A leap year, also known as an "intercalary year" or "bissextile" year, is a calendar year that contains an additional day, or in the case of a lunisolar calendar, an entire month, compared to a "common" year. The 366th day, or 13th month, is added to keep the calendar year synchronized with the astronomical or seasonal year. That's the basic definition.

And since astronomical events and seasons do not repeat in a whole number of days, calendars with a constant number of days each year will unavoidably drift over time. By inserting "intercalating" an additional day or month into particular years, the drift between a civilization's dating system and the physical properties of the Solar System (its flotsam and jetsam, if you will) can be corrected, and order restored to the Force. But I digress. This is a word/math salad to me, so I will try to break it down for you and share some fun facts about Leap Year. Buckle up!

February 29 is the extra added day. Also, why February?

Let's find out. First, let's remind folks why we have a Leap Day. (Why February has that extra "U" is for another post for another day!)

A calendar year is typically 365 days long. This is known as a "common" year, which refers to the number of days it takes the Earth to make one complete orbit around the Sun. Here's the but, and it's a big one: 365 is actually a rounded number! It takes Earth 365.242190 days to orbit the Sun, or 365 days, 5 hours, 48 minutes, and 56 seconds. This "sidereal" year is slightly longer than the calendar year, and that extra 5 hours, 48 minutes, and 56 seconds must be accounted for. If we didn't account for this spare time, the seasons would begin to drift, verrrry slowly, but it means having summer in December. This, of course, refers to the Northern Hemisphere. Summer in December is the norm in Australia and the Southern Hemisphere.

So, by adding an extra day every four years, our calendar years stay adjusted to the sidereal year. Well, except for some additional math. Over four years, the difference between the calendar years and the sidereal year is not precisely 24 hours. Instead, it's 23.262222 hours. Adding a leap day every four years makes the calendar longer by over 44 minutes. Over time, these extra 44+ minutes would also cause the seasons to drift. For this reason, not every four years is a leap year. What? Crazy right? But that's math. The rule is that if the year is divisible by 100, not 400, that leap year is skipped. For example, the year 2000 was a leap year, but the years 1700, 1800, and 1900 were not. The next time a leap year will be skipped is the year 2100. So, do not worry about adjusting your Google calendar for a long time. Are you still with me?

Now as to why February and not some other month:

"It has more to do with history, really," says Ben Gold, a professor of astronomy and physics at Hamline University. "It's mostly that the Romans didn't really like February very much." Back in the 8th century BC, the calendar was ten months long. "...Then they had winter, a long period of winter, which they didn't like very much and didn't even want to put it in months," says Gold. Eventually, the Romans tacked on January and February to the end of the year. February, the final month, got fewer days. Julius Caesar then tweaked the calendar to line it up with the Sun. In a decree, he added a Leap Day. The Leap Day itself didn't fix all the differences (or drifts), so 1582, Pope Gregory XIII created the Gregorian calendar and established February 29 as the official date. "It's the shortest," says Gold. "It's the leftover month nobody really wanted to begin with." There is also the unproven myth that Roman emperors stole days from February to add to the month names after them—July and August—it's probably untrue as the calendar was already set by then. Still, I like to imagine them fighting about it all the same.

One more reason to think about the Roman Empire!

Fun Facts about Leap Months! Entire Months of Leapness:

In the lunisolar Hebrew calendar, Adar Aleph, a thirteenth lunar month, or "Leap Month," is added seven times every 19 years to the twelve lunar months in common years to stave off its calendary drift! In leap years, it is preceded by a 30-day intercalary month named Adar Aleph (Aleph being the first letter of the Hebrew alphabet), also known as "Adar Rishon" (First Adar) or "Adar I." The 13th month, or second Adar, is called Adar Bet (Bet is the second letter of the Hebrew alphabet). Occasionally, instead of Adar I and Adar II, "Adar" and "Ve'Adar" are used (Ve means 'and' so the fun term And-Adar).

In the Solar Hijri (and Bahá'í calendars), a leap day is added on an "as needed" basis to ensure that the following year begins on the March equinox. The Solar Hijri calendar begins on the March equinox as determined by the astronomical calculation for the Iran Standard Time meridian and has years of 365 or 366 days. The ancient Iranian Solar calendar is one of the world's oldest and the most accurate solar calendars in use today. Since the calendar uses astronomical calculations for determining the vernal equinox, it has no intrinsic error. The first six months have 31 days, the next five have 30 days, and the last month has 29 days in common years but 30 days in leap years. One more fact about this calendar: the ancient Iranian New Year's Day called Nowruz always falls on the March equinox. While Nowruz is celebrated by communities in various countries, from the Balkans to Mongolia (and right here in Los Angeles), the Solar Hijri calendar remains only in official use in Iran.

The Chinese calendar is also lunisolar, so a leap year also has an extra month, often called an embolismic month after the Greek word for it. In the Chinese calendar, the leap month is added according to a rule that ensures that month 11 always contains the northern winter solstice. The intercalary month takes the same number as the preceding month; for example, if it follows the second month, it's "leap second month."

And there are also Leap seconds! Yes! Leap seconds:

"The length of a day is also occasionally corrected by inserting a leap second into Coordinated Universal Time (UTC), which is the primary time standard globally used to regulate clocks and time, because of variations in the Earth's rotation period. Unlike leap days, leap seconds are not introduced on a regular schedule because variations in the length of the day are not entirely predictable."

So now you know! Wasn't that fun? There might even be some new book titles or band names to borrow. I like Embolismic! or Intercalary Year. Sounds like a techno-math-rock band with a side of noise-rock realness! Sidereal Year might be a fun name, too, though it would need to be twee pop with at least one member playing tambourine or triangle.

The library has 658 books and movies using the title Leap Year. Calendar as a search term (once you spell it correctly), brings up even more books, including Calendar: Humanity's Epic Struggle to Determine a True and Accurate Year—a somewhat ominous title, but a great place to start if this blog post doesn't have enough Intercalary info to quench your thirst—The delightful 2010 movie Leap Year starring Amy Adams might be a fun title to check out if you really want some Leap Year themed media to watch on February 29. Bonus—It's also filed under subject: Leap Year Drama (another potential band name). There are musical selections that you can find in Freegal though not my made up bands. You can search by genre. However, you'll find your Twee under Indie and your Math-Rock in a variety of curated playlists). Find even more media through hoopla.

More fun facts:

If you are born on February 29, you are known as a Leapling or Leaper.

February 29 is not a legally recognized day, so those folks turning 18 on February 29 can't vote until March 1 and want a drink on your 21st birthday? Well, you'll have to wait until March 1 for that, too.