What is a Leap Year?

Every one of you has heard about leap years that come every four year. When I was child I knew that there is a leap year that comes every four years which consists of 366 days a year instead of 365 days, but do not know the real reason. When I was in my first semester of Computer Science course, my programming teacher gave me the assignment to write a program to find leap years. He also told us that the year which is divided by 4 or 400 is a leap year while the year which is divided by 100 is not a leap year, but does not explain the reason and asked us to do your own research and find the solution. Then I do some research on the internet and try to understand what leap year is?

In this article I’ll explain Leap Year as simply as possible in three steps.

Step 1

The Earth takes 24 hours to complete rotation around its axis and 365 days around the sun. But the problem here is that earth takes actually 365.25 days every year to complete its rotation around the sun, which means quarter day every year.

What to do with this quarter day? We just ignore this extra quarter day for 3 years. So after 4 years, quarter day will become complete 1 day. To solve this problem we consider 1 extra day after every 4 year, so the year now contains 366 days instead of 365 days

“Every year which is divisible by 4 is a Leap Year.”

That is why, years 2012, and 2016 are leap years.

Step 2

But the problem has not yet been solved. One year actually contains a little less then extra quarter day. So one year actually takes 365.2422 days to complete its rotation around the sun. We add one extra day after every four years, but it will overcorrected our calendar by a day every 100 years.

To solve this problem, we skip 1 leap year after every 100 days, which means any year divisible by 4 is a leap year and any year divisible by 100 is not a leap year. By skipping 1 leap year we have solved the problem.

“Every year which is divisible by 100 is not a Leap Year.”

Due to this 1900, 1800 and 1700 were not leap years.

Step 3

But wait there is another problem, when we continuously take out 1 day in every 100 years we will now overcorrected our calendar in the opposite direction. The day that we skip in every 100 years has now been added in every 400 years. Which means,

Every century year which is divisible by 400 is a Leap Year

Now we say that the year 2000 is a leap year not because it is divisible by four 4 but because it is divisible 400.

I hope you understand the concept of leap year, please reply if you have any problem.