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Leap Year Is Skipped Every 100 Years, But Not Every 400 Years

The Earth doesn’t quite keep an exact 24 hour day due to the speed at which it rotates. A complete rotation of the Earth actually takes only 23 hours, 56 minutes and 4.1 seconds. We try to account for this by adding, or not adding, leap years.

The Earth rotates 365.242375 times per year. Since the actual astronomical year doesn’t quite match up to the calendar system of 365 days, we have a leap year (also called an intercalary year or abissextile year) every 4 years in which we add a day to catch up and make sure the seasons continue to align with the correct months on the calendar.

However, just adding a 24 hour day every 4 years still doesn’t quite work out either. To further account for the slight inaccuracy leap year is skipped every 100 years. Then to make an additional adjustment it is not skipped every 400 years.

leap-year-cheat-sheet

How do you know if it’s a leap year or a skipped leap year?

If the year is evenly divisible by 4 it is a leap year. Unless it is a century year that is not evenly divisible by 400. Further breakdown:

  • Leap years are any year that can be evenly divided by 4 (election years in the United States as an easy way to remember) such as 2004, 2008, 2012, 2016, 2020, etc. unless we’re skipping:
  • Leap year is skipped on century years that can be evenly divided by 100 such as 1700, 1800, 1900, 2100, 2200, 2300, etc. unless we’re not skipping:
  • Leap year is not skipped on century years that can be evenly divided by 400 such as 1600, 2000, 2400, etc.

Therefore, this past century year (2000) was a leap year because it is divisible by 4 and also evenly divisible by 400. Next century year (2100) will be a skipped leap year because even though it is divisible by 4, it is not divisible by 400. In 2100, instead of adding a day in February like we normally would it will be treated like a common year or non-leap year.

Sources

  • Leap Years (link is external). Rod Pierce, Math Is Fun. 2 December 2015. Accessed 26 December 2015.
    • Summary: Explanation of the math behind why leap years exist and why they occur when they do. Including graph of seasonal drift to further illustrate the need for leap years.