The Earth’s spin axis and the equinox move, so “the” ECI frame has to be pinned down precisely.
Motion of the Spin Axis
- Celestial motion of the spin axis:
- Precession: 25,800 years
- Nutation: shorter-period oscillation (up to 18.6 years)
- Terrestrial motion, sub-daily:
- Polar motion: ~10 m
- Tidal & relativistic effects: ocean/Earth tides cause sub-daily variations in the Earth rotation speed (UT1)
Inertial Frames
- GCRF: geocentric celestial reference frame
- EME2000 (formerly J2000): Earth mean equator & equinox of J2000
- TEME: true equator, mean equinox
- Epoch time of the inertial frame:
- January 1, 2000 12:00 TT (terrestrial time)
- EME2000 and GCRF have ~20 mas (milliarcsecond) difference; we will not worry about the difference
- Mean equator, mean equinox: EME2000, GCRF
- Special reference frame TEME (true equator, mean equinox): used for mean elements (explained later), considered much less accurate
Mean and True
- “Mean”: includes only precession (25,800 year period); the nutation is averaged
- “True”: the actual, instantaneous direction including both precession and nutation
- “Mean of date”, “true of date”:
- Of date: coordinates and reference planes at the instantaneous moment of observation
- Of epoch: fixed at a specific fundamental reference time, such as J2000.0
- The same distinction gives mean vs apparent sidereal time: GMST and GAST