Why do we have a leap year? What would happen if we didn’t? Smithsonian scientist explains

Check your calendars, California. We get an extra day this month.

Whether you’ve realized it or not, 2024 is a leap year. Every four years (typically), a leap year occurs in February — making it 29 days long instead of the usual 28.

Bob Craddock, a scientist in the Center for Earth and Planetary Studies at the Smithsonian Institution in Washington, D.C., said leap years are “a little complicated.”

“Most people believe that leap years occur every four years, but that’s not the case,” Craddock said in a phone interview with The Sacramento Bee on Wednesday afternoon. “Leap years sometimes skip a year.”

Here’s the origin behind leap years and how they help California’s seasons:

Why does February only have 28 days to begin with?

According to Public Broadcasting Service Southern California, February typically only gets 28 days because of the Romans.

“Even numbers were bad luck in ancient Rome, so (the king) started by removing a day from all the even-numbered months,” the video published from PBS SoCal explains. This happened around 753 B.C.

With March through December being odd-numbered months — and shorter than what we use now — the calendar year amounted to only 298 days, essentially omitting the winter days.

To cover the 12 cycles of the moon, the king added 56 days to the 298. Since that would have resulted in an even number, 354, the king rounded his year up to 355.

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The king split these 57 days into two new months — January and February — and “tacked them on to the end of the year,” leaving February with only 28 days, the video explains.

Though 28 was an even number, the month was dedicated to “spiritual purification” so the Romans at the time did not mind, according to information from the video.

You’ll notice we’re still missing a few days compared to what we have in our calendars now. That’s because most countries observe a Gregorian calendar, which follows the sun — not the moon — similar to the Roman Julian calendar which was implemented in 46 B.C. to clear up some of the confusion.

Why do leap years exist?

While now a traditional calendar year is usually about 365 days long to match the time it takes for Earth to orbit the Sun, “365 is actually a rounded number,” according to the Smithsonian’s website.

It takes Earth 365.242190 days — or 365 days five hours, 48 minutes and 56 seconds — to orbit the Sun, and the “extra time needs to be accounted for somehow,” the website says.

“The problem is that it takes about an additional five hours, 48 minutes and 56 seconds for the earth to complete one orbit around the sun, so that’s about six hours,” Craddock said.

Roughly six hours every year for four years is 24 hours — or one day.

“You add a day every four years and that adjusts for the time it takes the sun to complete an orbit, or a calendar year,” he said.

However, leap years are sometimes skipped.


“If you do the math, that five hours, 48 minutes and 56 seconds isn’t exactly six hours, so we’re actually overcompensating,” Craddock said. “We have to make up for that difference too.”

He said the rule is if a “year is divisible by 100 and not divisible by 400,” then the leap year is skipped to adjust the time difference given to years with the extra day.

The next time a leap year will be skipped is in 2100, Craddock said.

What would happen if leap years didn’t exist?

If we didn’t adjust for that six-hour difference in our calendar year, he said the calendar years would start about six hours earlier every year.

“What that ends up doing is that events like the summer solstice on our calendars would start going forward in time,” Craddock said.

To keep our seasons in sync, he said we add an extra day every four years to ensure we still have summer in June and not December over the course of many years.

Why is it called a leap year?

“If you were to have your birthday on a Monday, then the next year it should occur on a Tuesday, because the calendar has an odd number of days,” Craddock said.

With a leap year, you add a day.

Due to the extra day given during a leap year, he said your birthday “leaps” over the day it was supposed to be on.

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