|February 29 in recent years|
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.
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. In a leap year, February 29 is the 60th day of the Gregorian calendar, 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. It is the last day of February for leap years.
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, so 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.
Leap days can present a particular problem in computing known as the leap year bug when February 29 is not handled correctly in logic that accepts or manipulates dates. For example, this has happened with ATMsand Microsoft's cloud system Azure.
Although most modern calendar years have 365 days, a complete revolution around the Sun (one solar year) takes approximately 365 days, 5 hours, 48 minutes, and 46 seconds (or, for simplicity's sake, approximately 365 days and 6 hours, or 365.25 days).An extra 23 hours, 15 minutes, and 4 seconds thus accumulates every four years (again, for simplicity's sake, approximately an extra 24 hours, or 1 day, every four years), requiring that an extra calendar day be added to align the calendar with the Sun's apparent position. Without the added day, in future years the seasons would occur later in the calendar, eventually leading to confusion about when to undertake activities dependent on weather, ecology, or hours of daylight.
Solar years are actually slightly shorter than 365 days and 6 hours (365.25 days), which had been known since the 2nd century BC when Hipparchus stated that it lasted 365 + 1/ − 1/ days, but this was ignored by Julius Caesar and his astronomical adviser Sosigenes. The Gregorian calendar corrected this by adopting the length of the tropical year stated in three medieval sources, the Alfonsine tables, De Revolutionibus, and the Prutenic Tables, truncated to two sexagesimal places, 365 14/33/ days or 365 + 1/ − 3/ days or 365.2425 days. The length of the tropical year in 2000 was 365.24217 mean solar days, Adding a calendar day every four years, therefore, results in an excess of around 44 minutes every four years, or about 3 days every 400 years. To compensate for this, three days are removed every 400 years. The Gregorian calendar reform implements this adjustment by making an exception to the general rule that there is a leap year every four years. Instead, a year divisible by 100 is not a leap year unless that year is also divisible by 400. This means that the years 1600, 2000, and 2400 are leap years, while the years 1700, 1800, 1900, 2100, 2200, and 2300 are not leap years.
The Gregorian calendar repeats itself every 400 years, which is exactly 20,871 weeks including 97 leap days (146,097 days). Over this period, February 29 falls on Sunday, Tuesday, and Thursday 13 times; Friday and Saturday 14 times; and Monday and Wednesday 15 times. Except for a century mark that is not a multiple of 400, consecutive leap days fall in order Sunday, Friday, Wednesday, Monday, Saturday, Thursday, Tuesday, and repeats again.
The calendar of the Roman king Numa Pompilius had only 355 days (even though it was not a lunar calendar) which meant that it would quickly become unsynchronized with the solar year. An earlier Roman solution to this problem was to lengthen the calendar periodically by adding extra days to February, the last month of the year. February consisted of two parts, each with an odd number of days. The first part ended with the Terminalia on the 23rd, which was considered the end of the religious year, and the five remaining days formed the second part. To keep the calendar year roughly aligned with the solar year, a leap month, called Mensis Intercalaris ("intercalary month"), was added from time to time between these two parts of February. The (usual) second part of February was incorporated in the intercalary month as its last five days, with no change either in their dates or the festivals observed on them. This followed naturally, because the days after the Ides (13th) of February (in an ordinary year) or the Ides of Intercalaris (in an intercalary year) both counted down to the Kalends of March (i.e. they were known as "the nth day before the Kalends of March"). The Nones (5th) and Ides of Intercalaris occupied their normal positions.
The third-century writer Censorinus says:
When it was thought necessary to add (every two years) an intercalary month of 22 or 23 days, so that the civil year should correspond to the natural (solar) year, this intercalation was in preference made in February, between Terminalia [23rd] and Regifugium [24th].
