We Don't Have Leap Years Every 4 Years

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 this year, from the AP:

• Why do we have leap years? They exist, 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.

• Do they happen every four years? 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. It was decreed that years divisible by 100 not follow the four-year leap day rule unless they are also divisible by 400, 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.
• What would happen if we stopped having them? 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," says one expert. "Christmas will be in summer. There will be no snow. There will be no feeling of Christmas."
• How annoying is it to be born on a leap day? It can be kind of a pain from a paperwork perspective for the estimated 5 million people worldwide who were born on a leap day. Some governments and others requiring forms to be filled out and birthdays to be stated stepped in to declare what date was used by leaplings for such things as drivers licenses, whether Feb. 28 or March 1. Technology has made it far easier for leap babies to jot down their Feb. 29 milestones, though there can be glitches in terms of health systems, insurance policies, and with other businesses and organizations that don't have that date built in.

The next leap years are 2028, 2032, and 2036. See the AP for more, including a history of how exactly the idea of a leap year came about and evolved.

(More leap year stories.)