Leap year

Last updated

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) compared to a common year. The 366th day (or 13th month) is added to keep the calendar year synchronised with the astronomical year or seasonal year. [1] Since astronomical events and seasons do not repeat in a whole number of days, calendars having a constant number of days each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. By inserting ("intercalating") an additional day—a leap day—or month—a leap month—into some years, the drift between a civilization's dating system and the physical properties of the Solar System can be corrected.

Contents

An astronomical year lasts slightly less than 3651/4 days. The historic Julian calendar has three common years of 365 days followed by a leap year of 366 days, by extending February to 29 days rather than the common 28. The Gregorian calendar, the world's most widely used civil calendar, makes a further adjustment for the small error in the Julian algorithm. Each leap year has 366 days instead of 365. This extra leap day occurs in each year that is a multiple of 4, except for years evenly divisible by 100 but not by 400.

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 Bahá'í 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, Christmas Day (25 December) fell on a Friday in 2020, Saturday in 2021, Sunday in 2022, and Monday in 2023, but then will "leap" over Tuesday to fall on a Wednesday in 2024.

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.

Julian calendar

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 four 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 three 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.

Prior to Caesar's creation of what would be the Julian calendar, February was already the shortest month of the year for Romans. In the Roman calendar (after the reform of Numa Pompilius that added January and February), all months except February had an odd number of days 29 or 31. This was because of a Roman superstition that even numbers were unlucky. [5] When Caesar changed the calendar to follow the solar year closely, he made all months have 30 or 31 days, leaving February unchanged except in leap years.

Gregorian calendar

An image showing which century years are leap years in the Gregorian calendar Leap Centuries.jpg
An image showing which century years are leap years in the Gregorian calendar

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 four 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]

1800 calendar, showing that February had only 28 days. 1800 calendar.jpg
1800 calendar, showing that February had only 28 days.

Whereas the Julian calendar year incorrectly summarised Earth's tropical year as 365.25 days, the Gregorian calendar makes these exceptions to follow a calendar year of 365.2425 days. This more closely resembles a mean tropical year of 365.2422 days. Over a period of four centuries, the accumulated error of adding a leap day every four years amounts to about three extra days. The Gregorian calendar therefore omits three leap days every 400 years, which is the length of its leap cycle. This is done by omitting 29 February in the three century years (multiples of 100) that are not multiples of 400. [9] [10] The years 2000 and 2400 are leap years, but not 1700, 1800, 1900, 2100, 2200 and 2300. By this rule, an entire leap cycle is 400 years which total 146,097 days, and the average number of days per year is 365 + 14  1100 + 1400 = 365 + 97400 = 365.2425. [11] (This rule could be applied to years before the Gregorian reform to create a proleptic Gregorian calendar, [12] though the result would not match any historical records.)

Gregoriancalendarleap solstice.svg
This graph shows the variations in date and time of the June Solstice.

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.

Leap day in the Julian and Gregorian calendars

A Swedish pocket calendar from 2008 showing 29 February Calendar-leapyeardate.jpg
A Swedish pocket calendar from 2008 showing 29 February
February 1900 calendar showing that 1900 was not a leap year Japanese Calendar with Verses by Osman Edwards 1900 February.jpg
February 1900 calendar showing that 1900 was not a leap year

The intercalary day that usually occurs every four 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]

Early Roman practice

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 five days of February were moved to the last five 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 ]

Julian reform

In Caesar's revised calendar, there was just one intercalary day nowadays called the leap day  to be inserted every fourth year, and this too was done after 23 February. To create the intercalary day, the existing ante diem sextum Kalendas Martias (sixth day (inclusive: i.e. what we would call the fifth day before) before the Kalends (first day) of March, i.e. what we would call 24 February) was doubled, [18] producing ante diem bis sextum Kalendas Martias [a second sixth day before the Kalends. This bis sextum ("twice sixth") was rendered in later languages as "bissextile": the "bissextile day" is the leap day, and a "bissextile year" is a year which includes a leap day. [19] This second instance of the sixth day before the Kalends of March was inserted in calendars between the "normal" fifth and sixth days. By legal fiction, the Romans treated both the first "sixth day" and the additional "sixth day" before the Kalends of March as one day. Thus a child born on either of those days in a leap year would have its first birthday on the following sixth day before the Kalends of March. In a leap year in the original Julian calendar, there were indeed two days both numbered 24 February. This practice continued for another fifteen to seventeen centuries, even after most countries had adopted the Gregorian calendar.[ citation needed ]

