Why do leap years happen every four years? Let's answer the question that keeps us up at night
You already know that a leap year refers to a year with one extra day, during which February has 29 days instead of 28 (the 29th is known as Leap Day). And if you know that, then you also know that this occurs every four years. But what you probably don’t know is why leap years happen in the first place. After all, a random extra day in a year can seem very, well, random.
As it turns out, leap years aren’t random at all. They’re in place for a reason. Our calendar year is defined as the time it takes for the Earth to orbit around the Sun once. It takes the Earth about 365 days to do this, or more specifically, 365 1/4 days. See how it’s a bit off? Adding an extra day every four years ensures that the Earth is in the same point of its orbit, at the same time of the calendar year, every year.
The reason for a leap year is to keep seasons in place.
Before we used a leap year calendar, the seasons changed. So for example, over a period of three hundred years, the month of June could change from summertime to wintertime if we didn’t use a leap year. However, it’s unlikely that you would notice the change in your lifetime, as it happens gradually.
So why does that extra day happen in February? Julius Caesar and the late Roman Emperor, Caesar Augustus, are to blame. The calendar created by Julius Caesar gave February 30 days and August 29 days. When Augustus came into power, he wanted his own month (August) to have more days. So, he casually added two days to August (no big deal), one of which came from February, leaving it the shortest month of the year, aka the month we add an extra day to every four years.
A leap year isn’t happening again until 2020, so we have some time before we see another February 29th on the calendar.