A leap year (or intercalary year) is a year containing an extra day, week or month in order to keep the calendar year synchronised with the astronomical or seasonal year. For example, February would have 29 days instead of just 28. Seasons and astronomical events do not repeat at an exact number of days, so a calendar which had the same number of days in each year would over time drift with respect to the event it was supposed to track. By occasionally inserting (or intercalating) an additional day or month into the year, the drift can be corrected. A year which is not a leap year is called a common year. According to the Gregorian calendar, the calendar year begins on January 1 and ends on December 31. ... A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... The seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, or the flowering of a species of plant. ... Intercalation is the insertioffn of an extra day, week or month into some calendar years to make the calendar follow the seasons. ... A common year is a calendar year of exactly 365 days and so is not a leap year. ...

Leap years (which keep the calendar in sync with the year) should not be confused with leap seconds (which keep clock time in sync with the day). A leap second is a one-second adjustment to civil time in order to keep it close to the mean solar time. ...

Gregorian calendar

The Gregorian calendar, the current standard calendar in most of the world, adds a 29th day to February in all years evenly divisible by 4, except for centennial years (those ending in -00), which receive the extra day only if they are evenly divisible by 400. Thus 1600, 2000 and 2400 are leap years but 1700, 1800, 1900 and 2100 are not. The Gregorian calendar is the calendar that is used nearly everywhere in the world. ... February is the second month of the year in the Gregorian Calendar. ...

The reasoning behind this rule is as follows:

• The Gregorian calendar is designed to keep the vernal equinox on or close to March 21, so that the date of Easter (celebrated on the Sunday after the 14th day of the Moon that falls on or after 21 March) remains correct with respect to the vernal equinox.
• The vernal equinox year is currently about 365.242375 days long.
• The Gregorian leap year rule gives an average year length of 365.2425 days.

This difference of a little over 0.0001 days means that in around 8,000 years, the calendar will be about one day behind where it should be. But in 8,000 years' time the length of the vernal equinox year will have changed by an amount which can't be accurately predicted (see below). So the Gregorian leap year rule does a good enough job. Illumination of Earth by Sun on the day of equinox The vernal equinox (or spring equinox) marks the beginning of astronomical spring. ... March 21 is the 80th day of the year in the Gregorian Calendar (81st in leap years). ... This article is about the Christian festival. ... March 21 is the 80th day of the year in the Gregorian Calendar (81st in leap years). ...

This graph shows the variation between the seasonal year versus the calendar year due to unequally spaced 'leap days' rules. See Iranian calendar to contrast with a calendar based on 8 leap days every 33 years.

Seasonal error of Gregorian calendar Originally created by Tom Ruen, 2003, using MSExcel graph, labeled with MSPaint Converted from Image:Gregoriancalendarleap. ... The Iranian calendar (also known as Persian calendar or the Jalaali Calendar) is a solar calendar currently used in Iran and Afghanistan. ...

Rules for determining when to have a leap year

In order to get a closer approximation, it was decided to have a leap day 97 years out of 400 rather than once every 4 years. To implement the model, it was provided that years divisible by 100 would be leap years only if they were divisible by 400 as well. So, in the last millennium, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. In this millennium, 2100, 2200, 2300 and 2500 will not be leap years, but 2400 will be. The years that are divisible by 100 but not 400 are known as "exceptional common years".

Which day is the leap day?

The Gregorian calendar is a modification of the Julian calendar first used by the Romans. The Roman calendar originated as a lunar calendar (though from the 5th century BC it no longer followed the real moon) and named its days after three of the phases of the moon: the new moon (calends, hence "calendar"), the first quarter (nones) and the full moon (ides). Days were counted down (inclusively) to the next named day, so 24 February was ante diem sextum calendas martii ("the sixth day before the calends of March"). The Roman calendar changed its form several times in the time between the foundation of Rome and the fall of the Roman Empire. ... It has been suggested that lunar year be merged into this article or section. ... (2nd millennium BC - 1st millennium BC - 1st millennium) The 5th century BC started on January 1, 500 BC and ended on December 31, 401 BC. // Overview The Parthenon of Athens seen from the hill of the Pnyx to the west. ... February 24 is the 55th day of the year in the Gregorian Calendar. ...

