The Google Doodle for February 29, 2016, celebrates a rare occasion: a leap day.
The leap year happens every four years, according to Google.
Here’s why we have a leap day every four years and what you need to know about the day:
1. The Leap Day Keeps the Calendar in Sync With the Earth’s Rotation Around the Sun
The leap day occurs to keep our calendar in sync with the Earth’s rotation around the sun.
“Without Leap Day, we’d be out of sync by about six hours per year,” Google explains.
You can watch scientist Neal deGrasse Tyson explain the leap day to National Geographic in the video above or by clicking here.
“We on Earth, as we orbit the sun, we know how to calculate a year,” he says. “It’s how long it takes to come back to where you were, that’s a year. But it turns out if you cut the number of days we live into the year it doesn’t divide evenly. So there’s not an even number of days in the time it takes for the Earth to go around the sun.”
2. There Are Some Years Where the Leap Day Is Skipped
To keep the calendar organized, there are some rules that call for the leap day to be skipped during certain four-year periods, according to TimeandDate.com.
According to the website, to be a leap year, it must be able to be evenly divided by 4. If it it can’t be divided by 100, it is not a leap year, unless the year is also evenly divisible by 400. So, the years 2000 and 2400 are leap years, while 1800, 1900, 2100, 2200, 2300 and 2500 are not.
3. The Leap Year Dates Back to Ancient Egypt & Rome
According to National Geographic, the leap day concept goes back to ancient times. Egypt was using a 365-day calendar during the time of Roman emperor Julius Caesar, with an extra day each four years to correct it.
After initially trying a “Year of Confusion,” a 445-day year to correct the missed time, Caesar also adopted the leap day calendar.
4. The System Was Perfected by Pope Gregory XII’s Astronomers in 1582
Pope Gregory XII’s astronomers perfected the leap year in 1582 when the Gregorian calendar was introduced.
In 1582, Pope Gregory XIII promoted the use of a calendar originally calculated by an Italian astronomer, Aloysius Lilius, and tweaked by a Jesuit mathematician, Christopher Clavius. The calendar’s innovation was to shave just under 11 minutes off the year, making it 365 days, 5 hours, 10 minutes and 48 seconds long. That is offset by the addition of one day every four years, in years divisible by four, with the exception of century years — unless they are divisible by 400.
The calendar was initially opposed by some, including Protestants who were worried it was a trick to get them to convert to Catholicism and landlords who thought they were being cheated out of a week of rent, according to the news site.
5. There Are About 4.1 Million People Around the World Born on February 29
The chances of being a “leaper,” or a person born on February 29, are 1 in 1,461, according to MedicalDaily.com.
“Because there are 1,460 days in four years plus one day for the leap year, the odds are one in 1,461. That said, there are an estimated 200,000 leapers in the United States and about five million worldwide,” the website explains.
Some famous leapers include Pope Paul III, rapper Ja Rule, poet John Byrom and actor Alex Rocco.