Orbital plane (astronomy)
The orbital plane is the plane of the orbit of the celestial body.
The orbital plane of large celestial body is largely constant over the inertial space of the fixed stars. However, with moons and satellites it is subject to significant perturbations due to the oblateness of the central body.
The constancy of the orbital plane and the orbital inclination is rigorously only if the kepler 's laws are fully valid. However, this presupposes precisely spherical planet and the lack of any atmosphere and other forces. In particular, when other body ( three-body problem ), the orbital plane is subject to periodic and secular changes that can make a few seconds of arc per year for the planet. For smaller bodies ( asteroids, comets ) and moons or satellites, ranging from minutes of arc per year up to several degrees per day. The periodic fluctuations can be averaged over long periods of time; the resulting average plane of the web is referred to as the Laplace plane.
Reference plane to define the six Keplerian orbital elements is either the ecliptic plane ( the orbital plane of the Earth), or the Earth's equator. The orbital planes of the eight major planets are only 1-3 degrees to the ecliptic tilted - with the exception of Mercury at 7 degrees.