Anyone who'd like to "feel" the Earth slowing down can do so on Saturday evening, June 30, 2012.(GMT) 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.
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