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°.
- United States Naval Observatory Nautical Almanac Office; UK Hydrographic Office, HM Nautical Almanac Office (2008). The Astronomical Almanac for the Year 2010. United States Government Publishing Office. p. M5. ISBN 978-0-7077-4082-9.
- "LEVEL 5 Lexicon and Glossary of Terms".
- "The Ecliptic: the Sun's annual path on the Celestial Sphere".