Leap day makes it a bisextile year

Bust of Julius Caesar

Bust of Julius Caesar from Wikipedia

Tomorrow 29 February 2008 is an extra day inserted into our calendar. Years when this occurs are known popularly known as leap years, but technically known as bisextile years. Why? And why is the extra day in February instead, say, at the end of December?

Let’s first look at why Julius Caesar (pictured above) introduced leap years every four years way back 46 years before our era. The Earth takes approximately 365.25 days to circle the Sun. The simplest way of approximating this is to have a cycle of three years of 365 days and one year of 366 days. In reality, the Earth takes 365 days 5 hours 48 minutes 45.2 seconds to circle Sun and so the Julian Calendar had an error of over 11 minutes each year. Initially, this error was too small to concern anyone, but over a long period this error accumulated and led to a modification of the calendar in 1582. However, that is another story….

The extra day each four years was placed in February as in the previous Roman calendar a whole extra month called Mercedonius was inserted into that month. It was inserted after the Feast of Terminalia which was held on what would call 23 February. Julius Caesar followed the tradition and made his extra day a repeat or extension of 24 February. The Romans though counted backwards and inclusively so to them that day was not the 24th, but the sixth day before the first day or Kalends of March (they would say, in a phrase that must have just tripped off the tongue: ante diem bis VI Kal. Mar where “bis” means “twice”). Hence the technical name for a leap year is bisextile year.

Enjoy the extra day. If you are single and female go ahead and pop the question!

One response to “Leap day makes it a bisextile year

  • I am in constant awe at the ancients’ understanding of ‘hands on’ celestial mechanics. These subtleties must have taken centuries of observation to discern.

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