For legal purposes, the two days of the bis sextum were considered to be a single day, with the second sixth being intercalated; but in common practice by the year 238, when Censorinus wrote, the intercalary day was followed by the last five days of February, a. d. VI, V, IV, III and pridie Kal. Mart. (the days numbered 24, 25, 26, 27, and 28 from the beginning of February in a common year), so that the intercalated day was the first of the doubled pair. Thus the intercalated day was effectively inserted between the 23rd and 24th days of February. All later writers, including Macrobius about 430, Bede in 725, and other medieval computists (calculators of Easter), continued to state that the bissextum (bissextile day) occurred before the last five days of February.[ citation needed ]

In England, the Church and civil society continued the Roman practice whereby the leap day was simply not counted, so that a leap year was only reckoned as 365 days. Henry III's 1236 Statute De Anno et Die Bissextili [lower-alpha 1] instructed magistrates to treat the leap day and the day before as one day. [20] [19] The practical application of the rule is obscure. It was regarded as in force in the time of the famous lawyer Sir Edward Coke (1552–1634) because he cites it in his Institutes of the Lawes of England . However, Coke merely quotes the Act with a short translation and does not give practical examples.

... and by (b) the statute de anno bissextili, it is provided, quod computentur dies ille excrescens et dies proxime præcedens pro unico dii, so as in computation that day excrescent is not accounted. [21]

29 February

Replacement (by 29 February) of the awkward practice of having two days with the same date appears to have evolved by custom and practice; the etymological origin of the term "bissextile" seems to have been lost. [14] In England in the fifteenth century, "29 February" appears increasingly often in legal documents  although the records of the proceedings of the House of Commons of England continued to use the old system until the middle of the sixteenth century. [14] It was not until the passage of the Calendar (New Style) Act 1750 that 29 February was formally recognised in British law. [22] [lower-alpha 2]

Liturgical practices

In the older Roman Missal, feast days falling on or after 24 February are celebrated one day later in a leap year. MissaleLeapYear.jpg
In the older Roman Missal, feast days falling on or after 24 February are celebrated one day later in a leap year.

In the liturgical calendar of the Christian churches, the placement of the leap day is significant because of the date of the feast of Saint Matthias, which is defined as the sixth day before 1 March (counting inclusively). The Church of England's Book of Common Prayer was still using the "two days with the same date" system in its 1542 edition; [24] it first included a calendar which used entirely consecutive day counting from 1662 and showed leap day as falling on February 29. [25] In the 1680s, the Church of England declared 25 February to be the feast of St Matthias. [26] Until 1970, the Roman Catholic Church always celebrated the feast of Saint Matthias on a. d. VI Kal. Mart., so if the days were numbered from the beginning of the month, it was named 24 February in common years, but the presence of the bissextum in a bissextile year immediately before a. d. VI Kal. Mart. shifted the latter day to 25 February in leap years, with the Vigil of St. Matthias shifting from 23 February to the leap day of 24 February. This shift did not take place in pre-Reformation Norway and Iceland; Pope Alexander III ruled that either practice was lawful. [27] Other feasts normally falling on 25–28 February in common years are also shifted to the following day in a leap year (although they would be on the same day according to the Roman notation). The practice is still observed by those who use the older calendars.[ citation needed ]

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.

Folk traditions

A spinster eagerly awaits the upcoming leap day, in this 1903 cartoon by Bob Satterfield. Bob Satterfield cartoon about leap year traditions.jpg
A spinster eagerly awaits the upcoming leap day, in this 1903 cartoon by Bob Satterfield.

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.[ citation needed ] [lower-alpha 3] 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 ]

According to Felten: "A play from the turn of the 17th century, 'The Maydes Metamorphosis,' has it that 'this is leape year/women wear breeches.' A few hundred years later, breeches wouldn't do at all: Women looking to take advantage of their opportunity to pitch woo were expected to wear a scarlet petticoat   fair warning, if you will." [29]

In Finland, the tradition is that if a man refuses a woman's proposal on leap day, he should buy her the fabrics for a skirt. [30]

In France, since 1980, a satirical newspaper titled La Bougie du Sapeur is published only on leap year, on 29 February.[ citation needed ]

In Greece, marriage in a leap year is considered unlucky. [31] One in five engaged couples in Greece will plan to avoid getting married in a leap year. [32]

In February 1988 the town of Anthony, Texas, declared itself the "leap year capital of the world", and an international leapling birthday club was started. [33]

Birthdays

A person born on February 29 may be called a "leapling" or a "leaper". [34] 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.

