Time
From 1889 until 1967 the unit of time was defined as a certain fraction of the mean solar day, the average time interval between successive arrivals of the sun at its highest point in the sky. The present standard, adopted in 1967, is much more precise. It is based on an atomic clock, which uses the energy difference between the two lowest energy state of cesium atom. Electromagnetic radiation (microwaves) of precisely the proper frequency causes transitions from one of these states to the other. One second is defined as the time required for 9, 192, 631, 770 cycles of this radiation.
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