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February 29, also known as leap day or leap year day, is a date added to most years that are divisible by 4, such as 2016, 2020, and 2024. A leap day is added in various solar calendars (calendars based on the Earth's revolution around the Sun), including the Gregorian calendar standard in most of the world. Lunisolar calendars (whose months are based on the phases of the Moon) instead add a leap or intercalary month.^[1]

In the Gregorian calendar, years that are divisible by 100, but not by 400, do not contain a leap day. Thus, 1700, 1800, and 1900 did not contain a leap day; neither will 2100, 2200, and 2300. Conversely, 1600 and 2000 did and 2400 will. Years containing a leap day are called leap years. Years not containing a leap day are called common years. February 29 is the 60th day of the Gregorian calendar in such a year with 306 days remaining until the end of the year. In the Chinese calendar, this day will only occur in years of the monkey, dragon, and rat.

A leap day is observed because the Earth's period of orbital revolution around the Sun takes approximately six hours longer than 365 whole days. A leap day compensates for this lag, realigning the calendar with the Earth's position in the Solar System; otherwise, seasons would occur later than intended in the calendar year. The Julian calendar used in Christendom until the 16th century added a leap day every four years; but this rule adds too many days (roughly three every 400 years), making the equinoxes and solstices shift gradually to earlier dates. By the 16th century the vernal equinox had drifted to March 11, and the Gregorian calendar was introduced both to shift it back by omitting several days, and to reduce the number of leap years via the aforementioned century rule to keep the equinoxes more or less fixed and the date of Easter consistently close to the vernal equinox.^[1][2]

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.

For 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. These extra days occur in each year which is an integer multiple of 4 (except for years evenly divisible by 100, which are not leap years unless evenly divisible by 400). Similarly, 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. In the Bahá'í Calendar, a leap day is added when needed to ensure that the following year begins on the March equinox.

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.^[2][3] For example, Christmas Day (December 25) fell on a Tuesday in 2012, Wednesday in 2013, Thursday in 2014, and Friday in 2015, but then leapt over Saturday to fall on a Sunday in 2016.

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. Unlike leap days, leap seconds are not introduced on a regular schedule because variations in the length of the day are not entirely predictable.

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.

西曆之閏年[編輯]

西暦額我畧曆係基於地球繞太陽公轉一週嘅時間作為一年，再畫分為12個月。

以一日24小時計（小時嘅最小單位秒由銫-133原子嘅一種特定躍遷定義），公轉周期實際上為365日又四份之一，四年加埋嘅誤差有差唔多一日，所以每四年補一日，而呢一年就叫做閏年（但係如果年份係100嘅倍數，就要俾400除得盡，先致係閏年，否則就係平年。所以1800年、1900年、2100年、2200年、2300年都係平年，而2000年、2400年，就係閏年）。

唐曆嘅閏年[編輯]

唐曆係陰陽曆，即係同時考慮太陰（月亮嘅別稱）同太陽嘅運行來制定。

月球繞地球公轉周期為一個「月」，呢個係中文「月」呢個時間單位嘅由來。月球繞地球公轉嘅周期為29日到30日之間，一「年」有十二或者十三個月。同時，地球繞太陽公轉周期為之一「歲」，以每年冬至為一歲嘅第一日。所以傳統上，冬至嗰日會做節慶祝，甚至有話：「冬至大過年」。

留意：「歲」同「年」係唔同嘅單位，歲嘅長度係固定嘅，大約係365.24日；而年嘅長度就唔固定，平年得十二個月，所以會短啲，而閏年就有十三個月，會長啲。設置閏年，係為咗令「年」嘅平均長度符合「歲」嘅長短而引入嘅誤差補值。閏年多出來嗰個月叫閏月。

重有：唐曆係個天文曆，月、年、歲、節氣呢啲事項，係跟住天文觀測而定，無固定嘅方程式來斷定，只可以根據最新嘅天文觀測數據來推斷未來嘅事項，而且會時不時跟住新收集嘅數據而更新。

維基同享有多媒體嘅嘢： 閏年