1. 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

2. 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

3. 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

4. TAI maintained by BIPM (~250 clocks)
Atomic Time
1 second = duration of 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