It’s January 1, 2009, only three more months until April Fools Day. Have you thought up a good joke yet? But we mustn’t get ahead of ourselves. And that isn’t just a figure of speech.

Reportedly just before midnight last night, one second was added to 2008; a “leap second.” This was done by counting down “57, 58, 59, 60, 00,” with “60” representing the leap second. So whether we knew it or not, and chances are we didn’t, everyone on earth experienced a rare 61 second minute.

Due to its inconstant speed and characteristic wobble, the Earth’s rotation is slowing, meaning that in order to keep the sun overhead at noon, atomic time must be aligned with astronomical time which requires the addition of a “leap second” every so often.

When did time become so precise? The answer is 1967, when the world switched from the imperfect astronomical timescale to Coordinated Universal Time (UTC), an atomic timescale clock which is used for broadcasting time signals across the world. The UTC’s accuracy is critical to the operation of GPS and the Internet, which is why it must be adjusted every so often.

So you missed last year’s leap second, when will the next one happen? No one knows…yet. Whenever the difference between UTC and UT1 (UT1 is proportional to the true rotation angle of the Earth with respect to a fixed frame of reference) approaches 0.6 seconds, the people who keep track of such things, collectively known as the International Earth Rotation and Reference Systems Service (IERS), announce the insertion of a leap second in “Bulletin C”, which is published about every six months. Leap seconds aren’t exclusive to New Years Eve however; they can happen either on December 31st or June 30th.

If you’re interested and you find yourself with nothing better to do the next time we have a leap second you can watch it happen here: http://www.time.gov/timezone.cgi?Mountain/d/-7/java