The set leap day was introduced in Rome as a part of the Julian reform in the 1st century BCE. As before, the intercalation was made after February 23. The day following the Terminalia (February 23) was doubled, forming the "bis sextum"—literally 'twice sixth', since February 24 was 'the sixth day before the Kalends of March' using Roman inclusive counting (March 1 was the Kalends of March and was also the first day of the calendar year). Inclusive counting initially caused the Roman priests to add the extra day every three years instead of four; Augustus was compelled to omit leap years for a few decades to return the calendar to its proper position. Although there were exceptions, the first day of the bis sextum (February 24) was usually regarded as the intercalated or "bissextile" day since the 3rd century CE. [ citation needed ] although this has only been formally enacted in Sweden and Finland. In Britain, the extra day added to leap years remained notionally the 24th, until the Calendar (New Style) Act 1750 set it as 29 February.February 29 came to be regarded as the leap day when the Roman system of numbering days was replaced by sequential numbering in the late Middle Ages,
A person born on February 29 may be called a "leapling", a "leaper", or a "leap-year baby".Some leaplings celebrate their birthday in non-leap years on either February 28 or March 1, while others only observe birthdays on the authentic intercalary date, February 29.
The effective legal date of a leapling's birthday in non-leap years varies between jurisdictions.
In the United Kingdom and its former colony Hong Kong, when a person born on February 29 turns 18, they are considered to have their birthday on March 1 in the relevant year.
In New Zealand, a person born on February 29 is deemed to have their birthday on February 28 in non-leap years, for the purposes of Driver Licensing under §2(2) of the Land Transport (Driver Licensing) Rule 1999.The net result is that for drivers aged 75, or over 80, their driver licence expires at the end of the last day of February, even though their birthday would otherwise fall on the first day in March in non-leap years. Otherwise, New Zealand legislation is silent on when a person born on February 29 has their birthday, although case law would suggest that age is computed based on the number of years elapsed, from the day after the date of birth, and that the person's birth day then occurs on the last day of the year period. This differs from English common law where a birthday is considered to be the start of the next year, the preceding year ending at midnight on the day preceding the birthday. While a person attains the same age on the same day, it also means that, in New Zealand, if something must be done by the time a person attains a certain age, that thing can be done on the birthday that they attain that age and still be lawful.
In Taiwan, the legal birthday of a leapling is February 28 in common years:
Thus, in England and Wales or in Hong Kong, a person born on February 29 will have legally reached 18 years old on March 1. If they were born in Taiwan they legally become 18 on February 28, a day earlier.
In the United States, according to John Reitz, a professor of law at the University of Iowa, there is no "... statute or general rule that has anything to do with leap day."Reitz speculates that "March 1 would likely be considered the legal birthday in non-leap years of someone born on leap day," using the same reasoning as described for the United Kingdom and Hong Kong. However, for the purposes of Social Security, a person attains the next age the day before the anniversary of birth. Therefore, Social Security would recognize February 28 as the change in age for leap year births, not March 1.
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 only their leap-year birthdays.[ citation needed ]
A similar device is used in the plot of Gilbert and Sullivan's 1879 comic opera The Pirates of Penzance : as a child, Frederic was apprenticed to a band of pirates until his 21st birthday. Having passed his 21st year, he leaves the pirate band and falls in love. However, since he was born on February 29, his 21st birthday will not arrive until he is eighty-eight (since 1900 was not a leap year), so he must leave his fiancée and return to the pirates.
Since 1967, February 29 has been the official birthday of Superman, but not Clark Kent.
There is a popular tradition known as Bachelor's Day in some countries allowing a woman to propose marriage to a man on February 29.If the man refuses, he then is obliged to give the woman money or buy her a dress. In upper-class societies in Europe, if the man refuses marriage, he then must purchase 12 pairs of gloves for the woman, suggesting that the gloves are to hide the woman's embarrassment of not having an engagement ring. In Ireland, the tradition is supposed to originate from a deal that Saint Bridget struck with Saint Patrick.
In the town of Aurora, Illinois, single women are deputized and may arrest single men, subject to a four-dollar fine, every February 29.
In Greece, it is considered unlucky to marry on a leap day.
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 reconcilie the calendar year with the astronomical cycle certain years contain extra days.
The traditional Chinese calendar, is a lunisolar calendar which reckons years, months and days according to astronomical phenomena. In China it is defined by GB/T 33661–2017, "Calculation and promulgation of the Chinese calendar", issued by the Standardisation Administration of China on 12 May 2017.
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 both days and months.
The Julian calendar, proposed by Julius Caesar in AUC 708, was a reform of the Roman calendar. It took effect on 1 January AUC 709 , by edict. It was designed with the aid of Greek mathematicians and Greek astronomers such as Sosigenes of Alexandria.
A leap year is a calendar year that contains an additional day added to keep the calendar year synchronized with the astronomical year or seasonal year. 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 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.
