Every four years, a leap year occurs. No, it’s not a worldwide event where people take a leap by changing a habit nor is it a fun activity where everyone jumps at the same time. Instead, a leap year is when there is an extra day to the shortest month, February, and 2020 happens to be one. But why exactly do we have leap years?
A day is calculated by the number of hours it takes for the Earth to make a full rotation on its axis, and a year is
calculated by the number of days it takes for the Earth to orbit the sun. In practice, it was established that the Earth takes 365 days to orbit the sun, with each day taking 24 hours. However, it actually takes 365.24 days to orbit the sun, or roughly a quarter of a day longer. Quick math shows that every four years, this little addition equates to almost a full day, which is added as Feb. 29.
However, this doesn’t occur every single fourth year. The slight difference of 0.24 days every year is not exactly a quarter of a day (that would be 0.25 days), and this difference eventually adds up. In fact, we lose three days every 400 years from this and skip a leap year every now and then as a result.
There are rules as to when to skip leap years. First, a year to add an extra day to has to be divisible by four. Second, a leap year cannot occur in a year divisible by 100. However, there is an exception to that rule: that would be the third rule. If a year can be divided by 400, a leap day can be added. This is why the two last leap years in years divisible by 100 happened in 1600 and 2000.
And there it is! Leap years and their exceptions!
Graphic by @sundaeghost