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 below. Consequently, the calendar drifts out '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 AUC 709 (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. 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, fell on a Saturday in 2021, falls on a Sunday in 2022 and 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. Each Gregorian leap year has 179 even-numbered days and 187 odd-numbered days. The 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. These extra days occur in each year which 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 (called intercalating in technical terminology) an additional 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.
#Leap years list mod
The year's position in the cycle is given by the formula ((year + 8) mod 28) + 1).This article needs additional citations for verification.
As the Julian calendar repeats after 28 years that means it will also repeat after 700 years, i.e. Like all leap year types, the one starting with 1 January on a Friday occurs exactly once in a 28-year cycle in the Julian calendar, i.e. Template:List of calendars Julian Calendar Gregorian leap years starting on Friday Decade Thus, the overall occurrence is thus 3.75% (15 out of 400). Leap years that begin on Friday, along with those that start on Sunday, occur most frequently: 15 out of the 97 (≈ 15.46%) total leap years in a 400-year cycle of the Gregorian calendar.
#Leap years list iso
ISO 8601-conformant calendar with week numbers forĪny leap year starting on Friday (dominical letter CB)Īpplicable years Gregorian Calendar Presented as common in many English-speaking areas Calendar for any leap year starting on Friday,
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