Leap Year: Who Chose February 29 As The Leap Day?

2024 is a Leap Year

In the year 2024, there is February 29 which is considered the leap day every leap year, but who really decided or chose this day?

Some may think that it is because it would be odd to have January 0 or December 32, and it is more appealing in the calendar to just add another day after February 28. However, the 29th of February every four years has something to do with timekeeping, astronomy, and the evolving attempts to align the two through mathematics, based on the article in TIME.

In an effort to ensure that lunar and solar schedules remain compatible and consistent as seasons come and go, it was said that intercalation, or the insertion of days in a calendar, has undergone various trials and tests across civilizations.

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Across cultures, this practice has been varying. The Egyptian calendar year featured 12 30-day months, with five epagomena (days) appended at every year’s end. 

In the Chinese calendar, lunisolar timekeeping is being observed. It means that an extra month is added every three years. This allows those years to celebrate two spring months to welcome in the new year or “double spring.

This is also the same in Vikrami and Hebrew calendars. A month is added once every three years or so, following the moon’s 19-year cycle of phases. Islam believes in a lunar calendar that has a 30-year cycle in which 11 of those years have an extra day added to the month.

February 29 as the leap day can be traced back to ancient Rome. Romulus, the first king of Rome, had the Roman Republican calendar around 738 B.C. Back then, the year started with Martius (now called March). It was just composed of 10 months, not including the winter because people were not working at that time.

By the 7th century B.C., Numa Pompilius, the second Roman king, as he was frustrated with the irregularities, decided that it was time to start formally counting winter months. With that, Ianuarius (January) and Februarius (February) were added. However, even though the additions were made, still it was not working.

This led them to add 27- or 28-day 13th month. It was called Mercedonius, or sometimes called Intercalaris, to shift their measure of time back in sync with the sun. They would insert the extra month after Feb. 23. That cut short February by five days. This was made so that they would immediately follow the celebration of Terminalia, an annual festival on Feb. 23 that honors the ancient Roman god of boundaries Terminus.

During the reign of Julius Caesar, he ordered to establish the new solar calendar. It was created with the help of Greek astronomer Sosigenes, an adviser to Egypt’s Cleopatra. This took effect in the year 45 B.C. after a corrective 445-day ultimus annus confusionis (“last year of confusion”). They based this on the math that a year should consist of exactly 365 days and 6 hours. It also started the belief that every four 365-day years those extra six hours would total one extra day.

Back then, the intercalary day was added after Feb. 23. It was done by extending Feb. 24 to 48 hours. The Julian calendar also started the January 1 as the year’s beginning. However, even though a leap day was added every four years, the solar year was still short by 11 minutes annually. This caused the 10-day difference from the actual solar cycle by the 16th century.

Then, Gregory XIII introduced a new calendar in the 1570s which was called the Gregorian calendar and this is being used today. This adjusted the every-four-years rule for leap years to exclude centurials (i.e., 1700, 1800, 1900…) except those divisible by 400 (i.e., 1600, 2000, 2400). Back then, Catholics were repeating the Feb. 24 rather than adding a new day.

February 29 was made the quadrennial intercalary day through the Calendar (New Style) Act in 1752 and this also January 1 as the new year.

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