Leap Year

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.

The Earth's motion 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.

This means that 2000 and 2400 are leap years, while 1800, 1900210022002300 and 2500 are NOT leap years.

The year 2000 was somewhat special as it was the first instance when the third criterion was used in most parts of the world since the transition from the Julian to the Gregorian Calendar.

Gregorian calendar seasons difference
Pope Gregory XIII (1572-1585) of Gregorian Cal...
Pope Gregory XIII (1572-1585) of Gregorian Calendar fame by Camillo Rusconi (1658-1728) (Photo credit: mharrsch)
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