The orbital plane of large celestial bodies is largely constant compared to the inertial space of the fixed stars . In moons and satellites , however, it is subject to significant orbital disturbances due to the flattening of the central body.
The constancy of the plane of the orbit and the inclination of the orbit is only given in all strictness if Kepler's laws are fully valid. However, this presupposes precisely spherical planets and the absence of any atmosphere or other forces. Particularly when there are other bodies ( three-body problem ), the orbital plane is subject to periodic and secular changes, which in planets can amount to a few arc seconds per year. For smaller bodies ( asteroids , comets ) and for moons or satellites, they range from arc minutes per year to several degrees per day. The periodic fluctuations can be averaged over long periods of time; the resulting mean orbit plane is called the Laplace plane .
Reference plane to define the six Kepler - orbital elements is either the ecliptic (the orbital plane of the Earth) and the Earth's equator . The orbital planes of the eight large planets are inclined only 1 to 3 degrees to the ecliptic - with the exception of Mercury with 7 degrees.