Like the Olympics, the World Cup and the American Presidential election a leap year comes around every four years, when an extra day (leap day) is added to our calendar, February 29th.
Or does it?
Leap Years are needed to keep our calendar in alignment with the Earth’s revolutions around the sun.
It takes the Earth approximately 365.242199 days (a tropical year) to circle once around the Sun.
Note: The illustration is not to scale.
However, the Gregorian calendar has only 365 days in a year, so if we didn’t add a day on February 29 nearly every 4 years, we would lose almost six hours off our calendar every year. After only 100 years, our calendar would be off by approximately 24 days!
How do we calculate Leap Years?
In the Gregorian calendar 3 criteria must be met to be a leap year:
The year is evenly divisible by 4;
If the year can be evenly divided by 100, it is NOT a leap year, unless;
The year is also evenly divisible by 400. Then it is a leap year.