February 29, 2009 will be the longest day since 1992. The reason for this lingering day is an adjustment that must be made to the world’s clocks in order to account for the earth’s slowing rotation. Such adjustments aren’t anything new. 34 adjustments have been made since 1972. The U.S. Naval Observatory in Washington, D.C., will take care of the atomic clocks under their charge with a party at 6:59:60 p.m., the moment when the second will be made official.
Unlike the first atomic clock of 1949, (see gallery,) modern atomic clocks depend upon very precise readings from space as the video above explains.