per day. Atomic time is constant, and to keep our clocks in sync, we periodically add a leap second. If we didn’t do this, eventually the clocks would say noon when it’s 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, which was introduced by Julius Caesar in 45 BCE.


Ego of the emperors
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 calendar confusion
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. And you thought daylight saving time was confusing.


• 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)

Follow Dave Epstein on Twitter @growingwisdom

1752 september leap year.png

Follow Dave Epstein on Twitter @growingwisdom

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