For legal purposes, legal birthdays depend on how local laws count time intervals.

Taiwan

The Civil Code of Taiwan since 10 October 1929, [35] implies that the legal birthday of a leapling is 28 February in common years:

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. [36]

Hong Kong

Since 1990 non-retroactively, Hong Kong considers the legal birthday of a leapling 1 March in common years: [37]

  1. The time at which a person attains a particular age expressed in years shall be the commencement of the anniversary corresponding to the date of [their] birth.
  2. Where a person has been born on February 29 in a leap year, the relevant anniversary in any year other than a leap year shall be taken to be March 1.
  3. This section shall apply only where the relevant anniversary falls on a date after the date of commencement of this Ordinance.

Revised Julian calendar

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.

Baháʼí calendar

The Baháʼí calendar is a solar calendar composed of 19 months of 19 days each (361 days). Years begin at Naw-Rúz, 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.

Bengali, Indian and Thai calendars

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.

The Thai solar calendar uses the Buddhist Era (BE) but has been synchronised with the Gregorian since AD 1941.

Chinese calendar

The Chinese calendar is lunisolar, so a leap year has an extra month, often called an embolismic month after the Greek word for it. In the Chinese calendar, the leap month is added according to a rule which ensures that month 11 is always the month that contains the northern winter solstice. The intercalary month takes the same number as the preceding month; for example, if it follows the second month (二月) then it is simply called "leap second month" i.e. simplified Chinese :闰二月; traditional Chinese :閏二月; pinyin :rùn'èryuè.

Hebrew calendar

The Hebrew calendar is lunisolar with an embolismic month. This extra month is called Adar Rishon (first Adar) and is added before Adar , which then becomes Adar Sheini (second Adar). According to the Metonic cycle, this is done seven times every nineteen years (specifically, in years 3, 6, 8, 11, 14, 17, and 19). This is to ensure that Passover (Pesah) is always in the spring as required by the Torah (Pentateuch) in many verses [38] relating to Passover.

In addition, the Hebrew calendar has postponement rules that postpone the start of the year by one or two days. These postponement rules reduce the number of different combinations of year length and starting days of the week from 28 to 14, and regulate the location of certain religious holidays in relation to the Sabbath. In particular, the first day of the Hebrew year can never be Sunday, Wednesday or Friday. This rule is known in Hebrew as "lo adu rosh" (לא אד״ו ראש), i.e., "Rosh [ha-Shanah, first day of the year] is not Sunday, Wednesday or Friday" (as the Hebrew word adu is written by three Hebrew letters signifying Sunday, Wednesday and Friday). Accordingly, the first day of Passover is never Monday, Wednesday or Friday. This rule is known in Hebrew as "lo badu Pesah" (לא בד״ו פסח), which has a double meaning — "Passover is not a legend", but also "Passover is not Monday, Wednesday or Friday" (as the Hebrew word badu is written by three Hebrew letters signifying Monday, Wednesday and Friday).

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.

Years consisting of 12 months have between 353 and 355 days. In a k'sidra ("in order") 354-day year, months have alternating 30 and 29 day lengths. In a chaser ("lacking") year, the month of Kislev is reduced to 29 days. In a malei ("filled") year, the month of Marcheshvan is increased to 30 days. 13-month years follow the same pattern, with the addition of the 30-day Adar Alef, giving them between 383 and 385 days.

