In 2020, February gets an extra day. Instead of 28 days, this year February will have 29 days. Almost everyone if familiar with the concept of leap year, but the reasoning behind it is a little complicated. For example, most people believe that leap year occurs once every four years, but that’s not always the case. What’s going on and why do we have leap year?

A calendar year is typically 365 days long. These so called “common years” loosely define the number of days it takes the Earth to complete one orbit around the Sun. But 365 is actually a rounded number. It takes Earth 365.242190 days to orbit the Sun, or 365 days 5 hours 48 minutes and 56 seconds. This “sidereal” year is slightly longer than the calendar year, and that extra 5 hours 48 minutes and 56 seconds needs to be accounted for somehow. If we didn’t account for this extra time, the seasons would begin to drift. This would be annoying if not devasting, because over a period of about 700 years our summers, which we’ve come to expect in June in the northern hemisphere, would begin to occur in December!

By adding an extra day every four years, our calendar years stay adjusted to the sidereal year, but that’s not quite right either. Some simple math will show that over four years the difference between the calendar years and the sidereal year is not exactly 24 hours. Instead, it’s 23.262222 hours. Rounding strikes again! By adding a leap day every four years, we actually make the calendar longer by over 44 minutes. Over time, these extra 44+ minutes would also cause the seasons to drift in our calendar. For this reason, not every four years is a leap year. The rule is that if the year is divisible by 100 and not divisible by 400, leap year is skipped. The year 2000 was a leap year, for example, but the years 1700, 1800, and 1900 were not. The next time a leap year will be skipped is the year 2100.

And why is it called “leap year?” Well, a common year is 52 weeks and 1 day long. That means that if your birthday were to occur on a Monday one year, the next year it should occur on a Tuesday. However, the addition of an extra day during a leap year means that your birthday now “leaps” over a day. Instead of your birthday occurring on a Tuesday as it would following a common year, during a leap year, your birthday “leaps” over Tuesday and will now occur on a Wednesday.

And if you happen to be born on leap day February 29, that doesn’t mean you only celebrate a birthday every four years. Every three years, you get to celebrate your birthday on March 1 and continue to grow old like the rest of us.

Thanks to leap year, our seasons will always occur when we expect them to occur, and our calendar year will match the Earth’s sidereal year.