knorthern knight writes: The second used to be defined as 1/86,400 th of a 24-hour day. But ocean tides, pounding against shorelines, slow down earth's rotation, so that a day gets 1.4 milliseconds longer each century. This seems small, but it would affect scientific constants (speed of light, etc). Since 1967, the International System of Units (SI) has defined the second as the duration of 9192631770 cycles of radiation corresponding to the transition between two energy levels of the caesium-133 atom. http://en.wikipedia.org/wiki/Atomic_clock#Mechanism Every few years, a "leap second" is added as necessary, either on June 30th or December 31st. Enjoy that extra second of sleep.