It's not every year that you see February 29 on the calendar -- and it's not technically every four years either.
Thursday is officially leap day. For some, its birthdate, or a day for special deals and freebies. But for others, its all about the science and math behind the number of days it takes Earth to orbit around the sun.
Here's a breakdown of what leap year is, how it impacts seasons, and the science behind why it matters.
What is a leap year?
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A leap year means there's an extra day in the calendar.
"A calendar year is typically 365 days long," an article from Smithsonian Magazine titled "The Science of Leap Year" reads. "These so called 'common years' loosely define the number of days it takes the Earth to complete one orbit around the Sun. But 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."
What would happen if we didn't have leap years?
That extra time known as a "sidereal" year must be accounted for somehow, the magazine says. Otherwise, seasons could be impacted.
"If we didn’t account for this extra time, the seasons would begin to drift," the article goes on to say. "This would be annoying if not devastating, because over a period of about 700 years our summers, which we’ve come to expect in June in the northern hemisphere, would begin to occur in December."
Younas Khan, a physics instructor at the University of Alabama at Birmingham, said eventually, Christmas could wind up at a different time entirely if we did away with leap years.
“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.”
Why is February 29 a leap day?
During a leap year, the month of February has an extra day added to it. So this year, there will be 29 days in February, rather than 28. That 29th day is also referred to as a "leap day."
When are leap years, and why don't they happen every four years exactly?
Leap years happen almost every four years. That means 2024 and 2028 will both be leap years.
But still, there are some exceptions to the once-every-four-years rules.
"Some simple math will show that over four years the difference between the calendar years and the sidereal year is not exactly 24 hours," the article goes on to say. "Instead, it’s 23.262222 hours."
If we added a "leap day" every four years, we'd actually make the calendar longer by over 44 minutes, Smithsonian said.
"Over time, these extra 44+ minutes would also cause the seasons to drift in our calendar," the article goes on to say. "For this reason, not every four years is a leap year."
How do you calculate a leap year?
The rule is that if the year is divisible by 100 and not divisible by 400, leap year is skipped, the magazine says.
"The year 2000 was a leap year, for example, but the years 1700, 1800, and 1900 were not," the article states. "The next time a leap year will be skipped is the year 2100."
Why is it called a 'leap year'?
A typical year is 52 weeks and one day long. "That means, if your birthday were to occur on a Monday one year, the next year it should occur on a Tuesday."
But, an extra day during a leap year means your birthday now "leaps" over a day, Smithsonian said.
What if you are born during a leap year?
It's not that those birthdays are celebrated only every four years. Rather, those born on a leap day typically celebrate on March 1, the magazine said.
According to the Almanac, people born on a leap day during a leap year are known as "leaplings." Some cultures see the birthday as a sign of good luck.
From a paperwork perspective though, it can be a pain.
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 organization that don't have that date built in.
There are about 5 million people worldwide who share the leap birthday out of about 8 billion people on the planet. Shelley Dean, 23, in Seattle, Washington, chooses a rosy attitude about being a leapling. Growing up, she had normal birthday parties each year, but an extra special one when leap years rolled around. Since, as an adult, she marks that non-leap period between Feb. 28 and March 1 with a low-key “whew.”
This year is different.
“It will be the first birthday that I’m going to celebrate with my family in eight years, which is super exciting, because the last leap day I was on the other side of the country in New York for college,” she said. “It's a very big year.”
