How Long Is a Year?

You may know that a year, or the time it takes the Earth to make one complete revolution, is roughly 365 days. Did you know that it’s not exactly 365?

It actually takes the Earth a little longer than 365 days to finish its journey around the Sun. It’s closer to 365.25 days. That means if we only have 365 days in our calendar year, we will fall behind 1/4 day each year, until our calendar is so far behind the seasons that we have snow in July!

The solution: Every 4 years, we add an extra day to the calendar. We call it a leap year. This helps us reset our calendars to better match Earth’s path through space, so our seasons stay somewhat consistent in the long run. Every year that is a multiple of 4 is a leap year…mostly.

If you were old enough to read the news in 2000, you might remember that 2000 was a leap year, as you’d expect, even though 1900 wasn’t. It was a hot news item at the time.

Earth’s revolution around the Sun is not exactly 365.25. It’s closer to 365.2422, or a tiny bit below 365-and-a-quarter. To fix this tiny little bit, we don’t have a leap year in years that end in “00”. But that’s not enough, so sometimes we add the leap year back in. Like in 2000.

Even though we have a leap year almost every 4 years, sometimes we skip one, just to make our numbers line up a little more closely.

Source: https://pumas.nasa.gov/files/04_21_97_1.pdf