February 29 only appears every four years on the calendar because it is a leap year. This system was put in place to adjust the solar calendar from 365 days to the tropical year of 365.2422 days.
Our calendar is based on a year linked to the Earth's revolution around the Sun. This duration is actually about 365.2422 days and not exactly 365: inconvenient, right? If we didn't make some adjustments, our seasons would gradually shift, and Christmas would end up in the middle of summer in a few centuries. To correct this discrepancy, we add a bonus day every four years: the famous February 29. A simple and clever way to set the clocks right with the actual solar year.
The Earth revolves around the Sun in about 365.25 days. The problem is: our calendar normally counts exactly 365 days. This extra quarter of a day each year accumulates little by little. Without correction, after a while, you would celebrate Christmas in the middle of summer! To fix this, we add a day every four years—the famous February 29. This small adjustment helps to regularly realign the calendar with the astronomical reality of the seasons.
The current calendar, called the Gregorian calendar, has set simple rules: every 4 years, there is a leap year with an extra day, February 29. But unlucky are the years that end a century: only those divisible by 400 remain leap years. For example, 1900 did not have its February 29 (even though it is divisible by 4!), while 2000, which is divisible by 400, took the opportunity to be a leap year. This small adjustment helps avoid gradually shifting the seasons on our calendars, keeping it well aligned with the solar year.
Having an extra day every four years obviously has some rather strange practical effects. Already on the administrative side, certain software or computer systems are thrown off by this unusual date: the infamous February 29 bug. The same goes for birthdays: those born on February 29 often have to celebrate either on February 28 or March 1, except in a leap year! Some cultures also have particular traditions related to this date. For example, in several European countries, especially Ireland, February 29 is the only day when, traditionally, women can propose to their partners without being seen as eccentric. Finally, this date holds a somewhat special place in the collective imagination, often viewed as an opportunity to do things differently or to try something new.
The Julian calendar, created by Julius Caesar in 45 B.C., already used the principle of adding one day every four years, but it had an accumulated error that was corrected with the introduction of the Gregorian calendar in 1582.
The probability of being born on February 29 is approximately 1 in 1,461. Thus, the global population includes about 5 million people born on this particular date.
In Ireland, there is a humorous tradition that only on February 29th can women propose to their fiancé. This custom dates back to the 5th century and is said to be associated with Saint Brigid.
Some countries, like Sweden, have experienced chaotic periods of calendar adjustment. In 1712, to restore an accurate calendar, the Swedes even exceptionally included a 30th of February!
Most modern computer systems incorporate precise calendars that automatically recognize leap years. However, certain software errors (known as leap year bugs) can occur, particularly when programs use incorrect simplifications, leading to issues such as misdated entries or occasional computational errors.
When a person is born on February 29, their birthday does not occur every year. In non-leap years, they typically celebrate their birthday either on February 28 or March 1, depending on custom, personal preference, or the legislation of the country where they reside.
Yes, to align precisely with the actual solar year, certain years that are multiples of 100 (for example, 1900) are not leap years, unless they are also multiples of 400 (in which case the year is a leap year, as was the case in the year 2000). This helps the calendar stay in sync with the seasons over the long term.
Indeed, other calendar systems also have periodic adjustments to align the calendar with astronomical cycles. For example, the Islamic lunar calendar regularly adds extra months, while the Hebrew calendar combines lunar and solar cycles with a complex system of intercalary months to stay aligned with the seasons.
The term "leap year" comes from the Latin "bis sextus," which means "twice sixth." The Romans added an extra day after February 24, called the sixth day before the Kalends of March. This additional day was thus referred to as "bis sextus," which is the origin of the word "leap year."
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