Islamic calendars

The observed and calculated versions of the lunar Islamic calendar do not have regular leap days, even though both have lunar months containing 29 or 30 days, generally in alternating order. However, the tabular Islamic calendar used by Islamic astronomers during the Middle Ages and still used by some Muslims does have a regular leap day added to the last month of the lunar year in 11 years of a 30-year cycle. [39] This additional day is found at the end of the last month, Dhu al-Hijjah, which is also the month of the Hajj. [40]

Jalaali Leap Year.svg

The Solar Hijri calendar is the modern Iranian calendar. It is an observational calendar that starts on the spring equinox (Northern Hemisphere) and adds a single intercalated day to the last month (Esfand) once every four or five years; the first leap year occurs as the fifth year of the typical 33-year cycle and the remaining leap years occur every four years through the remainder of the 33-year cycle. This system has less periodic deviation or jitter from its mean year than the Gregorian calendar and operates on the simple rule that New Year's Day must fall in the 24 hours of the vernal equinox. [41] The 33-year period is not completely regular; every so often the 33-year cycle will be broken by a cycle of 29 years. [42]

The Hijri-Shamsi calendar, also adopted by the Ahmadiyya Community, is based on solar calculations and is similar to the Gregorian calendar in its structure with the exception that its epoch is the Hijra. [43]

Coptic and Ethiopian calendars

The Coptic calendar has 13 months, 12 of 30 days each, and one at the end of the year of 5 days, or 6 days in leap years. The Coptic Leap Year follows the same rules as the Julian Calendar so that the extra month always has six days in the year before a Julian Leap Year. [44] The Ethiopian calendar has twelve months of thirty days plus five or six epagomenal days, which comprise a thirteenth month. [45]

See also

Notes

  1. Statute concerning [the] leap year and leap day
    The day of the leap year, and the day before, shall be holden for one day. [20]
  2. Though it appears in earlier Government proclamations, such as one of 1619. [23]
  3. Virtually no laws of Margaret survive. Indeed, none concerning her subjects are recorded in the twelve-volume Acts of the Parliaments of Scotland (1814–75) covering the period 1124–1707 (two laws concerning young Margaret herself are recorded on pages 424 & 441–2 of volume I).

Sources

Related Research Articles

Generally speaking, a calendar year begins on the New Year's Day of the given calendar system and ends on the day before the following New Year's Day, and thus consists of a whole number of days. A year can also be measured by starting on any other named day of the calendar, and ending on the day before this named day in the following year. This may be termed a "year's time", but not a "calendar year". To reconcile the calendar year with the astronomical cycle certain years contain extra days. The Gregorian year, which is in use in most of the world, begins on January 1 and ends on December 31. It has a length of 365 days in an ordinary year, with 8760 hours, 525,600 minutes, or 31,536,000 seconds; but 366 days in a leap year, with 8784 hours, 527,040 minutes, or 31,622,400 seconds. With 97 leap years every 400 years, the year has an average length of 365.2425 days. Other formula-based calendars can have lengths which are further out of step with the solar cycle: for example, the Julian calendar has an average length of 365.25 days, and the Hebrew calendar has an average length of 365.2468 days. The Lunar Hijri calendar is a lunar calendar consisting of 12 months in a year of 354 or 355 days. The astronomer's mean tropical year, which is averaged over equinoxes and solstices, is currently 365.24219 days, slightly shorter than the average length of the year in most calendars.

<span class="mw-page-title-main">Hebrew calendar</span> Lunisolar calendar used for Jewish religious observances

The Hebrew calendar, also called the Jewish calendar, is a lunisolar calendar used today for Jewish religious observance and as an official calendar of Israel. It determines the dates of Jewish holidays and other rituals, such as yahrzeits and the schedule of public Torah readings. In Israel, it is used for religious purposes, provides a time frame for agriculture, and is an official calendar for civil holidays alongside the Gregorian calendar.

Intercalation or embolism in timekeeping is the insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of days or months.

The Julian calendar is a solar calendar of 365 days in every year with an additional leap day every fourth year. The Julian calendar is still used as a religious calendar in parts of the Eastern Orthodox Church and in parts of Oriental Orthodoxy as well as by the Amazigh people.

<span class="mw-page-title-main">Lunisolar calendar</span> Calendar with lunar month, solar year

A lunisolar calendar is a calendar in many cultures, incorporating lunar calendars and solar calendars. The date of lunisolar calendars therefore indicates both the Moon phase and the time of the solar year, that is the position of the Sun in the Earth's sky. If the sidereal year is used instead of the solar year, then the calendar will predict the constellation near which the full moon may occur. As with all calendars which divide the year into months there is an additional requirement that the year have a whole number of months. In some case ordinary years consist of twelve months but every second or third year is an embolismic year, which adds a thirteenth intercalary, embolismic, or leap month.

