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5 Things You May Not Know About Leap Day

February 20, 2020 in History

By Stephen Wood

The extra day tacked on to every fourth year is a subtle admission that even something as regular and simple as a calendar can be more complicated than we think.

Nearly every four years, we add an extra day to the calendar in the form of February 29, also known as Leap Day. Put simply, these additional 24 hours are built into the calendar to ensure that it stays in line with the Earth’s movement around the Sun. While the modern calendar contains 365 days, the actual time it takes for Earth to orbit its star is slightly longer—roughly 365.2421 days. The difference might seem negligible, but over decades and centuries that missing quarter of a day per year can add up. To ensure consistency with the true astronomical year, it is necessary to periodically add in an extra day to make up the lost time and get the calendar back in synch with the heavens.

1. Many ancient calendars had entire leap months

Many calendars, including the Hebrew, Chinese and Buddhist calendars, are lunisolar, meaning their dates indicate the position of the Moon as well as the position of Earth relative to the sun. Since there is a natural gap of roughly 11 days between a year as measured by lunar cycles and one measured by the Earth’s orbit, such calendars periodically require the addition of extra months, known as intercalary or interstitial months, to keep them on track.

Intercalary months, however, were not necessarily regular. Historians are still unclear as to how the early Romans kept track of their years, mostly because the Romans themselves may not have been entirely sure. It appears that the early Roman calendar consisted of ten months plus an ill-defined winter period, the varying length of which caused the calendar to become unpegged from the solar year. Eventually, this uncertain stretch of time was replaced by the new months of January and February, but the situation remained complicated. They employed a 23-day intercalary month known as Mercedonius to account for the difference between their year and the solar year, inserting it not between months but within the month of February for reasons that may have been related to lunar cycles.

To make matters even more confusing, the decision of when to hold Mercedonius often fell to the consuls, who used their ability to shorten or extend the year to their own political ends. As …read more

Source: HISTORY

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