After every 4 years, an extra day is added to the calendar. This day is called a leap day. A leap day falls on the 29th of February.
A day is measured by the time taken by the earth to make one spin on its own axis. In other words, it takes 24 hours for the earth to make a complete rotation on its own axis.
A year is measured by how long the earth takes to orbit the sun. It usually takes 365 days for the earth to orbit the sun. However, the exact time taken by the earth is actually 365.24219 days. The additional 0.24 days is approximately a quarter of a day. In other words, it takes 365 days and an additional (1/4)th of a day for the earth to make a full rotation around the sun.
A quarter of a day added after 4 years becomes a full day. To keep everything consistent with the real astronomical year, a full day is added to the shortest month of the fourth year. This day is called a leap day. A year containing a leap day is called a leap year.
There are two rules to be kept in mind while determining a leap year:
- A leap day can only be added if the year is divisible by 4. For example, 2020 is a leap year because the number 2020 is divisible by 4. However, 2019 is not a leap year because the number 2019 is not divisible by 4.
- A leap year cannot fall on a year that’s divisible by 100. However, if the year is divisible by both 100 and 400, the leap day is added and a leap year is considered. For example, even if 2000 is divisible by 100, the year 2000 is still considered as a leap year since 2000 is divisible by 400 too. However, the years 1700 and 1800 were not considered leap years because those numbers were divisible by 100 but not by 400.
The concept of a leap year was started by Pope Gregory XIII in 1600. The current civil calendar we use nowadays is called the Gregorian calendar because it was this former head of the Catholic Church who commissioned this calendar.
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