Terrestrial Time
One commonly used ECI frame is defined with the Earth's Mean Equator and Equinox at 12:00 Terrestrial Time on 1 January 2000. It can be referred to as "J2K", "J2000" or "EME2000." The x-axis is aligned with the mean equinox. The z-axis is aligned with the Earth's rotation axis or celestial North Pole. The y-axis is rotated by 90° East about the celestial equator.