Friday, February 29th, 2008

Telling Time: Leap Year, Leap Day

Got the time? The NIST-F1 Cesium Fountain Clock will neither gain nor lose one second in more than 60 million years. copyright Geoffrey Wheeler

This Friday is a date that comes only once every 28 years - Friday, February 29th. This year is a leap year, with an extra day added at the end of the month of February to help the modern Gregorian calendar sync up with the astronomical calendar. It's needed because the actual time between two yearly solar events, such as the vernal equinox, works out to be 365.2422 days -- resulting in almost one extra day every over the course of four years.

Of course, since it's not almost one day, the rules for leap years are a little more complicated than 'every fourth year.' If a year is divisible by 4, it's a leap year -- unless it's also divisible by 100, in which case it's not a leap year -- unless it is divisible by 400, in which case it is a leap year. (Meaning that while 1800 and 1900 were not leap years, the year 2000 was.) Confused? Let's not even talk about leap seconds. Join us in this segment for a look at leap years, leap days, and the finer points of telling time.