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Why Do We Have Leap Years? Get Ready for a Math Lesson

Updated: Mar 21

We can’t stop time or turn back time to go back to the past, but in 2024 we do get a little bit of extra time. That’s because 2024 is a leap year, which means we get an extra day, or if you prefer to look at it another way, an extra 1,440 minutes or 86,400 seconds 

Leap years generally occur every four years. Our extra day, called a leap day, is always on Feb. 29. But why do we get an extra day every four years? That involves 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. It takes a little longer, about 365.2422 days. That extra little bit amounts to about six hours a year. So every four years we add one extra day because over the course of four years that extra time adds up to 24 hours, or a full day. 

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. This must be accounted for too. This 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. That means 2100 and 2200 will not be leap years, 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. The seasons would lose their hallmark characteristics and be more difficult to track. Eventually, leaves changing color wouldn’t happen in October and November. We would see those changes in December and then January and on and on.

So why do we add this extra day in February? This involves 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. So that extra day was added at the end of the year.  

Over time, calendars evolved and changed, but not everyone in the world was using the same calendar. In the 1700s, a calendar called the Gregorian calendar began to be universally used. That is the calendar we still use today. In the Gregorian calendar, each new year begins on Jan. 1, and every four years we add a leap day on Feb. 29.

Now that you know the math and history behind leap years, all that's left is to enjoy your extra day. The forest preserves are full of trails perfect for hiking and biking.


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