Ecliptic: Apparent path of the Sun on the celestial sphere
The ecliptic is the plane of Earth's orbit around the Sun.
The Sun appears to move against the background of stars because the Earth orbits the Sun.
The invariable plane is the average plane of the orbits of the planets. Properly, it is defined as the plane passing through its barycenter (center of mass). The present ecliptic plane is inclined to the invariable plane by about 1.5°.
References
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