Since 45 BC, February in a leap year had two days called "the sixth day before the calends of March". The extra day was originally the second of these, but since the third century it was the first. Hence the term bissextile day for 24 February in a bissextile year. Centuries: 2nd century BC - 1st century BC - 1st century Decades: 90s BC 80s BC 70s BC 60s BC 50s BC - 40s BC - 30s BC 20s BC 10s BC 0s BC 0s Years: 50 BC 49 BC 48 BC 47 BC 46 BC 45 BC 44 BC 43 BC 42 BC... (2nd century - 3rd century - 4th century - other centuries) Events The Sassanid dynasty of Persia launches a war to reconquer lost lands in the Roman east. ... February 24 is the 55th day of the year in the Gregorian Calendar. ...

Where this custom is followed, anniversaries after the inserted day are moved in leap years. For example, the former feast day of Saint Matthias, 24 February in ordinary years, would be 25 February in leap years. Saint Matthias is the Apostle chosen by the remaining eleven apostles to replace Judas Iscariot, following Judas betrayal of Jesus and suicide (Acts 1:21 - 26). ... February 24 is the 55th day of the year in the Gregorian Calendar. ... February 25 is the 56th day of the year in the Gregorian Calendar. ...

This historical nicety is, however, in the process of being discarded: The European Union declared that, starting in 2000, 29 February rather than 24 February would be leap day, and the Roman Catholic Church also now uses 29 February as leap day^{[citation needed]}. The only tangible difference is felt in countries that celebrate feast days. February 29 is the 60th day of a leap year in the Gregorian Calendar, with 306 days remaining. ... February 24 is the 55th day of the year in the Gregorian Calendar. ... Catholic Church redirects here. ... February 29 is the 60th day of a leap year in the Gregorian Calendar, with 306 days remaining. ... The calendar of saints is a traditional Christian method of organising a liturgical year on the level of days by associating each day with a saint, and referring to the day as the saints day of that saint. ...

Julian calendar

The Julian calendar adds an extra day to February in years divisible by 4. The Julian calendar was introduced in 46 BC by Julius Caesar and took force in 45 BC (709 ab urbe condita). ...

This rule gives an average year length of 365.25 days. The excess of about 0.0076 days with respect to the vernal equinox year means that the vernal equinox moves a day earlier in the calendar every 130 years or so. A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ...

Revised Julian Calendar

The Revised Julian calendar adds an extra day to February in years divisible by 4, except for years divisible by 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 the 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. The Revised Julian calendar is a calendar that was considered for adoption by the Eastern Orthodox churches at a synod in Istanbul in May 1923. ...

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 tropical year is slightly longer, the Revised Julian calendar does not do as good a job as the Gregorian calendar of keeping the vernal equinox on or close to 21 March. A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ... March 21 is the 80th day of the year in the Gregorian Calendar (81st in leap years). ...

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 complicated 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 then it is simply called "leap second month". A lunisolar calendar is a calendar whose date indicates both the moon phase and the time of the solar year. ... The Chinese calendar (Traditional Chinese: 農曆; Simplified Chinese: 农历; pinyin: nónglì) is a lunisolar calendar incorporating elements of a lunar calendar with those of a solar calendar. ... Intercalation is the insertion of an extra day or month into some calendar years to make the calendar follow the seasons. ... Diagram of the Earths seasons Solstice is an astronomical term regarding the position of the Sun in relation to the earths equator. ...

Hebrew calendar

The Hebrew calendar is also lunisolar with an embolistic month. In the Hebrew calendar the extra month is called Adar Alef (first Adar) and is added before Adar, which then becomes Adar bet (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. A lunisolar calendar is a calendar whose date indicates both the moon phase and the time of the solar year. ... The Hebrew calendar (Hebrew: ) or Jewish calendar is the annual calendar used in Judaism. ... Adar (אֲדָר, Standard Hebrew Adar, Tiberian Hebrew ʾĂḏār: from Akkadian adaru) is the sixth month of the ecclesiastical year and the twelfth month of the civil year on the Hebrew calendar. ... Adar (אֲדָר, Standard Hebrew Adar, Tiberian Hebrew ʾĂḏār: from Akkadian adaru) is the sixth month of the religious year and the twelfth month of the civil year on the Hebrew calendar. ... Adar (אֲדָר, Standard Hebrew Adar, Tiberian Hebrew ʾĂḏār: from Akkadian adaru) is the sixth month of the ecclesiastical year and the twelfth month of the civil year on the Hebrew calendar. ... The Metonic cycle or Enneadecaeteris in astronomy and calendar studies is a particular approximate common multiple of the year (specifically, the seasonal tropical year) and the synodic month. ...

