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Leap Day: why does February 29 appear every 4 years?

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February 29 is known as leap day because it only appears in leap years. (Freepik)

Every four years, an additional day is added to the Gregorian calendar. Its about February 29also know as leap daysince it appears in the years that carry this name.

The reason for this adjustment is related to the precise duration of a solar yearthat is, the time it takes for the Earth to complete its orbit around the Sun.

A solar year has, on average, a duration of 365 days, 5 hours, 46 minutes and 56 seconds. However, the Gregorian calendar states that a year has 365 days. This discrepancy of approximately a quarter of a day, while seemingly minor, would accumulate over time, causing a significant mismatch. For example, without the leap year adjustment, after 100 years, the calendar would be approximately 24 days ahead of the seasons.

To correct this deviation and ensure that festivals and seasons remain in sync with their corresponding astronomical periods, the concept of the leap-year. By adding an extra day, February 29, every four years, the aforementioned accumulation is compensated.

However, the leap year rule has exceptions for greater precision. Although an extra day is generally added every four years, there is a rule that excludes certain centenary years. If a year is a multiple of 100, it must also be divisible by 400 to be considered a leap year.. That is why the year 2000 was a leap year, but the year 1900 was not, despite being divisible by 4.

It takes the Earth 365.2422 days to complete one orbit around the Sun, which is why calendars adjust every four years worldwide – credit National Geographic

More than two millennia ago, in Ancient Rome, it was discovered that the calendar was not entirely synchronized with the solar year.

That is why Julius Caesar asked the astronomer Sosigenes to help him create an alternative to the Roman calendar that accurately represented the Earth’s rotation. This is how the Julian calendar emerged, named in honor of the person who proposed it.

The name leap comes from the Latin “ante diem bis sextum Kalendas Martias”, (sixth day before the calends of March in Spanish), that is, February 24. The phrase was summed up to “bis sextus”: leap year in Spanish.

However, due to some flaws in its structure, Pope Gregory XIII decided, through a papal bull, to “perfect” the calendar. This, called Gregorian calendarwas introduced in 1582 and lasts to this day.

One of the adjustments was that The additional day of leap years would be February 29 and not the 24th set by the Julian calendar.



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