Monday, February 25, 2008

Happy Intercalary Year

From the desk of Lyn Perry...

February 24, 2008
Happy Intercalary Year!
You'll know what I mean on the 29th of this month.

Most of the world operates according to the Gregorian Calendar, which was designed to keep the vernal equinox on or close to March 21 (an Easter thing, for those who are curious). The vernal equinox year is about 365.242374 days long (and increasing). Obviously, this astronomical occurrence does not repeat at an exact number of full days. Therefore, our calendar (which is made up of an exact number of full days, go figure!) lags behind over time with respect to the vernal equinox. By occasionally inserting (or intercalating) an additional day into the year, this drift can be corrected. This is done every four years (called a leap year or intercalary year) - and has been since 45 BC, when the concept was introduced by Julius Caesar.

The Julian Calendar, however, was slightly too long - it calculated the year at 365.25 days and so over hundreds of years too many leap years were celebrated. In 1582, Pope Gregory XIII, decreed that the Julian Calendar be modified to correct this overcompensation. And at a stoke of a papal bull (see the graphic above of the Inter Gravissimas) 10 'extra' days disappeared! We still celebrate leap year every fourth year (in calendar years divisible by 4 - like 2008!), but now with some exceptions. Because the Gregorian Calendar has 365.2425 days (still a tad long relative to the vernal equinox) we have to skip the leap year every 100 years - except when the century year is exactly divisible by 400 (thus 1900 was not a leap year, but 2000 was).

Now if you can follow all that, you can surely enjoy the new stories at ResAliens - which have nothing to do with leap year. But Happy Intercalary Year anway, Lyn Perry
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