Happy Leap Day! (But, what's Leap Day?)

Once every four years, the calendar is stretched to give February one extra day. But, why?

Short answer? It's to make sure our calendar isn't off by a quarter of a day. The actual length of a year is 365 and a quarter days.

365.2421 to be exact.

So if we were to ignore that in one year, we would be off a quarter of a day. Second year we'd be off half a day. Then three quarters of a day, and after four years we'd be off an entire day.

Meaning over time, a long time, February might be more aligned with summer weather instead of winter. And that's bad, right?

And if Feb. 29 is your birthday, you have Julius Caesar to blame for its creation.

Ancient civilizations knew a solar year was about 365 days. Egyptians, for instance, knew because of the flooding of the Nile. Caesar began the concept of leap year with the Julian Calendar on Jan. 1 45 BCE, and consisted of a 366-day leap year every four years without exception.

However, a solar year is a tiny bit less than 365.25 days, meaning over centuries, the calendar still moved slightly off from the seasons.

This wasn't noticed until many centuries later but became an issue due to Easter, said Dr. Frank Maloney, associate professor of astronomy and astrophysics at Villanova University.

Maloney has taught for nearly 12 leap years, and he's an expert in calendars and timekeeping, calling them a "fundamental connection, as everyone has to agree what day and time it is."

"Nearly all calendars have a mystical or religious theological component," Maloney said.

In this case, Easter, which is always the first Sunday after the full moon that occurs on or after the spring equinox, was off by about 10 days.

To get Easter back in line, Pope Gregory XIII issued a papal bull -- a public decree issued by a pope of the Catholic church -- in 1582 declaring that a year was 365 days, 5 hours, 49 minutes in length (not 365 years, 6 hours as the Julian calendar states).

October of 1582 went from Oct. 4 to Oct. 15 to enact the switch on the calendar. Catholic countries mostly followed suit, but many other countries’ citizens resisted, fearing it was a trick.

Great Britain and her colonies didn't adopt the Gregorian calendar until 1752. So somebody like George Washington was born on one day in the Julian calendar, and had another birthday after the switch to the Gregorian. In the American colonies, September 1752 went from 2-14.

The latest country to make the switch to adopt was Greece in 1923. By then, they were 14 or so days off. Afghanistan, Iran, Nepal and Ethiopia still don't use the Gregorian.

Even the Gregorian calendar will need an adjustment every once in a while. It’s margin of error is about 27 seconds a year, meaning every 3,236 years we will be an extra day out of line. Perhaps the year 4818 (3,236 years after the Gregorian calendar was first used) will be the first non-multiple of four leap year to correct it!

If a country were to still be using the Julian calendar, you might be considered, in a way to be “traveling through time” if you were to travel back and forth to that country from one that uses the Gregorian calendar.