Tomorrow night — June 30, 2012 — the world's official timekeepers will add a leap second for the first time in 3½ years.
Anyone who'd like to "feel" the Earth slowing down can do so on Saturday evening, June 30, 2012. The final minute before midnight Greenwich Mean Time will contain 61 seconds — an adjustment needed to bring the world's clocks back into sync with Earth itself.
This is the first occasion in 3½ years when a "leap second" has been necessary, and only the third time since the start of the new millennium. In contrast, seven leap seconds were added during the 1990s — a clue that the slowdown is neither regular nor predictable. (Tidal friction within the Earth is the main cause, but there are fluctuations due to shifting proportions of water in the polar and equatorial regions and other factors.) For more on the purpose and history of the leap second, see this U.S. Naval Observatory press release or this more technical discussion.
Should leap seconds be abolished? Since 2003 the International Telecommunication Union (ITU), a United Nations agency, has debated doing just that. Communications engineers say it would simplify time distribution around the globe. But most astronomers favor the current (and time-honored) link of clock time to the Sun, which occasional leap seconds help to maintain. With member countries unable to agree, the ITU decided in January to revisit the issue in 2015.
How To "Observe" the Leap Second
The June 30th leap second will be inserted into the very last minute before Greenwich midnight, which corresponds to 8 p.m. Eastern Daylight Time (or 5 p.m. PDT) in North America. You might try listening to the Naval Observatory Master Clock by calling 202-762-1401 or visiting online — but good luck getting through if too many people have the same idea.
Or find a shortwave radio. A few minutes before the appointed hour, tune in a time-signal station like WWV (at 5, 10, 15, or 20 megahertz) or CHU (at 3.330, 7.335, or 14.670) and start counting ticks. For successive minutes you'll get 60, 60, 60, ... , and then 61 seconds, just before the top of the hour.
It's easy to lose count if signal reception fades, so I prefer to tie a small weight to a string 39 inches long and let it swing from a nail over an open doorway. I adjust the string's length (a slip knot helps) until the weight makes exactly 60 swings, back or forth, during a normal minute. Then, in the final moment before the witching hour, the weight will make one more jog and start the hour with a swing in the opposite direction.