A leap year (also known as an intercalary year or bissextile year) is a calendar year that contains an additional day (or, in the case of a lunisolar calendar, a month) added to keep the calendar year synchronized with the astronomical year or seasonal year.[1] Because astronomical events and seasons do not repeat in a whole number of days, calendars that have the same number of days in each year drift over time with respect to the event that the year is supposed to track. By inserting (called intercalating in technical terminology) an additional day or month into the year, the drift can be corrected. A year that is not a leap year is a common year.
In the Gregorian calendar currently in use worldwide (except perhaps the Russian andIranian calendars), there is a leap year every year divisible by four except for years which are both divisible by 100 and not divisible by 400. Therefore, the year 2000 will be a leap year, but the years 1700, 1800, and 1900 were not. The complete list of leap years in the first half of the 21st century is therefore 2000, 2004, 2008, 2012, 2016, 2020, 2024, 2028, 2032, 2036, 2040, 2044, and 2048.
The extra rule involving centuries is an additional correction to make up for the fact that one extra day every four years is slightly too much correction (
). This scheme results in the vernal equinox gradually shifting its date between March 19 and 21, being shifted once every leap year, and then being abruptly shifted in non-leap centuries (see figure above).
). This scheme results in the vernal equinox gradually shifting its date between March 19 and 21, being shifted once every leap year, and then being abruptly shifted in non-leap centuries (see figure above).
In the Gregorian calendar, 97 years out of every 400 are leap years, giving the total number of days in 400 years as
The leap year was introduced in the Julian calendar in 46 BC. However, around 10 BC, it was found that the priests in charge of computing the calendar had been adding leap years every three years instead of the four decreed by Caesar (Vardi 1991, p. 239). As a result of this error, no more leap years were added until 8 AD. Leap years were therefore 45 BC, 42 BC, 39 BC, 36 BC, 33 BC, 30 BC, 27 BC, 24 BC, 21 BC, 18 BC, 15 BC, 12 BC, 9 BC, 8 AD, 12 AD, and every fourth year thereafter (Tøndering), until the Gregorian calendar was introduced (resulting in skipping three out of every four centuries). The UNIX command calincorrectly lists 4 AD as a leap year (Vardi 1991).
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