IERS Earth orientation data published in Bulletins A and B are given at 0h UTC (Coordinated Universal Time). The time argument to be used with the IAU2000A precession/nutation model is TT (Terrestrial Time). UTC is defined as TAI (International Atomic Time) + the number of leap seconds since 1972. TT is conventionally realized by TT = TAI + 32.184 s. The error accumulated in TAI since 1977 (i.e. its possible deviation from TT - 32.184 s) can now reach 20 microseconds (which is negligible when computing X and Y, but not the Earth Rotation Angle or UT1).
Glossary:
- Coordinated Universal Time
- Earth Rotation Angle
- International Atomic Time (TAI)
- leap second
- Terrestrial Time
- Universal Time (UT1)
