The Gregorian calendar therefore omits three leap days every 400 years, which is the length of its leap cycle. Over a period of four centuries, the accumulated error of adding a leap day every four years amounts to about three extra days. This more closely resembles a mean tropical year of 365.2422 days. Whereas the Julian calendar year incorrectly summarized Earth’s tropical year as 365.25 days, the Gregorian calendar makes these exceptions to follow a calendar year of 365.2425 days. For example, the years 1700, 1800, and 1900 are not leap years, but the years 16 are. However, this correction is excessive and the Gregorian reform modified the Julian calendar’s scheme of leap years as follows:Įvery year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. Adding one extra day in the calendar every four years compensates for the fact that a period of 365 days is shorter than a tropical year by almost 6 hours.
Each leap year, the month of February has 29 days instead of 28.
In the Gregorian calendar, the standard calendar in most of the world, almost every fourth year is a leap year. The Julian calendar continued in use unaltered for about 1600 years until the Catholic Church became concerned about the widening divergence between the March Equinox and 21 March, as explained at Gregorian calendar, below. Consequently, even this Julian calendar drifts out of ‘true’ by about three days every 400 years. This algorithm is close to reality: a Julian year lasts 365.25 days, a mean tropical year about 365.2422 days. His rule for leap years was a simple one: add a leap day every four years. On 1 January 45 BC, by edict, Julius Caesar reformed the historic Roman calendar to make it a consistent solar calendar (rather than one which was neither strictly lunar nor strictly solar), thus removing the need for frequent intercalary months. Leap years can present a problem in computing, known as the leap year bug, when a year is not correctly identified as a leap year or when February 29 is not handled correctly in logic that accepts or manipulates dates.ġ4 External links Julian calendar Unlike leap days, leap seconds are not introduced on a regular schedule because variations in the length of the day are not entirely predictable. The length of a day is also occasionally corrected by inserting a leap second into Coordinated Universal Time (UTC) because of variations in Earth’s rotation period. For example, Christmas Day (December 25) fell on a Friday in 2020, Saturday in 2021, Sunday in 2022, and will be on a Monday in 2023, but then will leap over Tuesday to fall on a Wednesday in 2024. The term leap year probably comes from the fact that a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next, but the day of the week in the 12 months following the leap day (from March 1 through February 28 of the following year) will advance two days due to the extra day, thus leaping over one day in the week. In the Bahá’í Calendar, a leap day is added when needed to ensure that the following year begins on the March equinox. In the lunisolar Hebrew calendar, Adar Aleph, a 13th lunar month, is added seven times every 19 years to the twelve lunar months in its common years to keep its calendar year from drifting through the seasons. A leap year of 366 days has 52 weeks and two days, hence the year following a leap year will start later by two days of the week. This extra leap day occurs in each year that is an integer multiple of 4 (except for years evenly divisible by 100, but not by 400). A year that is not a leap year is a common year.įor example, in the Gregorian calendar, each leap year has 366 days instead of 365, by extending February to 29 days rather than the common 28. By inserting (“intercalating”) an additional day (a “leap day”) or month into some years, the drift between a civilization’s dating system and the physical properties of the Solar System can be corrected. Because astronomical events and seasons do not repeat in a whole number of days, calendars that have a constant number of days in each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. (February 2020)Ī 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. This article needs additional citations for verification.
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