How Many Days Are There in Leap Year and Why Do We Add a Day?

It’s pretty common knowledge that Leap Day is on Feb. 29, every four years. But what else do you know about leap years?

ENTITY is here to answer your leap year questions about the Gregorian calendar that Americans use.

First things first, how many days are there in leap year?

In years when Feb. 29 or Leap Day is added to the calendar, there are 366 days in the leap year. A traditional year in the Gregorian calendar has 365 days.

How often does leap year happen?

Leap years occur every four years to keep the Gregorian calendar in line with the solar year. A solar year is determined by the Earth making one full revolution around the sun.

However, the question how many days are there in leap year is somewhat misleading. Technically, each year has more than 365 days. A single year according to the solar year is 365 days, five hours, 48 minutes and 46 seconds to be exact.

For convenience, we discount the five hours, 48 minutes and 46 seconds each year. However, this doesn’t mean that time just disappears. To make up for it, we add a day every four years.

Who came up with the idea of leap year?

Julius Caesar created the concept of Leap Day in 45 B.C. After consulting top astronomers of the time, Caesar realized his original 364 day model was incorrect. They advised him that a true solar year is 365.25 days.

However, they weren’t exactly right either. A solar year is actually 11 minutes short of 365.25 days.

When is leap year again?

The last leap year was in 2016, which means the next leap year will be Feb. 29, 2020. This is considered a normal leap year.

However, when it comes to century years, like the year 2000, a strange rule comes into play.

Because of the 11 minute time difference, Pope Gregory XIII realized he would have to modify the leap year to keep the calendar in order.

Gregory created a system that eliminates three leap years every 400 years. Century years that are divisible by 400 have a leap year, if they aren’t, they don’t have one. So we welcomed the turn of the 21st century with a leap year. But when we enter the 22nd century in 2100, there will be no leap year.

And because he made this change, the name Gregorian calendar, the modern date system in use, is named after Pope Gregory XIII.

There’s also a leap second.

This might seem over meticulous and as if scientists were just being silly at this point. But, there is an actual concept called the leap second, which literally adds a second to a year.

We know what you’re thinking, how the hell can one second make a difference?

Well first, we add a leap second because we are giving the Earth a chance to catch up. Instead of this being determined by the Earth revolving around the sun, it has to do with Earth’s daily rotation.

The velocity of Earth’s rotation around its own axis and atomic time do not match. Essentially, the Earth moves just a tiny bit slower than our clocks can calculate time. So, an extra second is added to the end of June or the end of December every year. Instead of the clock moving from 23:59:59 to 00:00:00, it goes from 23:59:59 to 23:59:60 to 00:00:00.

Since the first leap second was added in 1972, the number has risen to 27 leap seconds added. This means Earth slowed down 27 seconds compared to atomic time since then.

The last leap second was added on New Year’s Eve in 2016.

Scientists are still debating the effectiveness of leap seconds. Proponents believe they help balance the faults in computerized clocks. Opponents believe they add complexity to something that is irrelevant to the average person’s life.

But now that you know how many days are in leap year, you can start planning what you’ll do with your newfound time. But don’t hold your breath on getting much done during the next leap second.