The International Earth Rotations and Reference Systems Service has announced a leap second will be inserted into the calendar immediately before the stroke of midnight on January 1, 2009.
If you’re a real geek and are sitting at your PC rather than partying on New Year’s Eve, run the
date command once per second around midnight London-time. The time as reported would progress like this:
Wed Dec 31 23:59:56 UTC 2008(?!)
Wed Dec 31 23:59:57 UTC 2008
Wed Dec 31 23:59:58 UTC 2008
Wed Dec 31 23:59:59 UTC 2008
Wed Dec 31 23:59:60 UTC 2008
Thu Jan 1 00:00:00 UTC 2009
Thu Jan 1 00:00:01 UTC 2009
Thu Jan 1 00:00:02 UTC 2009
A leap second is inserted into the calendar whenever the time as measured by a collection of atomic clocks (UTC) and the time as measured from the stars (UT1) differs by more than 0.9 seconds. If there is a difference, a second (or two, in extreme cases) can be added or subtracted from our “official” time. This brings our clock time (called TAI) as close as possible to the Earth’s rotation, while still maintaining a regular clock-tick second.
The U. S. Naval Observatory is the official time-keeping entity of the United States. They currently host 34 cesium clocks and 14 hydrogen-maser clocks that together provide an exceptionally stable second. The USNO Time Service Department hosts a wonderful reference for all things time. In particular, their summary systems of time is a good (if not confusing) overview of the different time scales.
There are a few standards for leap seconds. Leap seconds are only added at two points each year — the last second of June 30 or December 31. They’re announced at least a few months ahead of the leap second — this allows plenty of time to alert those who really do need to worry about this stuff. Leap seconds occur simultaneously at all points around the world. Because UTC is based on Greenwich mean time, the leap second will occur then. (For those of us on the east coast of the United States, five hours behind GMT, the leap second will occur at 18:59:60 EST on December 31, 2008.)
Most clocks aren’t precise enough to care. Your bedroom alarm clock probably deviates UTC by more than one second, unless you’re a complete time pedant (or you’re just really lucky). Being off by one additional second (or one fewer second, depending on whether your clock is slow or fast) isn’t going to make any considerable difference.
Some clocks do care. The GPS system’s clocks started at zero at midnight UTC on January 6, 1980. There have been fourteen leap seconds since this date, and so the GPS system runs fourteen seconds faster than UTC. After the next New Year’s party, GPS will be an even quarter-minute faster than UTC. The lesson? Don’t synch your atomic clock to your GPS receiver.
If you have one of those fancy-dancy radio-based atomic clocks and you happen to be standing near it at midnight, I’d love to see if at 23:59:59 UTC it hangs for two seconds instead of one. If you get video of this I’ll give you a dollar.