Holy Leap Year Batman!

As I mentioned in the academic challanege post this morning, today is Leap Day, and every 4 years when we get one of these it resurrects the age-old challenge of how we really should be keeping time on this happy planet we call home.  Julius Caesar recognized the need for a leap year every 4 years when he promulgated the Julian Calendar.  The Earth orbits the sun in slightly more than 365 days.  He estimated it to be about 365.25 days, thus the need for a leap year every 4 years. 

As a quick aside, everyone probably knows that the Romans were real jerks in the never-ending quest for world conquest (most people seeking world conquest usually are).  However, as mean and nasty as they were, they were so darn smart!  Caesar was just one example.  Think about all of the innovations that were years ahead of their time:

-Leap Years
-Road systems with mileposts (the Via Appia)
-Aqueducts for water supply
-Architectural advancements (the Colosseum)

From abcnews.go.com

In any event, Caesar recognized that if we did not add one extra day to the calendar each year, the seasons would eventually get out of phase with the times of the year where we are used to them.  For hundreds of years, the Julian Calendar based on a 365.25 day year was the norm.  However, even the Romans were not THAT precise.  By the time of the 16th Century, the spring equinox was moving earlier and earlier into March.  This matters because the Catholic Church uses the spring equinox in its calculation of Easter.  Why did this happen?  It turns out that Caesar's calculation of the 365.25 day year was slightly too long.  Instead, by the time of Pope Gregory XIII in 1582, it was determined that an Earth year was only 365.2425 days.  As a result, the pope declared that the the calendar would advance by 12 days (the Gregorian) to account catch up for the slightly slower movement of the Earth than once thought.  Yes, back in those days, the pope had the power to control the calendar.  Today, this is accounted for by the years divisible by 100 (1700, 1800, and 1900) NOT being leap years, even though they are also divisible by 4.  Years divisible by 400 (1600, 2000) remain being leap years in the Gregorian Calendar.  As a result, every 100 years, the Gregorian Calendar gets one day further in advance of the Julian Calendar (except in years divisible by 400) because in the Julian Calendar, all years divisible by 4 are leap years, so 3 times every 400 years, the Julian Calendar slows down by one day. 

Make sense?

If you think that is crazy enough, remember that we can now measure time accurately to the fractions of a millisecond, thanks to atomic clocks.  Scientists can now accurately measure not only the orbit of the Earth, but also its rotation on its axis.  The second used to be defined based on Earth's axial rotation, but along with the wave of redefinitions of SI units back in the 70s, the second was defined based on the oscillation of Cesium-133 atoms.  Because of this redefinition, a 24 hour period based on cesium is slightlty longer than than the solar day.  As a result, every so often, leap seconds need to be added to Coordinated Universal Time (UTC formerly GMT) to account for the differences in these hyper-accurate atomic clocks and the slowing of Earth's rotation.  The next leap second will be added at the end of June.  So when the clock is just about to strike midnight on June 30th in Greenwich, one extra second will be added to the official UTC time (notated like this: 30 June 2012 23:59:59, followed by 30 June 2012 23:59:60, followed by 1 July 2012 00:00:00). 

It's crazy how accurately measurements can be made nowadays.  The next leap year will be in 2016.  The next leap second could very well be added in December.  The only year in which 2 leap seconds were introduced was the first year of the program, 1972.