NEW YORK - Leap year. It’s a delight for the calendar and math nerds among us. So how did it all begin and why? Have a look at some of the numbers, history and lore behind the (not quite) every four year phenom that adds a 29th day to February.

BY THE NUMBERS

The math is mind-boggling in a layperson sort of way and down to fractions of days and minutes. There’s even a leap second occasionally, but there’s no hullabaloo when that happens.

The thing to know is that leap year exists, in large part, to keep the months in sync with annual events, including equinoxes and solstices, according to the Jet Propulsion Laboratory at the California Institute of Technology.

It’s a correction to counter the fact that Earth’s orbit isn’t precisely 365 days a year. The trip takes about six hours longer than that, NASA says.

Contrary to what some might believe, however, not every four years is a leaper. Adding a leap day every four years would make the calendar longer by more than 44 minutes, according to the National Air & Space Museum.

Later, on a calendar yet to come, it was decreed that years divisible by 100 not follow the four-year leap day rule unless they are also divisible by 400, the JPL notes. In the past 500 years, there was no leap day in 1700, 1800 and 1900, but 2000 had one. In the next 500 years, if the practice is followed, there will be no leap day in 2100, 2200, 2300 and 2500. Still with us?

The next leap years are 2028, 2032 and 2036.

WHAT WOULD HAPPEN WITHOUT A LEAP DAY?

Eventually, nothing good in terms of when major events fall, when farmers plant and how seasons align with the sun and the moon.

“Without the leap years, after a few hundred years we will have summer in November,” said Younas Khan, a physics instructor at the University of Alabama at Birmingham. “Christmas will be in summer. There will be no snow. There will be no feeling of Christmas.”

WHO CAME UP WITH LEAP YEAR?

The short answer: It evolved.

Ancient civilizations used the cosmos to plan their lives, and there are calendars dating back to the Bronze Age. They were based on either the phases of the moon or the sun, as various calendars are today. Usually they were “lunisolar,” using both.

Now hop on over to the Roman Empire and Julius Caesar. He was dealing with major seasonal drift on calendars used in his neck of the woods. They dealt badly with drift by adding months. He was also navigating a vast array of calendars starting in a vast array of ways in the vast Roman Empire.

He introduced his Julian calendar in 46 BCE. It was purely solar and counted a year at 365.25 days, so once every four years an extra day was added. Before that, the Romans counted a year at 355 days, at least for a time.

But still, under Julius, there was drift. There were too many leap years! The solar year isn’t precisely 365.25 days! It’s 365.242 days, said Nick Eakes, an astronomy educator at the Morehead Planetarium and Science Center at the University of North Carolina in Chapel Hill.

Thomas Palaima, a classics professor at the University of Texas at Austin, said adding periods of time to a year to reflect variations in the lunar and solar cycles was done by the ancients. The Athenian calendar, he said, was used in the fourth, fifth and sixth centuries with 12 lunar months.

That didn’t work for seasonal religious rites. The drift problem led to “intercalating” an extra month periodically to realign with lunar and solar cycles, Palaima said.

The Julian calendar was 0.0078 days (11 minutes and 14 seconds) longer than the tropical year, so errors in timekeeping still gradually accumulated, according to NASA. But stability increased, Palaima said.

The Julian calendar was the model used by the Western world for hundreds of years. Enter Pope Gregory XIII, who calibrated further. His Gregorian calendar took effect in the late 16th century. It remains in use today and, clearly, isn’t perfect or there would be no need for leap year. But it was a big improvement, reducing drift to mere seconds.

Why did he step in? Well, Easter. It was coming later in the year over time, and he fretted that events related to Easter like the Pentecost might bump up against pagan festivals. The pope wanted Easter to remain in the spring.

He eliminated some extra days accumulated on the Julian calendar and tweaked the rules on leap day. It’s Pope Gregory and his advisers who came up with the really gnarly math on when there should or shouldn’t be a leap year.