A month is a unit of time, used with calendars, that is approximately as long as a natural orbital period of the Moon; the words month and Moon are cognates. The traditional concept of months arose with the cycle of Moon phases; such lunar months ("lunations") are synodic months and last approximately 29.53 days, making for roughly 12.37 such months in one Earth year. From excavated tally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as the Paleolithic age. Synodic months, based on the Moon's orbital period with respect to the Earth–Sun line, are still the basis of many calendars today and are used to divide the year.

<span class="mw-page-title-main">Roman calendar</span> Calendar used by the Roman Kingdom and Roman Republic

The Roman calendar was the calendar used by the Roman Kingdom and Roman Republic. Although the term is primarily used for Rome's pre-Julian calendars, it is often used inclusively of the Julian calendar established by the reforms of the Dictator Julius Caesar and Emperor Augustus in the late 1st century BC.

The Revised Julian calendar, or less formally the new calendar and also known as the Milanković calendar, is a calendar proposed in 1923 by the Serbian scientist Milutin Milanković as a more accurate alternative to both Julian and Gregorian calendars. At the time, the Julian calendar was still in use by all of the Eastern Orthodox Churches and affiliated nations, while the Catholic and Protestant nations were using the Gregorian calendar. Thus, Milanković's aim was to discontinue the divergence between the naming of dates in Eastern and Western churches and nations. It was intended to replace the Julian calendar in Eastern Orthodox Churches and nations. From 1 March 1600 through 28 February 2800, the Revised Julian calendar aligns its dates with the Gregorian calendar, which had been proclaimed in 1582 by Pope Gregory XIII.

<span class="mw-page-title-main">Year</span> Time of one planets orbit around a star

A year is the time taken for astronomical objects to complete one orbit. For example, a year on Earth is the time taken for Earth to revolve around the Sun. Generally, a year is taken to mean a calendar year, but the word is also used for periods loosely associated with the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc. The term can also be used in reference to any long period or cycle, such as the Great Year.

<span class="mw-page-title-main">Coptic calendar</span> Egyptian liturgical calendar

The Coptic calendar, also called the Alexandrian calendar, is a liturgical calendar used by the Coptic Orthodox Church and also used by the farming populace in Egypt. It was used for fiscal purposes in Egypt until the adoption of the Gregorian calendar on 11 September 1875. This calendar is based on the ancient Egyptian calendar. To avoid the calendar creep of the latter, a reform of the ancient Egyptian calendar was introduced at the time of Ptolemy III which consisted of adding an extra day every fourth year. However, this reform was opposed by the Egyptian priests, and the reform was not adopted until 25 BC, when the Roman Emperor Augustus imposed the Decree upon Egypt as its official calendar. To distinguish it from the Ancient Egyptian calendar, which remained in use by some astronomers until medieval times, this reformed calendar is known as the Coptic or Alexandrian calendar. Its years and months coincide with those of the Ethiopian calendar but have different numbers and names.

A solar calendar is a calendar whose dates indicate the season or almost equivalently the apparent position of the Sun relative to the stars. The Gregorian calendar, widely accepted as a standard in the world, is an example of a solar calendar. The main other types of calendar are lunar calendar and lunisolar calendar, whose months correspond to cycles of Moon phases. The months of the Gregorian calendar do not correspond to cycles of the Moon phase.

<span class="mw-page-title-main">Zoroastrian calendar</span> Religious date system

Adherents of Zoroastrianism use three distinct versions of traditional calendars for liturgical purposes, all derived from medieval Iranian calendars and ultimately based on the Babylonian calendar as used in the Achaemenid empire. Qadimi ("ancient") is a traditional reckoning introduced in 1006. Shahanshahi ("imperial") is a calendar reconstructed from the 10th century text Denkard.

The epact used to be described by medieval computists as the age of a phase of the Moon in days on 22 March; in the newer Gregorian calendar, however, the epact is reckoned as the age of the ecclesiastical moon on 1 January. Its principal use is in determining the date of Easter by computistical methods. It varies from year to year, because of the difference between the solar year of 365–366 days and the lunar year of 354–355 days.

Calendar reform or calendrical reform is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar design.

<span class="mw-page-title-main">Birkat Hachama</span> Jewish blessing, thanking God for creating the sun. Recited once every 28 years.

