Stories about time - atomic clock

 Use this true story when teaching about time and distance:

Atomic Clock  Isidor Rabi (1898-1988), 
The atomic clock invented in 1949 measures natural resonance frequency of atoms. 
The National Institute of Standards and Technology started using an atomic clock called NIST F-1 in Boulder Colorado 1999 to define time:  A second is defined as 9,192,631,770 vibrations of a cesium atom. The clock will lose 1 second every 20 million years.