Global Geological And Climatic Events

Why do we have Leap Years?


From UPSC perspective, the following things are important :

Prelims level: Leap Year and the science behind it

Mains level: Not Much


The year 2020 is a ‘leap year’, meaning the month of February will have 29 days instead of 28, and the total number of days will be 366 instead of 365. This was also the case in 2016, and 2024 will again be a leap year.

Leap Years

  • A calendar is meant to correspond to the Earth’s seasons.
  • For this, the number of days in a calendar needs to match the time required by the Earth to orbit the Sun.
  • The time required by the Earth to complete its orbit around the Sun is approximately 365.242 days. But years are usually only 365 days.
  • To adjust for the extra 0.242 days in the orbital period, which becomes almost one full day in four years, the calendar adds an extra day once every four years.
  • This approximates the time to 365.25 days, which is close to the actual 365.242 days.

But is that not inaccurate?

  • Yes, it is. And further adjustments are made to the Gregorian calendar, the calendar we follow today.
  • The Gregorian calendar was introduced in 1582. Before that, the calendar followed was the Julian calendar, introduced in 45 BC.
  • The calendars were different in their treatment of leap years.
  • The Julian calendar had leap days every four years, but since it still did not accurately conform to the Earth’s precise orbit time, it kept falling behind with respect to natural seasons over the centuries.
  • By the 16th century, the Julian calendar had fallen out of tune with the natural seasons by almost 10 days.
  • To correct this discrepancy, Pope Gregory XIII in 1582 decreed that the day of October 4 that year would be followed directly by October 15 – thus covering up the error.
  • The Pope also modified the leap year system in the Julian calendar. That new system came to be known as the Gregorian calendar.

What is the new system?

  • In the Gregorian calendar, a century year (a year ending with 00) is not a leap year, even though it is a multiple of 4. Thus, the year 2100 will not be a leap year.
  • But even this does not provide total accuracy. To ensure that, some century years remain leap years. In the Gregorian calendar, leap years include those century years which are exactly divisible by 400.
  • Thus, 2000 remained a leap year even though it ended with 00.
  • The Gregorian calendar reduces the margin of error under the Julian calendar, thus keeping days more in tune with seasons.

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