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 day 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. Accordingly, the first day of Pesah is never Monday, Wednesday or Friday. This rule is known in Hebrew as "lo badu Pesah", which has a double meaning - "Pesah is not a legend", but also "Pesah is not Monday, Wednesday or Friday" (as the Hebrew word badu is written by three Hebrew letters signifying Monday, Wednesday and Friday). This artyicle concerns the Sabbath in Christianity. ... Passover, also known as Pesach or Pesah (פסח pesaḥ), is a Jewish holiday (lasting seven days in Israel and among some liberal Diaspora Jews, and eight days among other Diaspora Jews) that commemorates the exodus and freedom of the Israelites from Egypt; it is also observed by some Christians to...

One reason for this rule is that Yom Kippur, the holiest day in the Hebrew calendar, 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 (Yom Kippur can be on Saturday, however). Yom Kippur (יום כיפור yom kippūr) is the Jewish holiday of the Day of Atonement. ... This artyicle concerns the Sabbath in Christianity. ...

Hindu Calendar

In the Hindu calendar, which is a lunisolar calendar, the embolismic month is called adhika maas (extra month). It is the month in which the sun is in the same sign of the stellar zodiac on two consecutive dark moons. The Hindu calendar used in Vedic times has undergone many changes in the process of regionalization, and today there are several regional Indian calendars, as well as an Indian national calendar. ... A lunisolar calendar is a calendar whose date indicates both the moon phase and the time of the solar year. ...

Iranian calendar

The Iranian calendar also has a single intercalated day once in every four years, but every 33 years or so the leap years will be five years apart instead of four years apart. The system used is more accurate and more complicated, and is based on the time of the March equinox as observed from Teheran. The 33-year period is not completely regular; every so often the 33-year cycle will be broken by a cycle of 29 or 37 years. The Iranian calendar (also known as Persian calendar or the Jalaali Calendar) is a solar calendar currently used in Iran and Afghanistan. ... Tehran (also spelled Teheran) (تهران in Persian), population 8,000,000 (metropolitan: 10,000,000), is the capital of Iran and one of the major world cities. ...

Long term leap year rules

The accumulated difference between the Gregorian calendar and the vernal equinoctial year amounts to 1 day in about 8,000 years. This suggests that the calendar needs to be improved by another refinement to the leap year rule: perhaps by avoiding leap years in years divisible by 8,000.

(The most common such proposal is to avoid leap years in years divisible by 4,000 [1]. This is based on the difference between the Gregorian calendar and the mean tropical year. Others claim, erroneously, that the Gregorian calendar itself already contains a refinement of this kind [2].)

Hypothetical 128 year based leap years has been proposed, and it can be adopted directly without any modification to current leap year calculations until the year 2048. Centuries: 20th century - 21st century - 22nd century Decades: 1990s 2000s 2010s 2020s 2030s - 2040s - 2050s 2060s 2070s 2080s 2090s Years: 2043 2044 2045 2046 2047 - 2048 -2049 The year 2048 (MMXLVIII) in the Gregorian Calendar corresponds to 5808-5809 in the Hebrew Calendar. ...

However, there is little point in planning a calendar so far ahead because over a timescale of tens of thousands of years the number of days in a year will change for a number of reasons, most notably:

1. Precession of the equinoxes moves the position of the vernal equinox with respect to perihelion and so changes the length of the vernal equinoctial year.
2. Tidal acceleration from the sun and moon slows the rotation of the earth, making the day longer.

