Earth-centered inertial

Earth-centered inertial (ECI) coordinate frames have their origins at the center of mass of Earth and are fixed with respect to the stars.

In a nutshell: The extent to which an ECI frame is actually inertial is limited by the non-uniformity of the surrounding gravitational field.

The location of an object in space can be defined in terms of right ascension and declination which are measured from the vernal equinox and the celestial equator.

Nutation is the smaller amplitude shorter-period (< 18.6 years) wobble that is superposed on the precessional motion of the Celestial pole.

It is due to shorter-period fluctuations in the strength of the torque exerted on Earth's equatorial bulge by the sun, moon, and planets.

To show a location about Earth using the ECI system, Cartesian coordinates are used. The x – y plane coincides with the equatorial plane of Earth. The x -axis is permanently fixed in a direction relative to the celestial sphere , which does not rotate as Earth does. The z -axis lies at a 90° angle to the equatorial plane and extends through the North Pole . Due to forces that the Sun and Moon exert, Earth's equatorial plane moves with respect to the celestial sphere. Earth rotates while the ECI coordinate system does not.