Ephemeris Time - JPL Ephemeris Time Argument T_eph

JPL Ephemeris Time Argument T_eph

High-precision ephemerides of sun, moon and planets were developed and calculated at the Jet Propulsion Laboratory (JPL) over a long period, and the latest available were adopted for the ephemerides in the Astronomical Almanac starting in 1984. Although not an IAU standard, the ephemeris time argument T_eph has been in use at that institution since the 1960s. The time scale represented by T_eph has been characterized as a relativistic coordinate time that differs from Terrestrial Time only by small periodic terms with an amplitude not exceeding 2 milliseconds of time: it is linearly related to, but distinct (by an offset and constant rate which is of the order of 0.5 sec/yr) from the TCB time scale adopted in 1991 as a standard by the IAU. Thus for clocks on or near the geoid, T_eph (within 2 milliseconds), but not so closely TCB, can be used as approximations to Terrestrial Time, and via the standard ephemerides T_eph is in widespread use.