Monday, February 29, 2016

February 29th -- "Leap Day"

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 2008, 2012, 2016, 2020, and 2024. A leap day is added in various solar calendars (calendars based on the Earth's rotation around the Sun), including the Gregorian calendar standard in most the world. Lunisolar calendars (calendars based on the rotation 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, 2100, 2200, and 2300 will not contain a leap day, while 1600 and 2000 did, and 2400 will. Years containing a leap day are called leap years. February 29 is the 60th day of the Gregorian calendar in such a year, with 306 days remaining until the end of the year. In the Chinese calendar, this day will only occur in years of the monkey, dragon, and rat.

A leap day is observed because a complete revolution around the Sun takes approximately 6 hours longer than 365 days (8,760 hours). It compensates for this lag, realigning the calendar with the Earth's position in the Solar System; otherwise, seasons would occur in a different time than intended in the calendar year.

Leap Years

Although most modern calendar years have 365 days, a complete revolution around the Sun (one solar year) takes approximately 365 days and 6 hours. An extra 24 hours thus accumulates 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.

A solar year is actually slightly shorter than 365 days and 6 hours (365.25 days). As early as the 13th century it was recognized that the year is shorter than the 365.25 days assumed by the Julian calendar: the Earth's orbital period around the Sun was derived from the medieval Alfonsine tables as 365 days, 5 hours, 49 minutes, and 16 seconds (365.2425 days). The currently accepted modern figure is 365 days, 5 hours, 48 minutes, 45 seconds. Adding a calendar day every four years, therefore, results in an excess of around 44 minutes for those 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 was also exactly divisible by 400. This means that the years 1600, 2000, and 2400 are leap years, while the years 1700, 1800, 1900, 2100, 2200, 2300, and 2500 are not leap years.

Leap Second

The concepts of the leap year and leap day are distinct from the leap second, which results from changes in the Earth's rotational speed. But the basic problem is the same: the quotient of the larger measure of time by the smaller is a non-integer. There is no way to perfectly fit a whole number of days/months into a year, nor is there a way to perfectly fit a whole number of seconds into a day. Leap seconds and leap years are used to correct the resulting drift.


A person who is born on February 29 may be called a "leapling" or a "leap-year baby". In non-leap years, some leaplings celebrate their birthday on either February 28 or March 1, while others only observe birthdays on the authentic intercalary date, February 29.

Folk Traditions

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.

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