Later this week, we will experience the rarest day in the Gregorian calendar. Thursday will be Feb. 29, a date that only comes around during leap years.
Where did this quadrennial tradition come from? What does it have to do with our solar system? And what could it mean for billionaires hoping to colonize Mars? Read on to find out.
Leap years exist because the number of days it takes Earth to revolve around the sun is not perfectly even. If our planet’s orbit lasted exactly 365 days, we would have no need for leap years. But that revolution actually takes about 365.2422 days.
“There's a mismatch of about six hours,” said Lyle Tavernier, an educational technology specialist at NASA’s Jet Propulsion Laboratory in Pasadena who recently wrote about the math behind leap years. “We have to add in an extra day somewhere.”
February 29th marks the day our calendar uses to make up for that additional time. This leap day occurs about every four years (more on that later).
If we wiped leap days off the calendar, you probably wouldn’t notice major changes right away.
But over time, important seasonal markers — like solstices and equinoxes — would start to wander toward other parts of the year.
“The summer solstice would slowly drift into July and September, and then eventually into later in the year as well,” Tavernier said. “You could have a snowy July if we didn't do this.”
Generally speaking, leap years happen every four years. But sometimes, they don’t.
Remember when we said Earth takes about 365.2422 days to orbit around the sun? Well, if you take that extra bit of time — 5 hours, 48 minutes and 46 seconds — and multiply it by four, you can see that it still doesn’t quite add up to 24 hours.
To account for that mismatch, our calendar skips a leap year every so often. For example, 1896 was a leap year, and so was 1904 — but 1900 was not. Youngsters today might grow up to experience the next skipped leap year in 2100.
Here’s some easy math you can use to figure out if any given year will be a leap year.
Leap years happen every four years, unless the year is divisible by 100. However, if a year divisible by 100 is also divisible by 400, it will feature a leap year. That’s why the year 2000 featured a February 29th.
“You can do the math going forward and see that in the year 2400, we'll also have a leap year,” Tavernier said.
Leap years have existed in our calendar since the reign of Julius Caesar in 46 B.C. However, the Julian Calendar “overcorrected” for leap years by failing to account for the math we discussed above.
It wasn’t until 1582 with the creation of the Gregorian Calendar under Pope Gregory XIII that we started to skip a leap year every century or so. This is the system we continue to use today.
Cultures that use other calendar systems also account for leap years, in slightly different ways. Chinese and Hindu calendars feature leap months, rather than leap days. The Islamic calendar features 11 leap years in every 30-year cycle.
The short-lived French Republican Calendar in use from 1793 to 1805 following the French Revolution featured 12 months made up of three 10-day weeks. Five extra days were tacked onto the end of each year. But leap years featured a sixth extra day known as jour de la Révolution (Revolution Day).
If Elon Musk’s dream of colonizing Mars ever comes to fruition, humanity would still have to observe leap years. In fact, Martian leap years would happen a lot more often.
It takes about 668.6 Martian days for the Red Planet to revolve around the sun. Correcting for that imbalance would require more frequent leap years.
“In a 10-year span, Mars would have four non-leap years and six leap years,” Tavernier said. “Quite a bit different from the way that we insert a leap day into our calendar every four years.”