A lunisolar calendar is a calendar in many cultures whose date indicates both the Moon phase and the time of the solar year. If the solar year is defined as a tropical year, then a lunisolar calendar will give an indication of the season; if it is taken as a sidereal 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 this 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.
The International Fixed Calendar is a solar calendar proposal for calendar reform designed by Moses B. Cotsworth, who presented it in 1902. It divides the solar year into 13 months of 28 days each. It is therefore a perennial calendar, with every date fixed to the same weekday every year. Though it was never officially adopted in any country, entrepreneur George Eastman adopted it for use in his Eastman Kodak Company, where it was used from 1928 to 1989. It is sometimes also called the 13-month calendar or the equal-month calendar, but there are multiple alternative calendar designs that these descriptive labels apply to as well.
The Roman calendar was the calendar used by the Roman kingdom and republic. The term often includes the Julian calendar established by the reforms of the dictator Julius Caesar and emperor Augustus in the late 1st century BC and sometimes includes any system dated by inclusive counting towards months' kalends, nones, and ides in the Roman manner. The term usually excludes the Alexandrian calendar of Roman Egypt, which continued the unique months of that land's former calendar; the Byzantine calendar of the later Roman Empire, which usually dated the Roman months in the simple count of the ancient Greek calendars; and the Gregorian calendar, which refined the Julian system to bring it into still closer alignment with the tropical year.
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 type of calendar is a lunar calendar, whose months correspond to cycles of Moon phases. The months of the Gregorian calendar do not correspond to cycles of the Moon phase.
The computus is a calculation that determines the calendar date of Easter. Easter is traditionally celebrated on the first Sunday after the Paschal full moon, which is the first full moon on or after 21 March. Determining this date in advance requires a correlation between the lunar months and the solar year, while also accounting for the month, date, and weekday of the calendar. The calculations produce different results depending on whether the Julian calendar or the Gregorian calendar is used.
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.
A leap week calendar is a calendar system with a whole number of weeks every year, and with every year starting on the same weekday. Most leap week calendars are proposed reforms to the civil calendar, in order to achieve a perennial calendar. Some, however, such as the ISO week date calendar, are simply conveniences for specific purposes.
The Buddhist calendar is a set of lunisolar calendars primarily used in mainland Southeast Asian countries of Cambodia, Laos, Myanmar and Thailand as well as in Sri Lanka and Chinese populations of Malaysia and Singapore 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 lunisolar calendar and by the Thai solar calendar.
The Symmetry454 calendar (Sym454) is a proposal for calendar reform created by Irv Bromberg of the University of Toronto, Canada. It is a perennial solar calendar that conserves the traditional month pattern and 7-day week, has symmetrical equal quarters in 82% of the years in its 293-year cycle, and starts every month on Monday.
The intercalary month or epagomenal days of the ancient Egyptian, Coptic, and Ethiopian calendars are a period of five days in common years and six days in leap years in addition to those calendars' 12 standard months, sometimes reckoned as their thirteenth month. They originated as a periodic measure to ensure that the heliacal rising of Sirius would occur in the 12th month of the Egyptian lunar calendar but became a regular feature of the civil calendar and its descendants. Coptic and Ethiopian leap days occur in the year preceding Gregorian leap years.
Bissext, or bissextus is the day which is added to the Gregorian calendar every fourth year to compensate for the six-hour difference in length between the common 365-day year and the actual length of the solar year.
The Gregorian calendar is the calendar used in most of the world. It was introduced in October 1582 by Pope Gregory XIII as a minor modification of the Julian calendar, reducing the average year from 365.25 days to 365.2425 days, thus correcting for the drift against the solar year that the inaccuracy had caused during the intervening centuries.
The Hanke–Henry Permanent Calendar (HHPC) is a proposal for calendar reform. It is one of many examples of leap week calendars, calendars which maintain synchronization with the solar year by intercalating entire weeks rather than single days. It is a modification of a previous proposal, Common-Civil-Calendar-and-Time (CCC&T). With the Hanke–Henry Permanent Calendar, every calendar date always falls on the same day of the week.
The Solar Hijri calendar, also called the Iranian Hijri calendar or Shamsi Hijri calendar, and abbreviated as SH, is the official calendar of Iran and Afghanistan. It begins on the March equinox (Nowruz) as determined by astronomical calculation for the Iran Standard Time meridian and has years of 365 or 366 days.
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