We consider a year to consist of 365 days. The reality is that a solar year - the time it takes for the earth to complete one orbit around the sun - takes 365 and one quarter days. For those who want a precise number, that would be 365 days 5 hours 48 minutes and 56 seconds (365.242190 days) for one solar orbit. The difference between a calendar year and a solar year is only a few hours, so why does it matter? Those .242190 days begin to add up. If allowed to go unchecked forever, the seasons will begin to drift. The Smithsonian’s National Air and Space Museum website states that “this would be annoying if not devastating, 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!”

Way back to 1582 A.D. those extra hours already added up to 10 days. That was when Pope Gregory XIII created the Gregorian calendar, added a 29th day to February and created the leap year.

Were you, like me, taught that any year evenly divisible by 4 is a leap year? Well hold on a moment. It turns out that the “divisible by 4” rule is not always true. Why? The answer boils down to the rounding of numbers. Adding an extra day adds 24 hours to the calendar. On the other hand, over a period of 4 years the 5 hours 48 minutes and 56 seconds referenced above equals about 23 hours and 16 minutes — or 44 minutes less than 24 hours. This over compensation would begin to add up over time and would also cause a drifting of the seasons. The solution to this little dilemma brings us to the “century year” rule.

Any year that ends with two zeroes, i.e. 2000, is called a century year. In order for a century year to be a leap year it must be evenly divisible by 400 rather than by 4. So was 2000 a leap year? Yes it was, because 2000 divided by 400 = 5. Will 2100 be a leap year? No, because 2100 divided by 400 equals 5.25. The next century year that will also be a leap year is 2400.

Why is it called a leap year anyway? The length of our calendar year is 52 weeks and 1 day. Let’s say your birthday is March 1st. March 1st was a Sunday in 2020, a Monday in 2021, a Tuesday in 2022, and a Wednesday in 2023. Following that progression it will fall on a Thursday in 2024, right? Wrong. Your birthday will be on Friday in 2024 because it leaps ahead one day.

People born on February 29th are called Leaplings. These folks only get to celebrate on their actual birthday 1 year out of every 4. The other years they have the option of celebrating on either February 28th or March 1st. A few famous leaplings are Italian composer Gioachino Rossini, former Indian Prime Minister Morarji Desai, actor Dennis Farina and motivational speaker Tony Robbins.

To any Leaplings reading this, I hope that you enjoy celebrating on your actual birthday this year!