Weatherz School: Why we have Leap Day almost every four years

Why we have Leap Day almost every four years

Happy Leap Day! The 29th of February happens once every four years (with an exception) because our trip around the sun isn't a perfect 365 days.

It’s the 29th of February, a day we have almost every four years.

As you probably know, a year is defined by the time it takes for one trip around the sun. Our year is typically referred to as being 365 days.

To be more precise, it’s 365.2422 days. And yes, that small addition makes a difference.

If we didn’t have leap day, we would miss nearly 6 hours every year, which means in 100 years, our calendar would be 24 days off. The passage of just one century would move the spring equinox into February.

We can’t have that, so we add one day every four years. this makes for a yearly average of 365.25 days. Problem solved, right? Almost.

There’s still a rounding error that gives us 11 extra minutes every year. This means that after 100 years, our calendar would be 18 hours off.

That’s why we have leap year hopscotch. We skip a leap year if it falls on the start of a century, unless that year is divisible by 400.

That’s why we didn’t skip Leap Day in 2000. The last one we skipped was 1900, and the next one will be 2100.

This brings our yearly average to 365.2425 days. Going back to the calculator, we’re still off by about 26 seconds, which would take 3,333 years to be one day off.

Happy Leap Day!