Birkat Hachama refers to a rare Jewish blessing that is recited to the Creator, thanking God for creating the sun. The blessing is recited when the Sun completes its cycle every 28 years on a Tuesday at sundown. Jewish tradition says that when the Sun completes this cycle, it has returned to its position when the world was created. Because the blessing needs to be said when the Sun is visible, the blessing is postponed to the following day, on Wednesday morning.

The Buddhist calendar is a set of lunisolar calendars primarily used in Tibet, Cambodia, Laos, Myanmar, India, Sri Lanka, Thailand and Vietnam as well as in Malaysia and Singapore and by Chinese populations for religious or official occasions. While the calendars share a common lineage, they also have minor but important variations such as intercalation schedules, month names and numbering, use of cycles, etc. In Thailand, the name Buddhist Era is a year numbering system shared by the traditional Thai lunar calendar and by the Thai solar calendar.

<span class="mw-page-title-main">March equinox</span> When sun appears directly over equator

The March equinox or northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. The March equinox is known as the vernal equinox in the Northern Hemisphere and as the autumnal equinox or fall equinox in the Southern Hemisphere.

The Burmese calendar is a lunisolar calendar in which the months are based on lunar months and years are based on sidereal years. The calendar is largely based on an older version of the Hindu calendar, though unlike the Indian systems, it employs a version of the Metonic cycle. The calendar therefore has to reconcile the sidereal years of the Hindu calendar with the Metonic cycle's near tropical years by adding intercalary months and days at irregular intervals.

The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull Inter gravissimas issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day 'tropical' or 'solar' year that is determined by the Earth's revolution around the Sun.

Nisan-years is an ancient calendar system used around Mesopotamia. Its beginning was from the prehistorical era. Ever since Mesopotamia had historical writings, even before the First Babylonian dynasty of Hammurabi, its calendar used the Nisan-years.

