In an ordinary year, if you were to count all the days in a calendar from January to December, you’d count 365 days. But approximately every four years, February has 29 days instead of 28. So, there are 366 days in the year. This is called a leap year.

Credit: NASA/JPL-Caltech

Why do we have leap years?

A year is the amount of time it takes a planet to orbit its star one time. A day is the amount of time it takes a planet to finish one rotation on its axis.

Credit: NASA/JPL-Caltech

It takes Earth approximately 365 days and 6 hours to orbit the Sun. It takes Earth approximately 24 hours — 1 day — to rotate on its axis. So, our year is not an exact number of days.

Because of that, most years, we round the days in a year down to 365. However, that leftover piece of a day doesn’t disappear. To make sure we count that extra part of a day, we add one day to the calendar approximately every four years. Here’s a table to show how it works:

Year	Days in Year	Leap Year?
2017	365	No
2018	365	No
2019	365	No
2020	366	Yes

Because we’ve subtracted approximately 6 hours — or ¼ of a day — from 2017, 2018 and 2019, we have to make up that time in 2020. That’s why we have leap day!

Are leap years really that important?

Leap years are important so that our calendar year matches the solar year — the amount of time it takes for Earth to make a trip around the Sun. Subtracting 5 hours, 46 minutes and 48 seconds off of a year maybe doesn’t seem like a big deal. But, if you keep subtracting almost 6 hours every year for many years, things can really get messed up.

For example, say that July is a warm, summer month where you live. If we never had leap years, all those missing hours would add up into days, weeks and even months. Eventually, in a few hundred years, July would actually take place in the cold winter months!

Credit: NASA/JPL-Caltech