Leap 15.5
Janise Knollman
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 Solar Hijri and Bah' calendars, 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 1 March through 28 February 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, 1 March was on a Friday in 2024, then it will be on Saturday in 2025, Sunday in 2026, and Monday in 2027, but then will "leap" over Tuesday to fall on a Wednesday in 2028.
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 29 February is not handled correctly in logic that accepts or manipulates dates.
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. His rule for leap years was a simple one: add a leap day every 4 years. This algorithm is close to reality: a Julian year lasts 365.25 days, a mean tropical year about 365.2422 days.[4] Consequently, even this Julian calendar drifts out of 'true' by about 3 days every 400 years. 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.
In the Gregorian calendar, the standard calendar in most of the world,[6] almost every fourth year is a leap year. Each leap year, the month of February has 29 days instead of 28. Adding one extra day in the calendar every 4 years compensates for the fact that a period of 365 days is shorter than a tropical year by almost 6 hours.[7] However, this correction is excessive and the Gregorian reform modified the Julian calendar's scheme of leap years as follows:
Every 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. For example, the years 1700, 1800, and 1900 are not leap years, but the years 1600 and 2000 are.[8]
The Gregorian calendar was designed to keep the vernal equinox on or close to 21 March, so that the date of Easter (celebrated on the Sunday after the ecclesiastical full moon that falls on or after 21 March) remains close to the vernal equinox.[13] The "Accuracy" section of the "Gregorian calendar" article discusses how well the Gregorian calendar achieves this design goal, and how well it approximates the tropical year.
The intercalary day that usually occurs every 4 years is called leap day and is created by adding an extra day to February. This day is added to the calendar in leap years as a corrective measure because the Earth does not orbit the Sun in precisely 365 days. Since about the 15th century, this extra day has been 29 February, but when the Julian calendar was introduced, the leap day was handled differently in two respects. First, leap day fell within February and not at the end: 24 February was doubled to create, strangely to modern eyes, two days both dated 24 February.[14] Second, the leap day was simply not counted so that a leap year still had 365 days.[15]
The early Roman calendar was a lunisolar one that consisted of 12 months, for a total of 355 days. In addition, a 27- or 28-day intercalary month, the Mensis Intercalaris, was sometimes inserted into February, at the first or second day after the Terminalia a. d. VII Kal. Mar. (23 February), to resynchronise the lunar and solar cycles. The remaining days of Februarius were discarded. This intercalary month, named Intercalaris or Mercedonius, contained 27 days. The religious festivals that were normally celebrated in the last 5 days of February were moved to the last 5 days of Intercalaris. The lunisolar calendar was abandoned about 450 BC by the decemviri,[16] who implemented the Roman Republican calendar, used until 46 BC. The days of these calendars were counted down (inclusively) to the next named day, so 24 February was ante diem sextum Kalendas Martias ["the sixth day before the calends of March"] often abbreviated a. d. VI Kal. Mart. The Romans counted days inclusively in their calendars, so this was the fifth day before 1 March when counted in the modern exclusive manner (i.e., not including both the starting and ending day).[17] Because only 22 or 23 days were effectively added, not a full lunation, the calends and ides of the Roman Republican calendar were no longer associated with the new moon and full moon.[citation needed]
... and by (b) the statute de anno bissextili, it is provided, quod computentur dies ille excrescens et dies proxime prcedens pro unico dii, so as in computation that day excrescent is not accounted.[21]
In the Eastern Orthodox Church, the feast of St. John Cassian is celebrated on 29 February, but he is instead commemorated at Compline on 28 February in non-leap years. The feast of St. Matthias is celebrated in August, so leap years do not affect his commemoration, and, while the feast of the First and Second Findings of the Head of John the Baptist is celebrated on 24 February, the Orthodox church calculates days from the beginning of the current month, rather than counting down days to the Kalends of the following month, this is not affected. Thus, only the feast of St. John Cassian and any movable feasts associated with the Lenten or Pre-Lenten cycles are affected.
In Ireland and Britain, it is a tradition that women may propose marriage only in leap years. While it has been claimed that the tradition was initiated by Saint Patrick or Brigid of Kildare in 5th century Ireland, this is dubious, as the tradition has not been attested before the 19th century.[28] Supposedly, a 1288 law by Queen Margaret of Scotland (then age five and living in Norway), required that fines be levied if a marriage proposal was refused by the man; compensation was deemed to be a pair of leather gloves, a single rose, 1, and a kiss.[29][c] In some places the tradition was tightened to restricting female proposals to the modern leap day, 29 February, or to the medieval (bissextile) leap day, 24 February.[citation needed]
A person born on February 29 may be called a "leapling" or a "leaper".[36] In common years, they usually celebrate their birthdays on 28 February. In some situations, 1 March is used as the birthday in a non-leap year, since it is the day following 28 February.
Technically, a leapling will have fewer birthday anniversaries than their age in years. This phenomenon may be exploited for dramatic effect when a person is declared to be only a quarter of their actual age, by counting their leap-year birthday anniversaries only. For example, in Gilbert and Sullivan's 1879 comic opera The Pirates of Penzance, Frederic (the pirate apprentice) discovers that he is bound to serve the pirates until his 21st birthday (that is, when he turns 88 years old, since 1900 was not a leap year) rather than until his 21st year.
If a period fixed by weeks, months, and years does not commence from the beginning of a week, month, or year, it ends with the ending of the day which precedes the day of the last week, month, or year which corresponds to that on which it began to commence. But if there is no corresponding day in the last month, the period ends with the ending of the last day of the last month.[38]
The Revised Julian calendar adds an extra day to February in years that are multiples of four, except for years that are multiples of 100 that do not leave a remainder of 200 or 600 when divided by 900. This rule agrees with the rule for the Gregorian calendar until 2799. The first year that dates in the Revised Julian calendar will not agree with those in the Gregorian calendar will be 2800, because it will be a leap year in the Gregorian calendar but not in the Revised Julian calendar.
This rule gives an average year length of 365.242222 days. This is a very good approximation to the mean tropical year, but because the vernal equinox year is slightly longer, the Revised Julian calendar, for the time being, does not do as good a job as the Gregorian calendar at keeping the vernal equinox on or close to 21 March.
The Bahʼ calendar is a solar calendar composed of 19 months of 19 days each (361 days). Years begin at Naw-Rz, on the vernal equinox, on or about 21 March. A period of "Intercalary Days", called Ayyam-i-Ha, is inserted before the 19th month. This period normally has 4 days, but an extra day is added when needed to ensure that the following year starts on the vernal equinox. This is calculated and known years in advance.
The Revised Bengali Calendar of Bangladesh and the Indian National Calendar organise their leap years so that every leap day is close to 29 February in the Gregorian calendar and vice versa. This makes it easy to convert dates to or from Gregorian.
One reason for this rule is that Yom Kippur, the holiest day in the Hebrew calendar and the tenth day of the Hebrew year, now must never be adjacent to the weekly Sabbath (which is Saturday), i.e., it must never fall on Friday or Sunday, in order not to have two adjacent Sabbath days. However, Yom Kippur can still be on Saturday. A second reason is that Hoshana Rabbah, the 21st day of the Hebrew year, will never be on Saturday. These rules for the Feasts do not apply to the years from the Creation to the deliverance of the Hebrews from Egypt under Moses. It was at that time (cf. Exodus 13) that the God of Abraham, Isaac and Jacob gave the Hebrews their "Law" including the days to be kept holy and the feast days and Sabbaths.