Why do we have a Leap Day?

Why is it every four years?

You can read all about that on this page!

# Why Leap Day?

The way we measure time, in hours, days and months, has its origin in the position of the stars and the moon. As the earth not only revolves around the sun, it also rotates on its own axis, this leads to the seasons as we know it (Summer, Winter).

The earth rotates a total of 366.26 times on its own axis, in one rotation around the sun. This duration is also known as the sidereal year. The number of solar days, however, is only 365.26. This is caused as the rotation around the sun and around its own axis run in the same direction.

So our calender is based on the number of solar days. The rotation of the earth and thus the seasons are natural phenomena and do not adhere to man-made units such as days and hours (60 minutes). This leads to a little deviation from our calender and the seasons. Every year we ‘lose’ about 6 hours. After 100 years, our calender would be 24 hours off from our seasons. In the long term, January would even become a Summer month (for the Northern Hemisphere). So a correction is needed.

During the Roman Empire, the calendar contained 355 days. Julius Ceasar opted for a permanent solution to have a matching calender. He introduced the Julian calendar with February being the last month of the year. A day was added every for years, if that year was divisible by 4.

## That’s not all of it

Just when you think that adding one day every four years would solve all: you are wrong. When you do the calculation, you see that in one century 25 leap days are added to the calendar. And that is about 6 hours too much. So in the long run the calendar would deviate again from the seasons.

Therefore, the turns of a century, although they are divisible by 4, are not leap years after all.

Wait… that’s not all!

Over a period of several centuries we are just a bit short again. So we need another correction. This takes place at the turn of a century that is divisible by 400. So: the year 1900 was not a leap year, but 2000 was.

This last correction was introduced by Pope Gregory XIII and is now known as the Gregorian calendar which is used in almost all countries today.

## Other fun facts

## Leapling

When you are born on February 29 you are called a **leapling**. In the non-leap years we celebrate our birthday on February 28 or March 1st.

#### Next Leap Day 2024 is in:

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## Why Leap Year?

In non-leap years a certain date only moves one weekday. For example: if April 23 is on a Thursday, the next year it will be on a Friday.

But in leap years, it “**leaps**“.

It won’t be on a Friday, but a Saturday.

## Video explanation

If all the rotations of the earth are making your head spin, watch this video:

Created by CGP Grey

## Leap second

Since we have the atomic clock, timekeeping has become much more accurate. Due to irregularities in the rotation of the earth, it turns out that the atomic clock (that has no deviations at all) is not perfectly aligned with the earth.

To correct this, the leap second has been introduced. The first time was in 1972. The last one was on June 30, 2015. In total 26 leap seconds have been added.

Adding a leap second looks weird: if the clock is at 23:59:59, it does not jump to 0:00:00 as it usually does. First it will hit 23:59:60 and then will go to 0:00:00.