Here's a question that's sure to stump the crowd at your next holiday party: "How many hours are there in a day?"
Most everyone would instinctively answer, "It's 24, of course!" But that's not quite right.
More precisely, we Earthlings currently enjoy 24.0000003 hours in an average day. That tiny additional fraction at the end is a consequence of changing the official definition of a second (in 1967) from an astronomical standard to atomic time. So time is no longer reckoned by Earth's rotation. Instead, atomic clocks around the world are synced using Coordinated Universal Time, or UTC.
Meanwhile, our planet's spin is gradually slowing down (thanks to tidal friction). So to keep UTC in close step with the turning of the Earth, every now and then officials at the International Earth Rotation and Reference Systems Service add a "leap second" to Universal Time — and we'll get our next one on New Year's Eve at 23:59:59 UT, which corresponds to 6:59:59 p.m. Eastern Standard Time.
I'm not sure where I'll be partying at that moment, but I'm glad to have the chance to whoop it up just a bit more as I ring in 2009.
A press release announcing the forthcoming time warp points out that this is the 24th leap second added since 1972, the previous one having been added at the end of 2005.
The U.S. Naval Observatory in Washington, D.C., which issued the announcement, is the world's leader in maintaining and correcting time standards. Just last month, for example, the observatory unveiled a new "master clock" facility. The new digs emphasize function over form. Storage vaults for the dozens of atomic clocks on site (about a third of those operating worldwide) maintain temperature to within 0.1°C and humidity to within 3%.
Among those ultraprecise timekeepers is a next-generation Rubidium Atomic Frequency Fountain clock. The rubidium atoms inside oscillate more than six billion times per second, and the frequency stability can be maintained to fractions of a trillionth of a second. I'm told that a wristwatch version is not yet available.
Precision timekeeping for terrestrial and astronomical purposes can be quite involved; when your time permits, check out our online introduction to the subject. For the gory details on how the need for leap seconds is computed, click here.
Meanwhile, if all you want is the current time, the USNO offers several online methods to get it. And, notes Geoff Chester, the observatory's public-affairs officer, "We even continue to provide dial-up modem time, provided you can get to a command prompt and a phone dialer. The numbers are 202-762-1594 (Washington) and 719-567-6743 (Colorado)" — settings are 1200 baud, N81.
"And, yes, you can still call us for the time," Chester adds. Use 202-762-1401 and 202-762-1069 (Washington) or 719-567-6742 (Colorado).
Finally, time signals are continuously broadcast over shortwave radio at frequencies of 2.5, 5, 10, 15, and 20 megahertz. (As Steve Allen comments below, these broadcasts are a service of the National Institute of Standards and Technology.)
Observers "hear" in the Northeast usually dial in to the Canadian's time-signal broadcast at 7.335 megahertz, which is broadcast via the national Research Council's shortwave station CHU, located near Ottawa. But now we've gotten word that CHU will swtich to a new frequency, 7.850 megahertz.
There's just one small problem: the switchover occurs at 00:00 UT on January 1st. So if you tune in on the old frequency, you'll hear the leap second tick by — and then the broadcast will go dead!