English: To show a location about the earth using the Earth Centered Inertial (ECI) system, Cartesian coordinates are used. The x-y plane coincides with the earth's equatorial plane. The z axis lies at a 90° angle to the equatorial plane and extends through the North Pole. While the earth rotates, the ECI coordinate system does not. The x axis is permanently fixed in a direction relative to the celestial sphere, and the celestial sphere does not rotate like the Earth does. Due to forces exerted from the sun and moon, the earth's equatorial plane moves with respect to the celestial sphere. Since our measurements are calculated relative to satellite positions, this movement will cause error to position solutions if the ECI system were used.