Why do we have leap day?

Every four years, we have an extra day in the month of February. Have you ever wondered why leap day even exists? It all comes down to math.

Every year, the Earth completes one revolution around the Sun. We typically think of this as 365 days, right? Well, it’s actually 365.2422 days and those extra numbers past the decimal point DO matter!

If we didn’t have leap day, the extra 0.2422 days would equal 5.8 hours per year. Without that added day, after 100 years, our calendar would be 24 days off — meaning the first day of Spring would be at the end of February. We can’t have that!

The solution is adding an extra day to February every four years, allowing our “yearly total” to equal 365.25 days. Problem solved? Almost…

By subtracting 365.25 days and 365.2422 days, we are now left with a surplus of 0.0078 days. With this in place, in 100 years, our calendar would be off by 18 hours.

Let’s fix this with a game! Playing leap year hopscotch, we decide to skip a leap year if it falls at the start of a new century. The exception: if the year is divisible by 400. This makes our new yearly average 365.2425.

With leap day and now leap day hopscotch rules, we are now left with a surplus of about 25.9 seconds each year. At this rate, it would take 3,333 YEARS in order to be one day off.

We’ll just let them worry about that in three millenniums… Leap day, that’s why!