Why do we have leap years? How important are leap days (2024)

FAQs

Why do we have leap years? How important are leap days? ›

Leap years are necessary because the Earth's orbit around the Sun is not exactly 365 days. We add an extra day every four years to correct this discrepancy. The rule for determining a leap year is that it must be divisible by 4, unless it is also divisible by 100 but not by 400.

The extra day keeps calendars and seasons from gradually falling out of sync and impacting harvesting, planting and other cycles based on the seasons. Without Leap Days, in 100 years, calendars would be 24 days off, CBS Minnesota reported, and in 700 years, Northern Hemisphere summers would begin in December.

Leap years help to keep the 12-month calendar matched up with Earth's movement around the Sun. After four years, those leftover hours add up to a whole day. In a leap year, we add this extra day to the month of February, making it 29 days long instead of the usual 28.

On years without leap days, you get to celebrate your birthday on March 1 and continue to grow old like the rest of us. Thanks to leap year, our seasons will always occur when we expect them to occur, and our calendar year will match the Earth's sidereal year.

1. Leap day is a matter of timekeeping. Earth orbits the sun every 365 days, six hours and nine minutes—slightly more than our typical 365-day calendar. “One tropical year is not evenly divisible by an integer number of synodic days, so the remainder accumulates,” explained Stacy McGaugh.

While this might not seem like much of a difference, over four years, it works out to roughly a full day. This is where leap years come in. Without them, the summer season we generally experience in June would happen in December 700 years from now (as just one example from the Northern Hemisphere).

In some cultures, leap years represent opportunity. Elsewhere, leap days, months (and in some cases whole years) are considered bad luck and avoidance tactics are deployed to try and reverse any ill fortune.

It's a correction to counter the fact that Earth's orbit isn't precisely 365 days a year. The trip around the sun takes about six hours longer than that. “Without the leap year your seasons would drift,” Worthey said.

Without leap years, the Fourth of July would eventually be in the winter. Leap days, which are tacked on at the end of February every four years, ensure our calendars remain in sync with the Earth's orbit, said Shauna Edson, an astronomy educator at the National Air and Space Museum.

When it comes to legal documents, such as obtaining a passport or driver's license, the date February 29th is recognized as the official birthday for leaplings in most countries. However, some states in the U.S. allow the leapling to celebrate their birthday on either February 28th or March 1st on non-leap years.

What happens if you are born on February 29 legally? ›

If you're born in a leap year, when can you legally drink, vote, or drive? Legality in terms of drinking and voting is not impacted by leap years, even if someone is "technically" not 18 or 21. If you're born on February 29, your birthday would be observed after 11:59 p.m. on Feb. 28 — or March 1 — on non-leap years.

Earth takes about 365.2422 days to make one revolution around the Sun. That's about six hours longer than the 365 days that we typically include in a calendar year. As a result, every four years, we have about 24 extra hours that we add to the calendar at the end of February in the form of leap day.

You may be used to hearing that it takes the Earth 365 days to make a full lap, but that journey actually lasts about 365 and a quarter days. Leap years help to keep the 12-month calendar matched up with Earth's movement around the Sun. After four years, those leftover hours add up to a whole day.

Why and where are leap years considered bad luck? Some Greeks, Ukranians and Italians have long believed that getting married during a Leap Year is unlucky and destined to end in divorce. In Scotland, some people believe that being born on Leap Day is a terrible omen, signifying a life full of suffering for Leaplings.

Rare Occurrence

Babies born on February 29th are rare – in fact, it only happens once every four years – making it a reason to celebrate. It's estimated that approximately 4.1 million people around the world are born on a leap year.