Tonight will be longer by a second. International timekeepers are adding a leap second to the clock at midnight universal time, which is 7 p.m. CDT. Universal time will be 11:59:59 and then the unusual reading of 11:59:60 before it hits midnight.
A combination of factors, including Earth slowing down a bit from the tidal pull of the moon, and an atomic clock that’s a hair too fast, means that periodically timekeepers have to synchronize the official atomic clocks, said Daniel Gambis, head of the Earth Orientation Service in Paris that coordinates leap seconds.
The time it takes the Earth to rotate on its axis — the definition of a day — is about two milliseconds longer than it was 100 years ago,
said Geoff Chester, spokesman at the U.S. Naval Observatory and keeper of official U.S. atomic clocks. That adds up to nearly three-quarters of a second a year.
Timekeepers add leap seconds every now and then to keep the sun at its highest at noon, at least during standard time. This is the first leap second since January 2009 and the 25th overall. Gambis said the next one probably won’t be needed until 2015 or 2016.