History of Leap Year


The Leap Day, February 29, depicts a day that occurs only once every four years, every Leap Year or intercalary year when an extra day is inserted into the calendar. But not every fourth year; if that year ends in “00,” like 1900, then it is not a Leap Year. Except if that year ending in 00 is also divisible by 400, then it is a Leap Year. Unless it is a Tuesday and it is dark. OK, I made up that last rule.

So, years like 2024 are Leap Years, being divisible by 4. But 1900 is not a Leap Year, as it ends in 00. In the year 2000, you’ll remember the famous Y2K, when computer programmers only obeyed the first two rules and assumed that it wasn’t a Leap Year so that all the computers failed, and the world came to an end. That was a Leap Year, as it was divisible by 4, and though it ended in 00, it was divisible by 400 (indeed, it’s divisible five times, if you’re still with me.)


Calculation Correction for Leap Year

How did we get into this calculation conundrum? It has to do with a cumulative rounding error in trying to reconcile the Julian calendar with the tropical or astronomical calendar. The Julian calendar, established by Julius Caesar in 46 B.C., lasted from 45 B.C. until A.D. 1582 and stipulated that the year should be 365 days for three years in a row, with every 4th year having 366 days. This meant that an average year was 365.25 days. However, according to the tropical calendar, the year has 365.24219 days.

Tropical YearThis tropical (or seasonal) calendar recognizes that the year is marked by two successive passages of the Sun through the vernal equinox (equal nights). You and I know that the Sun does not pass through the Earth’s sky, but rather the Earth orbits the Sun – but it’s easier to explain this by considering this apparent motion of the Sun in our sky. And, of course, this is just the easy explanation.

A Leap Second is the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time. But that was determined back in 1960. Since then, the second has been defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the Cesium 133 atom… due, of course, to the interaction of the state of the nucleus and the state of the electron clouds. Perhaps that’s intuitively obvious to the most casual observer of the Newtonian dynamical theory of motion.


Leap Year: It Adds Up

BedeSo, where does this cumulative rounding error come in? Back in A.D. 730, an Anglo-Saxon monk named the Venerable Bede recognized that the Julian year was 11 minutes and 14 seconds too long, which would produce an error of about one day every 128 years.

But there were so many other things going on then, and the Venerable Bede didn’t have a blog, so nothing was done about it for 800 years.


Gregorian vs. Julian Calendar

Gregory XIII

In A.D. 1582 this accumulated error was estimated at ten days, and Pope Gregory XIII decreed that the day following Oct. 4 would be Oct. 15, pretty handy if you had a library book due during that time. This Gregorian calendar was adopted throughout much of the Catholic world but not everywhere.

Uncivilized parts of the British Empire, like the American Colonies, didn’t make the change until 1752, when September 2 was followed by September 14, and New Year’s Day was changed from 25 March to 1 January.

Ultimately, to make future adjustments for the error, which amounts to about three days every 400 years, it was decided that years ending in “00” would be normal years rather than Leap Years, except for those divisible by 400. Unless it is a Tuesday and it is dark.


Leap Year Traditions

There are several Celtic and British traditions associated with Leap Day, sometimes known as Ladies’ Privilege or Bachelor’s Day

The Irish and Scottish traditions harken back to a story about a negotiation between St. Patrick and St. Bridget of Ireland. Complaining that men took too long to propose marriage, St. Bridget suggested that women be given the chance to initiate. Patrick suggested once every seven years, but Bridget negotiated that down to once every four years.

The 2010 Amy Adams film Leap Year popularized this opportunity by telling the story of an American woman who traveled to Ireland to propose to her beau.


Star Wars and Leap Year?

There was a time when marriages were banned during the season of Lent. It behooved a woman to press her beau to marry before Shrove Tuesday (also known as Mardi Gras elsewhere) because Lent began the following day on Ash Wednesday.

Otherwise, she might be shamed throughout Lent – especially on Chalk Sunday, the first Sunday of Lent – by having her name added to the list of unmarried women known as the “Skellig List.” It was believed that Lent started later there – as it is off the Western coast of Ireland – providing one last chance to marry.

skelligs aerial 1 1200x765 (1)In the Star Wars film The Last Jedi, our hero, Luke Skywalker, is in self-exile on a remote island. These scenes were filmed on Skellig Michael, a hermitage off the western coast of County Kerry, Ireland.

In Scotland, a woman must wear a red petticoat to propose on Leap Day. If the beau should decline, a fine was imposed, though a kiss could be required. Legend holds that Bridget proposed to Patrick right after he allowed women to do so, but he declined, instead presenting her with a kiss and a silk skirt.


Leap Year Tradition in America

 say you'll marry me or i'll press the button!

American Leap Year postcard, 1908

Ralph Waldo Emerson in 1860 wrote about a “Leap Year Dance” where women invited men. In the mid-20th century, this was popularized in cartoon strips like L’il Abner with the “Sadie Hawkins Day Dance.” I was invited to one when I was 15, my first high school dance.


Bill Petro, your friendly neighborhood historian

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About billpetro

Bill Petro writes articles on history, technology, pop culture, and travel. He has been a technology sales enablement executive with extensive experience in Cloud Computing, Automation, Data Center, Information Storage, Big Data/Analytics, Mobile, and Social technologies.


  1. Steve Allen on February 26, 2008 at 4:31 pm

    A leap second is not that fraction of the tropical year for 1900; it is exactly one second of TAI. The length of the atomic second was initially chosen to match that fraction of the tropical year, but the uncertainty was 2 parts in 10 to the 9, and the rate of TAI was adjusted by 1 part in 10 to the 12 at the beginning of 1977.

  2. Bill Petro on February 26, 2008 at 6:48 pm

    Well Steve, apparently I’ve oversimplified to the point of error! Thanks for the comment.

  3. Austin Brummer on February 29, 2012 at 11:25 am

    Hey Bill, another quick note. Unfortunately the earth’s rotation is slowing down at a rate of .005 seconds per year. So, if you extrapolate the math out … carry the 2 … divide by 3.14 … In about 2,000,000,000 we will need to add another leap year to keep us in sync.

    • Bill Petro on February 29, 2012 at 4:42 pm

      Thanks Austin. I only update this article every 4 years, but I’ll put this in my To Do list to update the article when the cumulative rotational deterioration reaches a threshold of notice.

  4. Rekha on April 20, 2012 at 8:45 am

    Thank you for sharing.

  5. AROKIADOSS on June 2, 2019 at 7:57 am

    But till now i couldn’t get the correct answer of ending 00 is not leap year why, tell me briefly as possible

    • billpetro on June 3, 2019 at 3:16 pm

      The time it takes for the Earth to orbit the sun is actually 365.242199 days. Adding one extra day to every fourth year (leap year), we get an average of 365.25 days per year, which is fairly close to the actual number. To get closer still, adding a leap year every 400 years on the century mark brings the average length of the year to 365.2425 days.


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