References

  1. Meeus, Jean (1998), Astronomical Algorithms, Willmann-Bell, p. 62
  2. Harper, Douglas (2012), "leap year", Online Etymology Dictionary, archived from the original on 21 August 2012, retrieved 15 August 2012
  3. "leap year", Oxford US Dictionary, archived from the original on 13 September 2015, retrieved 6 January 2020
  4. Astronomical almanac online glossary, US Naval Observatory, 2020, archived from the original on 23 February 2022, retrieved 28 January 2022
  5. Hogeback, Jonathon, "Why Are There Only 28 Days in February?", Encyclopedia Britannica , retrieved 31 May 2023
  6. Dershowitz, Nachum; Reingold, Edward M. (2008), Calendrical calculations (3rd ed.), Cambridge: Cambridge University Press, p. 45, ISBN   978-0-521-88540-9, OCLC   144768713, The calendar in use today in most of the world is the Gregorian or 'new-style' calendar designed by a commission assembled by Pope Gregory XIII in the sixteenth century.
  7. Lerner, Ed. K. Lee; Lerner, Brenda W. (2004), "Calendar", The Gale Encyclopedia of Science, Detroit, MI: Gale
  8. Introduction to Calendars Archived 2019-06-13 at the Wayback Machine . (10 August 2017). United States Naval Observatory.
  9. United States Naval Observatory (14 June 2011), Leap Years, archived from the original on 15 October 2007, retrieved 9 April 2014
  10. Lerner & Lerner 2004, p. 681.
  11. Richards, E. G. (2013), "Calendars", in Urban, S. E.; Seidelmann, P. K. (eds.), Explanatory Supplement to the Astronomical Almanac (3rd ed.), Mill Valley CA: University Science Books, p. 598, ISBN   9781891389856
  12. Doggett, L.E. (1992), "Calendars", in Seidelmann, P. K. (ed.), Explanatory Supplement to the Astronomical Almanac (2nd ed.), Sausalito, CA: University Science Books, pp. 580–1
  13. Richards, E. G. (1998), Mapping time: The Calendar and its History, Oxford University Press, pp. 250–1, ISBN   0-19-286205-7
  14. 1 2 3 Pollard (1940), p. 188.
  15. Cheney (2000), p 145, footnote 1.
  16. According to Christian Ludwig Ideler (1825)
  17. Key, Thomas Hewitt (2013) [1875], Calendarium, University of Chicago, the intermediate days are in all cases reckoned backward upon the Roman principle already explained of counting both extremes.
  18. Pollard (1940), p. 186.
  19. 1 2 Cheney (2000), Page 145, Footnote 1.
  20. 1 2 Ruffhead, Owen (1763), The Statutes at Large, from Magna Charta to the End of the Last Parliament, Mark Basket, p. 20, retrieved 21 October 2021 (21 Hen, III)
  21. Coke, Edward (1628). "Cap. 2, Of Villenage". First Part of the Institutes of the Lawes of England . p.  136 left (Sect. 202, line 3).
  22. Pickering, Danby, ed. (1765), The Statutes at Large: from the 23rd to the 26th Year of King George II, vol. 20, Cambridge: Charles Bathurst, p. 194, retrieved 28 January 2020 (calendar at the end of the Act)
  23. Bond, John James (1875), "Preface", Handy Book of Rules and Tables for Verifying Dates With the Christian Era Giving an Account of the Chief Eras and Systems Used by Various Nations...' (4th ed.), London: George Bell & Sons, p.  xix
  24. Campion, Rev W M; Beamont, Rev W J (1870), The Prayer Book interleaved, London: Rivingtons, p. 31 via Archive.org
  25. Church of England (1762) [1662], Book of Common Prayer, Cambridge: John Baskerville via Archive.org
  26. Cheney (2000), p. 8.
  27. Liber Extra, 5. 40. 14. 1
  28. Mikkelson, B.; Mikkelson, D.P. (2010), "The Privilege of Ladies", The Urban Legends Reference Pages, snopes.com
  29. Felten, E. (23 February 2008), "The Bissextile Beverage", Wall Street Journal, archived from the original on 12 August 2017, retrieved 12 August 2017
  30. Hallett, S. (29 February 2012), "Leap Year Proposal: What's The Story Behind It?", Huffington Post, archived from the original on 21 October 2014, retrieved 21 December 2015
  31. "A Greek Wedding", Anagnosis Books, archived from the original on 10 February 2012, retrieved 12 January 2012
  32. "Teaching Tips 63", Developing Teachers, archived from the original on 2 March 2012, retrieved 12 January 2012
  33. Anthony – Leap Year Capital of the World, Time and Date, 2008, archived from the original on 9 November 2011, retrieved 6 November 2011
  34. "29 February: 29 things you need to know about leap years and their extra day", Mirror, 28 February 2012, archived from the original on 2 January 2016, retrieved 7 December 2015
  35. Legislative History of the Civil Code of the Republic of China, archived from the original on 28 December 2016, retrieved 19 July 2011
  36. "Article 121 Civil Code", Part I General Principles of the Republic of China, archived from the original on 4 March 2021, retrieved 19 July 2011
  37. Age of Majority (Related Provisions) Ordinance (Ch. 410 Sec. 5), Hong Kong Department of Justice, 30 June 1997, archived from the original on 18 May 2013, retrieved 19 July 2011 (Enacted in 1990).
  38. Exodus 23:15, Exodus 34:18, Deuteronomy 15:1, Deuteronomy 15:13
  39. The Islamic leap year, Time and Date AS, n.d., archived from the original on 3 March 2020, retrieved 29 February 2012
  40. Leap year trivia you might want to know, GMA News, n.d., archived from the original on 15 May 2013, retrieved 29 February 2012
  41. Bromberg, Irv, Fixed Arithmetic Calendar Cycle Jitter, University of Toronto, archived from the original on 24 October 2019, retrieved 24 October 2019
  42. Heydari-Malayeri, M. (2004), A Concise Review of the Iranian Calendar, Paris Observatory, arXiv: astro-ph/0409620 , Bibcode:2004astro.ph..9620H, archived from the original on 16 July 2011, retrieved 19 December 2010
  43. Hijri-Shamsi Calendar, Al Islam, 2015, archived from the original on 26 January 2017, retrieved 18 April 2015, The time frame in these months is the same as [...] the months of a Christian calendar.
  44. Fr Tadros Y Malaty (1988), The Coptic Calendar and Church of Alexandria., The Monastery of St. Macarius Press, The Desert of Scete, archived from the original on 13 September 2022, retrieved 13 September 2022
  45. "Ethiopia: The country where a year lasts 13 months", BBC News, 10 September 2021, archived from the original on 18 September 2022, retrieved 17 September 2022