In particular, the second component of change depends on such things as post-glacial rebound and sea level rise due to climate change. We can't predict these changes accurately enough to be able to make a calendar that will be accurate to a day in tens of thousands of years. Precession of the equinoxes refers to the precession of the Earths axis of rotation. ... This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ... It has been suggested that Tidal friction be merged into this article or section. ... Changes in the elevation of Lake Superior due to glaciation and post-glacial rebound Post-glacial rebound (sometimes called continental rebound, isostatic rebound or isostatic adjustment) is the rise of land masses that were depressed by the huge weight of ice sheets during the last ice age, through a process... Measurement of recent sea level rise from 23 long tide gauge records in geologically stable environments Changes in sea level since the end of the last glacial episode Sea level rise is an increase in sea level. ... Variations in CO2, temperature and dust from the Vostok ice core over the last 400 000 years Climate change refers to the variation in the Earths global climate or regional climates over time. ...

Marriage proposal

There is a tradition, said to go back to Saint Patrick and Saint Bridget in 5th century Ireland, whereby women may make marriage proposals only in leap years. The word tradition, comes from the Latin word traditio which means to hand down or to hand over. ... Saint Patrick Saint Patrick (386–March 17, 493, see below) was a missionary and is regarded as the patron saint of Ireland (along with Saint Brigid and Saint Columba). ... Saint Brigid of Ireland Saint Brigid of Ireland (Bridget, Bridgit, Brigit; in English St. ... Europe in 450 The 5th century is the period from 401 - 500 in accordance with the Julian calendar in the Christian Era. ...

Saint Patrick, having driven the snakes out of the bogs was walking along the shores of Lough Neagh, when he was accosted by Saint Bridget in tears, and was told that a mutiny had broken out in the nunnery over which she presided, the ladies claiming the right of proposing for marriage.

Saint Patrick said he would concede them the right every seventh year, when Saint Bridget threw her arms round his neck, and exclaimed, "Arrah, Pathrick, jewel, I daurn't go back to the girls wid such a proposal. Make it one year in four." Saint Patrick replied, "Bridget, acushla, squeeze me that way again, an' I'll give ye leap-year, the longest of the lot." Saint Bridget, upon this, popped the question to St Patrick himself, who, of course, could not marry: so he patched up the difficulty as best he could with a kiss and a silk gown.^[1]

According to a 1288 law by Queen Margaret of Scotland (also known as the Maid of Norway because she never set foot in Scotland), fines were levied if the proposal was refused by the man; compensation ranged from a kiss to a silk gown to soften the blow. Because men felt that put them at too great a risk, the tradition was in some places tightened to restricting female proposals to 29 February. Others regard this as an urban legend.^[2] Events February 22 - Nicholas IV becomes Pope. ... Margaret, Maid of Norway (1283—1290), was Queen of Scotland (1286-1290). ... February 29 is the 60th day of a leap year in the Gregorian Calendar, with 306 days remaining. ...

Birthdays

A person who was born on 29 February may be called a "leapling". In non-leap years they usually celebrate their birthday on 28 February or 1 March. February 29 is the 60th day of a leap year in the Gregorian Calendar, with 306 days remaining. ... Leaplings are people whose birthdays fall on February 29, a date occurring only during a Leap Year. ... February 28 is the 59th day of the year in the Gregorian calendar. ... March 1 is the 60th day of the year in the Gregorian calendar (61st in leap years). ...

There are many instances in children's literature where a person's claim to be only a quarter of their actual age turns out to be based on counting their leap-year birthdays. A similar device is used in the plot of the Gilbert and Sullivan operetta The Pirates of Penzance. Librettist W. S. Gilbert (1836–1911) and composer Arthur Sullivan (1842–1900) collaborated on a series of fourteen comic operas in Victorian England between 1871 and 1896. ... Operetta (literally, little opera) is a performance art-form similar to opera, though it generally deals with less serious topics. ... Poster announcing the copyright performance at the Bijou Theatre, Paignton The Pirates of Penzance, or The Slave of Duty, is a comic opera in two acts, with music by Arthur Sullivan and libretto by W. S. Gilbert. ...

References

1. ^ Ivor H. Evans, Brewer's Dictionary of Phrase and Fable, Cassell, London, 1988
2. ^ The Privilege of Ladies by Barbara Mikkelson

See also

• leap second
• Intercalation
• Leap week calendar