What is a leap year?


Grayson Joslin, Editor

Normally, the average calendar year contains 365 twenty-four hour days. This calendar, however, is not perfect. In some years, the average 365 day year is not enough. So, some years, like this year, 2020, will have add a 366th day to the calendar. This day – February 29th, is called “leap day”.

The question is then posed – “Why do we even need a leap day?”

The standard calendar assumes that it will take 365 days for the Earth to make one complete rotation around the sun. Instead the Earth takes 365 days, 5 hours, 48 minutes and 46 seconds to make one complete trip around the sun. This is called a solar, or a tropical, year. As the years progress with a 365 day calendar year and a 365 1/4 day solar year, the calendar loses sync with the solar year. If one extra day is not added to the calendar to make up for the lost time, in a few hundred years, the calendar will be flipped, with Christmas being celebrated in June instead of December. The solution to this lost time is the leap day, where every four years, an extra day is added to the calendar to make up for the almost one day lost to help align the calendar year and the solar year.

The leap year and leap day was first introduced by Julius Caesar, the famous leader of the Roman Republic, who also invented the Julian calendar. This calendar had flaws, with one of the most notable being leap days. Under this calendar, any evenly divisible by four would be a leap year. This, however, is too much. This results in the calendar being off by one day every 128 years.

In 1582, the Gregorian calendar, the most used calendar system in the world today, was introduced. It was seen as an improvement to the previous Julian calendar. The Julian calendar incorrectly assumed that the length of one year was 365.25 days. This is not the case, and the Julian calendar had fallen out of sync with the solar position of Earth by the time that 1582 came around. When the Gregorian calendar was implemented, the Julian calendar was eleven days behind reality.

With the introduction of the Gregorian calendar came three simple rules on how to determine which years were leap years:

  1. The year must be divisible by 4. Examples of this would be 1616, 1864 and 2020.
  2. If the year is divisible by 100, it is not a leap year. Years like 1700 and 1900 would not be considered leap years and would have the regular 365 days.
  3. If the year is evenly divisible by 400, however, then that year will be a leap year. 1900 and 2100 were not leap years, but 2000 was. The next leap year to follow this year will be in 2400.

Using these rules, the Gregorian calendar will be only be one day off in 8,000 years.

Leap Day is an important day in the calendar and its importance cannot be understated. Enjoy Leap Day this year, as you won’t be able to have another one for four years!