Is 2020 a Leap Year? Here's Why February Has an Extra Day

This February, there will be 29 days in the month—rather than the usual 28—making 2020 a "leap year." These 366-day leap years occur every four years. But why do we have them?

The simple answer is they are needed to keep the internationally accepted Gregorian calendar aligned with the actual time it takes Earth to complete one orbit of the Sun, what's known as the "tropical year."

This tropical year—or the length of time it takes for the Earth to go from one equinox to the next—is 365 days, 5 hours, 48 minutes and 45 seconds long.

"Leap years are required because we Earthlings decide to measure our 'time to orbit' by the number of 'Earth rotations,' or solar days," James O'Donoghue, a planetary scientist at the Japanese space agency JAXA, told Newsweek.

"We define a year as 365 days, but Earth actually takes 365.24 days orbit the Sun, so we're falling short every single year by 0.24 days! To fix this, every four years we add a leap day—February 29," he said.

However, while adding an extra day every four years corrects the alignment with the Gregorian calendar over the short term, further adjustments are required to keep it in line over the long term.

This extra adjustment would not be required if a tropical year was exactly 365.25 days—because multiplying it by four would give a nice round number. But because the length of the tropical year is just shy of this number in reality—approximately 365.242189 days—the additional correction is needed.

leap year
2020 is a leap year meaning February has an extra day. iStock

"As you can see that would be an over-correction, so there are further corrections required every 100 years," O'Donoghue said.

According to the rules of the Gregorian calendar, years which can be evenly divided by 100—such as 1900—are not counted as leap years even if they are meant to be. There are exceptions to this rule if a year is also evenly divisible by 400, such as the year 2000.

"If we failed to do this, our calendar would migrate away from our seasons, causing long-term headaches for people who plan life and work around the seasons," O'Donoghue said.

"If you think about it, it'd have to be a big coincidence for this rock we live on to rotate in a whole number of days! It could have been some fraction of anywhere between 0 to 1, and we got 0.24 days—not too bad," he said.

The following animation created by O'Donoghue clearly demonstrates why we have leap years and shows what would happen if we ignored them.

THIS is why we have Leap Years —
AND what happens if we don't!

— James O'Donoghue (@physicsJ) February 8, 2020