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Why do we have leap days? Get ready for a math lesson — and a history lesson

A block calendar showing the date changing to Feb. 29
(Photo via Shutterstock)

We cannot, of course, turn back time. And we can't stop time from ticking into the future. But this year is one of those lucky years when we get a little more time to savor, because 2024 is a leap year. 

In our quadrennial leap years, February has 29 days instead of the usual 28. Why do we get an extra day to relish every four years? Get ready for a math lesson.  The length of a year — usually 365 days — is based on how long it takes Earth to make one full trip around the sun. But it doesn't take exactly 365 days for Earth to complete one full revolution around the sun. It takes a little longer, about 365.2422 days, according to NASA. That extra little bit amounts to about six hours, so every four years we add a day because over the course of four years that extra time adds up to 24 hours, or a full day. 

Following along so far? Stick with us. It gets just a little more complicated. The six extra hours it takes for Earth to revolve around the sun is an approximation too. Adding 24 hours every four years adds an extra 45 minutes to each four-year cycle, and this must be accounted for too. These extra 45 minutes amounts to about three days every 400 years. To account for this, years that are divisible by 100 are only leap years if they are also divisible by 400, according to NASA. That means 2100 will not be a leap year, but 2000 was and 2400 will be. 

If we didn't add this extra day every four years, the start of our seasons would slowly shift later and later because the solstices and equinoxes would gradually be moved back each year, NASA reports. The seasons would lose their hallmark characteristics and be more difficult to track. Leaves changing each fall? Not so when fall starts in January. 

So why do we add this extra day in February? Get ready for a history lesson, because the practice dates back to ancient Rome. Leap day isn't in February because February is the shortest month. In ancient Rome, each year began in the month of Martius, which we now call March. But at that time, each year was only 10 months long and didn't include winter because ancient Romans didn't work in winter, Time reports. 

Over time, years that were only 10 months became complicated to account for because the years didn't match Earth's revolution around the sun and because the Roman calendar didn't match calendars used in other parts of the world, according to Time. In the 7th century BCE, the Roman king Numa Pompilius officially decreed that winter months would be included in the calendar, adding the months of Ianuarius and Februarius (what we now call January and February) to the end of the year, not the beginning as they are now. 

After the change, the Roman calendar still didn't line up with the seasons, so every few years Romans would add a 13th month to get back in sync with the sun. This extra month would be added after Feb. 23, which meant shortening the month by five days.

Then, when Julius Caesar was in power, he ordered the creation of a new calendar to match how long it took for Earth to revolve around the sun — 365 days and 6 hours. The Julian calendar, as it is called, took effect in 45 BCE, and it accounted for the extra day that was needed every four years. The extra day was added by making Feb. 24 48 hours instead of 24.

The Julian calendar was used for centuries, until Pope Gregory XIII introduced a new calendar in the 1570s. This calendar, known as the Gregorian calendar, is the one we use today, but its acceptance worldwide was slow, Time reports. The United Kingdom and its American territories did not adopt it until 1752, when the Calendar Act became law. Under the Calendar Act, the new year began on Jan. 1 and every four years would see the addition of an intercalary day, what we call a leap day, on Feb. 29.

Now that all the math and history of leap years has been sorted, all that's left is to enjoy your extra day. Need some inspiration? The forest preserves are full of trails perfect for hiking and biking. Looking for something a little more interactive? Join our Take It Outside challenge and spend your bonus day on a scavenger hunt through the preserves via the Goosechase app. Or if you want to spend some time enjoying the scenery from a comfy seat, we've got spots for that too.

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