Time=number of repetitions of a periodic phenomenon. Different types: Rotation of the Earth: sideral and universal Orbital motion of Moon and planets: ephemeris, dynamical Frequency of nuclear radiation: atomic time Catherine LeCocq SLAC USPAS, Cornell University Large Scale Metrology of Accelerators June 27 - July 1, 2005

Sideral time = hour angle of the vernal equinox Types Apparent: using the true vernal point: LAST=Local Apparent Sideral Time GAST=Greenwich Apparent Sideral Time Mean: using the mean vernal point: LMST and GMST Catherine LeCocq SLAC USPAS, Cornell University Large Scale Metrology of Accelerators June 27 - July 1, 2005

Universal time = hour angle of the mean Sun + 12 hours Mean Sun = fictitious Sun that moves in the plane of the equator with constant velocity Types: UT0 wrt true pole position UT1 contains the variations in the rotation of the Earth Catherine LeCocq SLAC USPAS, Cornell University Large Scale Metrology of Accelerators June 27 - July 1, 2005

TAI maintained by BIPM (~250 clocks) Atomic Time 1 second = duration of 9162631770 periods of the radiation corresponding to the transition between 2 hyperfine levels of the ground state of the Cesium 133 atom. TAI maintained by BIPM (~250 clocks) For daily use: UTC (Universal Time Coordinate) introduced in 1972: UTC=TAI-n (n is integer of seconds) aka GMT or Zulu Time Catherine LeCocq SLAC USPAS, Cornell University Large Scale Metrology of Accelerators June 27 - July 1, 2005